Advancing the Sustainability of Poplar-Based Agroforestry: Key Knowledge Gaps and Future Pathways

Abstract

Poplars (Populus L.) are fast-growing, widely distributed trees with high ecological, economic, and climate-mitigation value, making them central to diverse agroforestry systems worldwide. This study presents a comprehensive bibliometric and content-based review of global poplar-based agroforestry research, using Scopus and Web of Science databases and a PRISMA-guided screening process to identify 496 peer-reviewed publications, covering publications from 1987 to 2024. Results show a steady rise in scientific output, with a notable acceleration after 2013, dominated by agriculture, forestry, and environmental sciences, with strong international contributions and research themes focused on productivity, carbon sequestration, biodiversity, and economic viability. A wide range of Populus species and hybrids is employed globally, supporting functions from crop production and soil enhancement to climate mitigation and ecological restoration. Poplar-based systems offer substantial benefits for soil health, biodiversity, and carbon storage, but also involve trade-offs related to tree–crop interactions, such as competition for light reducing understory crop yields in high-density arrangements, management intensity, and regional conditions. Poplars provide a wide array of provisioning, regulating, and supporting ecosystem services, from supplying food, fodder, timber, and biomass to moderating microclimates, protecting soil and water resources, and restoring habitats, while supporting a broad diversity of agricultural and horticultural crops. However, several critical gaps—including a geographic research imbalance, socio-economic and adoption barriers, limited understanding of tree–crop interactions, and insufficient long-term monitoring—continue to constrain widespread adoption and limit the full realization of the potential of poplar-based agroforestry systems.

1. Introduction

Populus L. is a genus of fast-growing, dioecious, and deciduous trees native to the Northern Hemisphere, with a wide distribution across North America, Europe, and Asia. Around 96–118 species, along with numerous hybrids and countless clones of poplars, exist worldwide [1,2,3,4]. Their natural habitats are typically moist areas, like river valleys and swamps, although some species tolerate drier conditions. While historically significant for local use, they are increasingly grown in plantations in important forest economies (China, the United States of America, India, Italy, Argentina, Canada), for timber, paper, bioenergy, and carbon storage [5,6,7,8]. Poplars are especially appreciated for their fast growing rate [9,10,11], being also introduced worldwide in cities as one of the main tree species within the urban dendroflora [12,13,14].

Recently, growing attention has focused on the role of poplar plantations in climate change mitigation [15,16,17]. Climate change leads to increased frequency and intensity of extreme events, modified precipitation patterns [18], and influences water availability, which will affect the water cycle [19,20] and accelerate soil erosion [21], both in agricultural and forested areas [22,23], and influence the naturalness level of the forests [24,25,26]. These changes directly influence the provision not only of the provisioning, regulating, and supporting ecosystem services but also recreational activities [27].

One of the best solutions to mitigate climate change worldwide is agroforestry [28,29,30]. Integrating trees into agricultural systems through agroforestry is increasingly recognized as a key mitigation and adaptation strategy for climate change, thanks to its potential for carbon sequestration, enhanced ecosystem services, and reduced pressure on natural forests. For example, the Food and Agriculture Organization of the United Nations (FAO) recently described agroforestry as a “key climate solution” that can help reduce greenhouse gas emissions, enhance biodiversity, improve soil fertility, and support livelihoods across smallholder farms [31]. Moreover, the Intergovernmental Panel on Climate Change (IPCC) identifies agroforestry (within sustainable land management and agriculture/forestry linkages) as one of the response options with substantial mitigation potential: depending on scale-up, crop/livestock and agroforestry measures combined could reduce global annual emissions by 2.3–9.6 Gt CO₂-eq by 2050 [32]. Agroforestry is also one of the best solutions for food security, reducing poverty, and protecting the environment. It is based on a wide range of systems—agrosilviculture (boundary plantation, windbreaks, shelterbelts, hedgerows, alley cropping, intercropping, taungya system), hortisilviculture, agrihortisilviculture, silvopastoral (fodder banks, foraging forests), agrosilvopastoral, forest farming systems, multi-strata agroforestry systems, home gardens, recreation forestry, urban/rural food forests, landscaping, rehabilitation of degraded forests, and riparian forest buffers (corridors, galleries) [33,34,35,36]. The great diversity of methods creates a wide range of options around the world. By combining this wide array of solutions with the diverse ecological traits and economic uses of poplars, numerous opportunities arise to optimize global resources for the benefit of both society and the natural environment [37,38].

Several review articles about the role of trees under different conditions or management types [39,40,41,42,43], as well as review articles specifically on poplars [44,45,46,47,48], were published in recent years.

The aim of this article is to provide a comprehensive bibliometric and content-based review of global research on poplar (Populus spp.) integration within agroforestry systems. Specifically, the study seeks to identify trends, geographical patterns, and research hotspots related to poplar-based agroforestry; to document the diversity of Populus species and hybrids used worldwide; and to analyze their roles in productivity, carbon sequestration, biodiversity enhancement, and sustainable land management. By synthesizing bibliometric data and thematic findings, the review aims to highlight knowledge gaps, regional differences, and opportunities for future research and sustainable application of poplars in agroforestry systems. What differentiates this review from previous work is its combined approach: it integrates bibliometric analysis with a thematic, content-based review, providing both a quantitative overview of global research patterns and a qualitative synthesis of technical and ecological findings. This dual approach allows us to identify not only what has been studied and where, but also how research themes interconnect and where critical knowledge gaps remain, thus filling an important gap in the literature.

2. Materials and Methods

This study was conducted in two complementary phases. The first phase consisted of a bibliometric analysis aimed at investigating global research trends related to poplars and agroforestry. Publications were retrieved from two leading bibliographic databases: Scopus and the Science Citation Index Expanded (SCI-Expanded) within the Web of Science (WoS).

The initial search focused on the query “poplars and agroforestry,” alongside additional keywords and Boolean operators to ensure comprehensive coverage. Keywords included shelterbelts and windbreaks, combined using Boolean operators AND and OR to refine the search scope. These terms were chosen to capture a wide spectrum of agroforestry practices involving poplar species.

Following the initial retrieval, all records were screened and refined according to PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines [49].

• Search strings and database queries

The initial search focused on identifying publications explicitly related to poplar species (genus Populus) in combination with agroforestry or agroforestry-related practices. To ensure that the search captured records linking both elements, we applied a two-component Boolean structure consisting of the following:

(A) poplar-related terms AND (B) agroforestry-related terms

Thus, the overall logic was: (poplar-related keywords) AND (agroforestry/agroforestry-practice keywords). This structure minimized the retrieval of records that mentioned only agroforestry practices or only poplars without conceptual relevance to poplar-based agroforestry.

Full search strings and database implementation

Scopus (advanced search, TITLE-ABS-KEY):

(TITLE-ABS-KEY (“poplar*” OR “Populus”))

AND

(TITLE-ABS-KEY (“agroforestry” OR “shelterbelt*” OR “windbreak*”))

Web of Science—SCI-Expanded (topic search, TS):

S = (“poplar*” OR “Populus”)

AND

TS = (“agroforestry” OR “shelterbelt*” OR “windbreak*”)

Wildcards (*) were used to include plural and morphological variants. Field tags and syntax were adjusted according to the requirements of each database.

These queries were designed to capture both general references to poplars and agroforestry as well as specific practices commonly associated with poplar-based agroforestry systems. Syntax adjustments were applied to accommodate database-specific requirements while maintaining logical structure. Wildcards (*) were used to capture plural forms and morphological variants.

• Search parameters

Time span:unrestricted in the search settings (all years up to the search date: 15 October 2024). The earliest record retrieved after deduplication dated from 1987.

Document types: peer-reviewed articles and reviews only.

Exclusions: conference proceedings, editorials, correspondence, book chapters, and theses were filtered out during screening.

• De-duplication and data quality control

Duplicate records were removed using a two-step process:

• Automated duplicate detection based on DOI and exact title matching in Microsoft Excel.
• Manual screening of remaining records by title, first author, publication year, and journal to resolve missing or inconsistent DOI entries.

This process removed 316 duplicate records. The relatively high proportion of duplicates is expected because Web of Science and Scopus index many of the same journals and bibliographic sources, resulting in substantial overlap between search outputs. Previous reviews using both databases have reported similar duplicate rates, often ranging between 30% and 50%. Data quality control included verification of metadata fields (title, abstract, authorship, year), correction of OCR errors, and standardization of author and institution names. All modifications were documented in a data-audit log.

• Inclusion and exclusion criteria

A two-stage screening process was applied: title/abstract screening followed by full-text review. Criteria included the following:

Inclusion: peer-reviewed articles or reviews; English language; clear relevance to poplar-based agroforestry, defined as follows: any study where Populus species are a primary component of an agroforestry, silvopastoral, or tree-crop/grass system; or studies analyzing shelterbelts, where poplar trees constitute ≥ 20% of the woody component or are the main species under investigation; sufficient metadata available.

Exclusion: non-peer-reviewed items (editorials, correspondence, posters, theses, patents); studies not involving poplar-based agroforestry systems; studies where poplar appears only in a list of species, background text, or in <10% of the study content, and the agroforestry system analyzed did not include poplar as a study variable, treatment, or focus species; inaccessible full text; lack of abstract.

Full-text exclusions were coded using structured reasons:

(A) out of scope, (B) non-peer-reviewed/editorial, (C) no poplar/agroforestry data, (D) full text inaccessible, (E) insufficient methodological detail.

• Screening procedure

Two independent reviewers (Reviewer A and Reviewer B) performed the screening. At stage one, titles and abstracts were assessed independently. Records deemed potentially relevant by either reviewer progressed to full-text screening. Stage two involved an independent full-text review. Disagreements were resolved by a third senior reviewer (Reviewer C) through discussion and adjudication. All screening steps were conducted manually using Microsoft Excel, which was used to organize records, track decisions, and document reviewer agreements and conflicts.

Final selection and dataset. After applying the two-stage screening method and resolving disagreements, 496 publications met the inclusion criteria and were selected for bibliometric and qualitative content analysis (

Table 1. The most representative journals publishing articles on poplars and agroforestry.

Crt. No.	Review	Documents	Citations	Total Link Strength
1	Agroforestry Systems	94	2290	271
2	Agriculture Ecosystems & Environment	14	579	77
3	Ecological Engineering	11	744	73
4	Forests	17	207	64
5	Forest Ecology and Management	12	256	56
6	Biomass & Bioenergy	12	660	51
7	Current Science	10	127	45
8	Sustainability	7	70	43
9	Plant and Soil	9	361	37
10	Journal of Environmental Management	7	118	31
11	Agronomy	7	53	24
12	Plos one	3	64	14
13	Geoderma	5	312	8
14	Science of the Total Environment	3	53	8

, Figure 1).

The bibliometric analysis encompassed nine dimensions: (1) publication type, (2) research discipline, (3) year of publication, (4) geographic distribution of contributions, (5) authorship, (6) institutional affiliations, (7) journals, (8) publishers, and (9) keywords. Data processing and visualization were performed using the Web of Science Core Collection (version 5.35, Clarivate) [50], Scopus [51], Microsoft Excel (version 2024) [52], and Geochart (version 5.35) [53]. To further investigate bibliometric relationships, VOSviewer (version 1.6.20) [54] was used to generate maps of co-authorship networks, co-citation patterns, and keyword co-occurrence clusters, respectively.

The second phase involved a qualitative content analysis of the 496 screened publications. This step provided a deeper understanding of knowledge production on the topic and enabled the categorization of results into four thematic domains: (1) global research trends on poplars and agroforestry; (2) populus species used in agroforestry; (3) growth, biomass, and carbon stocks of poplars in agroforestry: implications for sustainability; (4) ecological characteristics and sustainability of poplar in agroforestry systems; (5) effects on biodiversity and ecosystem functions in poplar-based agroforestry systems; (6) agroforestry systems with poplars used (Figure 2).

3. Results

3.1. A Bibliometric Review

The bibliometric analysis identified a total of 496 publications on this topic. The vast majority consisted of research articles (454 records, 92%), followed by 17 review articles (3%), 16 proceedings papers (3%), and 9 book chapters (2%) (Figure 3).

The annual number of publications has increased steadily, with particularly pronounced growth after 2013. Over the past 14 years, the average yearly output has ranged between 30 and 40 publications (Figure 4).

The distribution of publications across research areas reveals the dominance of 3 categories among the 25 identified: Agriculture (225 articles), Forestry (151 articles), and Environmental Sciences/Ecology (101 articles), respectively (Figure 5).

Among the most prolific contributors to this topic, French-speaking researchers were particularly prominent. Julien Fortier, Benoit Truax, and Daniel Gagnon each authored 16 publications, followed closely by France Lambert with 15, respectively.

Researchers from 53 countries across 5 continents contributed to publications on this topic (Figure 6). The most represented countries were India (97 articles), Canada (73 articles), China (65 articles), and Germany (61 articles).

The countries can be grouped into six clusters, three of which include at least five countries. The first cluster comprises Canada, Egypt, France, Greece, Morocco, and South Africa; the second includes Belgium, Finland, Italy, Poland, South Korea, and the United States of America; and the third consists of England, the Netherlands, Portugal, Spain, and Switzerland, respectively (Figure 7).

Papers on this topic were published in 159 journals. The most prominent journal was Agroforestry Systems, with 94 articles, approximately 6 times more than the next most represented journal, followed by Forests (17 articles) and Agriculture, Ecosystems & Environment (14 articles), respectively (Table 1, Figure 8).

The most representative institutions for authors who have published on this topic were as follows: Punjab Agricultural University (with 38 articles), Indian Council of Agricultural Research (with 33 articles), and Chinese Academy of Sciences (with 19 articles). Among the 59 publishers that have issued articles on this topic, the most prominent were: Springer Nature (126 articles), Elsevier (96 articles), and MDPI (45 articles).

Agroforestry, poplar, biomass, and plantations were the most frequently used keywords in the published articles (

Table 2. The most frequently used keywords in articles on poplars and agroforestry.

Cur. No.	Keyword	Occurrences	Total Link Strength
1	Agroforestry	173	517
2	Poplar	98	326
3	Biomass	76	297
4	Plantations	44	190
5	Nitrogen	42	183
6	Dynamics	41	182
7	Carbon sequestration	43	169
8	Yield	45	167
9	Management	34	158
10	Sequestration	35	155
11	Agroforestry systems	42	149
12	Biomass production	35	144
13	Competition	26	109
14	Forest	28	108
15	Productivity	32	108
16	Carbon	26	98

).

The keywords can be grouped into three clusters. The first includes terms related to forest and management: afforestation, forest, ecosystem services, management, climate change, and carbon sequestration. The second includes terms related to poplars and agroforestry: Populus deltoides, trees, agroforestry systems, temperate agroforestry, growth, and yield. The third included terms related to biomass and energy: biomass, biomass production, bioenergy, energy, and productivity (Figure 9).

3.2. A Classical Review

3.2.1. Global Research Trends on Poplars and Agroforestry

The main research areas regarding the poplars and agroforestry systems are presented in

Table 3. Global research on poplar-based agroforestry: topics, regions, and key references.

Cur. No.	Area of Research	Region	Citing Article
1	Assessment of crop yield, productivity, and carbon sequestration in agroforestry systems	India	Adhikari et al., 2020 [36]
2	Awareness of adopters and non-adopters towards different aspects of poplar-based agroforestry	India	Rakesh Nanda et al., 2001 [55]
3	Doubling farmers’ income through Populus deltoides-based agroforestry systems	India	Chavan et al., 2022 [56]
4	Economics of poplar short-rotation coppice plantations	Germany	Schweier and Becker, 2013 [57]
5	Effects of tree competition on corn and soybean photosynthesis, growth, and yield	Canada	Reynolds et al., 2007 [58]
6	Energy dynamics in Populus deltoides G3 Marsh agroforestry systems	India	Chaturvedi and Das, 2005 [59]
7	Increased nitrogen-use efficiency of a short-rotation poplar plantation in elevated CO2 concentration	Italy	Calfapietra et al., 2007 [60]
8	Influence of agroforestry plant species on the infiltration of S-Metolachlor in buffer soils	France	Dollinger et al., 2019 [61]
9	Limits to recruitment of tall fescue plants in poplar silvopastoral systems	Argentina	Clavijo et al., 2010 [62]
10	Microbial biomass dynamics during the decomposition of leaf litter of poplar	India	Chander et al., 1995 [63]
11	Phosphorus storage and resorption in poplars	Canada	Da Ros et al., 2018 [64]
12	Poplar agroforestry: a re-evaluation of its economic potential on arable land	United Kingdom	Willis et al., 1993 [65]
13	Poplar in wetland agroforestry	China	Fang et al., 2010 [66]
14	Poplar tree for innovative plantation models	Italy	Bergante, 2022 [67]
15	Potentials of poplar and eucalyptus in Indian agroforestry for revolutionary enhancement of farm productivity	India	Bangarwa and Sirohi, 2018 [68]
16	Poplar (Populus deltoides) based agroforestry system	India	Deswal et al., 2014 [69]
17	Prospects for the use of walnut and poplar in agroforestry	Ukraine	Ivaniuk et al., 2023 [70]
18	Silvopastoral systems in temperate zones	Chile	Dube et al., 2016 [71]
19	Soil organic carbon sequestration by shelterbelt agroforestry systems	Canada	Dhillon and Rees, 2017 [72]
20	Stem taper equations for poplars growing on farmland	Sweden	Hjelm, 2013 [73]
21	Suitability of fast-growing tree species (Salix spp., Populus spp., Alnus spp.) for the establishment of economic agroforestry zones for biomass energy	Latvia	Daugaviete et al., 2022 [74]
22	Understory plant diversity and biomass in hybrid poplar riparian	Canada	Fortier et al., 2011 [75]
23	Vertical root separation and light interception in a temperate tree-based intercropping system	Canada	Bouttier et al., 2014 [76]
24	Yield and quality of sugarcane under poplar (Populus deltoides)-based rainfed agroforestry	India	Chauhan and Dhiman, 2003 [77]

.

Analysis of the published literature revealed that diverse research focuses on poplar-based agroforestry systems across multiple regions. The studies can be broadly categorized into four main areas: productivity and carbon sequestration, economic potential, plant–soil interactions, and system-specific innovations.

Productivity and carbon sequestration: A significant proportion of research has focused on assessing the impact of poplar-based agroforestry systems on crop yield, farm productivity, and carbon sequestration. In India, studies have demonstrated that integrating Populus deltoides into agroforestry systems can enhance crop productivity and potentially increase farmers’ income [18,56]. Similar assessments have been reported in Canada, focusing on the effects of tree competition on the photosynthesis, growth, and yield of intercrops such as corn and soybean [58], as well as on soil organic carbon sequestration by shelterbelt systems [72].

Economic assessments: Economic analyses of poplar-based systems represent another major research focus. Early studies in the UK evaluated the economic potential of poplar agroforestry on arable land [65], while more recent works in Germany and India have examined the profitability of short-rotation coppice plantations and the potential for doubling farmers’ income through improved system designs [57,68].

Plant–soil interactions and ecosystem services: Several studies have investigated the biophysical and ecological functions of poplar in agroforestry. Research has examined microbial biomass dynamics during leaf litter decomposition [63], phosphorus storage and resorption [64], nitrogen-use efficiency under elevated CO₂ [60], and the influence of tree species on pesticide infiltration in buffer soils [61]. The impact of poplar on understory plant diversity, root architecture, and light interception has also been explored in temperate systems [75,76].

Innovations and system-specific studies: Research has additionally highlighted the role of poplar in silvopastoral systems [62,71], wetland agroforestry [66], and the development of novel plantation models [67]. Studies have also explored the suitability of fast-growing tree species, including Populus spp., for biomass energy and sustainable land-use systems [74].

Geographically, India emerges as the most extensively studied region, reflecting the importance of poplar in national agroforestry initiatives. Canada, Italy, and other temperate countries also contribute significantly to research on ecological and productivity aspects, while economic and system-level studies are more widespread in Europe.

3.2.2. Populus Species Used in Agroforestry

From the analysis of several hundred articles, poplar genotypes, including species, hybrids, varieties, and clones, were identified as being utilized across different continents in agroforestry systems with annual and perennial plants of agricultural, horticultural, livestock, industrial, and medicinal relevance (

Table 4. Populus species used in agroforestry specifically for their ecological role.

Cur.No.	Species, Hybrid, Variety, Clone–Name, Region	Agroforestry System	Use	Country-Citation
	section Populus
1	P. adenopoda—Chinese aspen (E.Asia)	shelterbelts, windbreaks	R: fodder, timber, habitat loss, migration assistance.	CN—Ranjitkar et al., 2021 [78]; Tian et al., 2025 [79].
2a	P. alba var. pyramidalis	shelterbelts	R: greening arid zones; land reclamation.	CN—Xu et al., 2011 [80]; Tian et al., 2024 [81]
3	P. × canescens—gray poplar	shelterbelts, tree rows, gallery forests	R: habitat protection, sand control, carbon sequestration.	HU—Borovics et al., 2025 [82]; TR—Özcan et al., 2020 [83]; KZ—Kairova, 2023 [84].
4	P. davidiana—Korean aspen (E.Asia)	shelterbelts, windbreak	R: fodder, timber, bioenergy feedstock; sand fixation.	CN—Ranjitkar et al., 2021 [78].
5	P. grandidentata—bigtooth aspen (N.America)	farm woodlands	R: water control, SRC biomass energy, plant succession.	US—Perry et al., 2001 [85].
6	P. sieboldii—Japanese aspen (E.Asia)	shelterbelts, extension forestry	R: sand control, wood, waste landfills reclamation	CN—Gao et al., 2024 [86]; KR—Kim & Lee, 2005 [87].
7	P. tremula—common aspen (Europe, N.Asia)	shelterbelts, wood-pastures, alley cropping	R: wastewater purification; AC: oilseed rape, winter wheat; WP: grazing	EE—Heinsoo, 2014 [88]; DE—Zitzmann, 2025 [89]; FI—Oldén, 2016 [90].
8	P. tremuloides—quaking aspen (E.N.America)	hedgerows, silvopastoral system	R: soil erosion, windbreaks, carbon sequestration, crop productivity	CA—Gross et al., 2022 [91]; CA—Chen et al., 2025 [92]; US—Powell & Bork, 2006 [93].
	section Aigeiros
10	P. fremontii—Fremont cottonwood (W.-N.Am.)	shelterbelts, river corridors	R: biodiversity (bird nesting), soil humidity.	US—Nishida et al., 2013 [94].
11	P. nigra L. var. pyramidalis	Shelterbelts	R: antidesertification; farmland protection	KZ—Kairova, 2023 [84].
	section Tacamahaca
12	P. angustifolia—willow-leaved poplar (C.-N.Am.)	roadside, river corridors	R: riparian zones.	US—Goodrich et al., 2008 [95].
16	P. hsinganica	forest farming	R: sand control, desert greening.	CN—Wen & Li, 1996 [96].
17	P. intramongolica	riparian forest buffers	R: sand control, desert greening.	CN—Wen & Li, 1996 [96].
18	P. kangdingensis	riparian forest buffers	R: drought resistance, land reclamation.	CN—Yin et al., 2005 [97].
19	P. koreana—Korean poplar (NE Asia)	SRC forest blocks, urban forestry	R: pollution mitigation, carbon sequestration, urban greening	RU—Sultanova et al., 2025 [98].
20	P. laurifolia—laurel-leaf poplar (central Asia)	shelterbelts, gallery forests, landscaping	R: biodiversity, phytoremediation of coal deposits and industrial dumps.	UA—Lavrov et al., 2021 [99]; KZ—Kairova, 2023 [84].
21	P. maximowiczii—Japanese poplar (NE As.)	shelterbelts, roadside	R: farmland protection, urban green areas.	JP—Kasuya et al., 2010 [100]; RU- Kaganov, 2021 [101].
21a	P. maximowiczii × P. balsamifera	plantations	R: herb species with conservation status	CA—Boothroyd-Roberts et al., 2013 [102].
22	P. simonii (NE Asia) P. simonii ‘Fastigiata’	shelterbelts, by-water shelterbelts	R: sand fixation, wind protection, farmland shelter, decarbonization.	CN—Liu et al., 2022 [103]; Gao et al., 2024; [86] PL—Łukaszkiewicz et al., 2020 [104].
23	P. suaveolens—Korean poplar (NE Asia)	shelterbelts	R: winter hardiness and drought resistance in forest-steppe.	RU—Tsarev, 1980 [105].
24	P. szechuanica—Sichuan poplar (NE Asia)	shelterbelts	R: wood, wind and sand fixation, soil and water conservation.	CN—Xin et al., 2019 [106].
26	P. yunnanensis—Yunnan poplar (E.Asia)	polluted land reclamation	R: desertification, phytoremediation, greening cities, roadside.	CN—Xiao et al., 2023 [107].
	section Leucoides
27	P. lasiocarpa—Chinese necklace poplar (E.Asia)	hedgerow	R: corridor networks, landscape, diversity.	CN—Tang et al., 2014 [108].
	section Turanga
29	P. pruinosa—gray-leaved poplar (Asia)	gallery forests, in arid territories	R: nutritional, regulating, and cultural ecosystem services, soil-strengthening.	KZ—Dimeyeva et al., 2023 [109]; UZ—Reimov, 2005 [110].
30	P. ilicifolia—African poplar (E.Africa)	river galleries	R: IUCN vulnerable, wood, fodder	CN—Chen et al. 2020 [111].
	section Abaso
31	P. brandegeei (Mexico)	river forest buffers	R: riparian forests.	MX—Breceda et al., 1997 [112].
	Intersectional hybrids
e	P. × beijingensis (Asia)	shelterbelts	R: sand fixation, water conservation.	CN—Chunya et al., 2018 [113]; CN—Fan et al., 2010 [114].
I	P. × xiaozhuanica (Asia)	shelterbelts	R: wind protection.	CN—Wang et al., 2021 [115].

Am.: America; AC: alley cropping; IC: intercropping; WP: wood-pastures; R: role, service.

).

A total of 32 Populus species and 21 hybrids were identified in the reviewed literature as components of agroforestry systems worldwide (Table 4 and

Table 5. Populus species used in agrihortisilviculture and agrosilvopastoral systems.

No.	Species, Hybrid, Variety, Clone–Name, Region	Agroforestry System	Use	Country-Citation
	section Populus
2	P. alba—white poplar (S.Eu—C.Asia)	shelterbelts, taungya system	R: microclimate regulation, dune stabilization; TS: pepper, strawberry.	RO—Enescu 2025 [35]; IR—Khodakarimi 2016 [116]; HU—Frank & Vityi 2016 [117].
5a	P. alba × P. grandidenta ‘Crandon’	agroforestry, alley cropping	AC: oat, red clover, red fescue, orchardgrass, hairy vetch.	US—Headlee et al., 2019 [118]; US—Delate et al., 2005 [119].
8a	P. tremula × P. tremuloides	intercropping	IC: reed canary grass, festulolium, galega	LV—Bardule et al., 2016 [120].
	section Aigeiros
9	P. deltoides—eastern cottonwood (E.-N.Am.)	alley cropping, taungya system	IC/AC: div. cereals, vegetables, seeds, condiments, arom. herbs, fruits, sugarcane.	IN—Chavan et al., 2022 [121]; IN—Agarwal et al., 2023 [122].
	P. deltoides ‘Australiano 106/60’ P. deltoides ‘I78’	silvopastoral system	SP: tall fescue, cocksfoot, ryegrass, timothy canarygrass, rescuegrass, swamp rice-grass, clover, alfalfa, oats, triticale; IC: browntop, soft brome, barley grass.	AR—Casaubon et al., 2018 [123]; CH—Dube et al., 2016 [71]; NZ—Guevara-Escobar et al., 2000 [124]; Kemp et al., 2001 [125].
9a	P. deltoides × P. nigra DN3308; DN-177	alley cropping	IC: soybean, barley, buckwheat, winter rye, winter wheat, maize, hay.	CA—Rivest et al., 2009 [126]; Bouttier et al., 2014. [76].
11	P. nigra L.—black poplar (Europe)	IN- agrisilviculture s., silvopastoral	IC: div. cereals, vegetables, seeds, condiments, fruits	IR—Henareh et al., 2025 [127]; US- Snell et al., 2017 [128].
11a	P. × canadensis (P. deltoides × P. nigra)—hybrid black poplar P. × canadensis AF2	shelterbelts, alley cropping, taungya system alley cropping	SB/IC/AC: div. cereals, vegetables, seeds, condiments, fruits, medicinal herbs, fodder, Christmas trees; AC(RSC): sorghum, soybean.	BG—Kachova & Ferezliev, 2020 [129]; RO—Mihaila et al., 2021 [130].
	section Tacamahaca
13	P. balsamifera—Balsam poplar (N.-N.America)	agrosilviculture, silvopasture	FP/IC: div. cereals, vegetables, seeds, condiments, fruits	IN—Fatima et al., 2022 [131]; CA—Gross et al., 2022 [91]; KZ—Kairova, 2023 [84].
14	Populus cathayana—(NE.Asia)	shelterbelts, block forests, intercropping	IC: amaranth; R: sand fixing.	CN—Jun et al., 2014 [132]; Gong et al., 2018 [133]; MN—Su et al., 2021 [134].
14a	P. cathayana × canadansis ‘xin lin 1’	intercropping	IC: soybean, cilantro, cabbage, Chinese cabbage, corn, watermelon.	CN—Lu et al., 2022 [135].
15	P. ciliata—Himalayan poplar (Asia)	intercropping, roadside, land slides	IC: fodder; R: wood, fodder, reclamation lands.	IN—Shandil, 2005 [136]; Naithani & Nautiyal, 2012 [137].
19a	P. koreana × P. trichocarpa ‘Koreana’	alley cropping, silvoarable, hedgerows	AC: silage maize, spring barley, winter wheat, oilseed rap;	DE—Linnebank & Zitzmann, 2025 [138].
21b	P. maximowiczii × P. nigra ‘NM6’	alley cropping	AC: native tallgrass-forb-legume polyculture, foder	US—Gamble et al., 2014 [139].
21c	P. maximowiczii × P. trichocarpa ‘NE-42’	intercropping	IC: white clover, melilot	CZ—Mrnka et al., 2024 [140]; DK—Manevski et al., 2019 [141].
	‘Hybride 275’	alley cropping	AC: maize, barley, wheat, oilseed rap.	DE—Reuse & Langhof, 2025 [142].
	‘OP42’	animal-plant system	SP: pig farm.	DK—Manevski et al., 2019 [141].
25	P. trichocarpa—W. balsam poplar (W.-N.Am.)	shelterbelts, alley cropping	R: shade, wind protection, field protection, AC: barley.	US—DeBell, 1990 [143]; DE—Majaura et al., 2025 [144].
25a	P. trichocarpa × P. trichocarpa Trichobel	silvoarable, alley cropping	AC: wheat, winter barley, cocksfoot, timothy, red fescue, white clover.	UK—Burgess et al., 2003 [145].
25b	P. trichocarpa × P. koreana ‘P-468’	intercropping	IC: white clover, melilot.	CZ—Mrnka et al., 2024 [140].
	section Leucoides
	section Turanga
28	P. euphratica—Euphrates poplar (N.Af, SW-C.Asia)	shelterbelts, inter-cropping, river gallery	R: farmland protection, sand control; IC: fodder/forage, Tarim red deer food.	CN—Tian et al., 2024 [81]; Qiao et al., 2006 [146]; Thomas et al., 2000 [147].
	section Abaso
32	P. mexicana (Mexico)	shade crops	SC: coffee agroecosystems.	MX—Ortiz-Ceballos et al., 2020 [148].
	Intersectional hybrids
a	P. × tomentosa—Chinese white poplar	windbreak, intercropping	IC: vegetables, peanut, wheat, soybean, watermelon; R: fodder, timber, fiber.	CN—Ranjitkar et al., 2021 [78]; Jiang & Qin, 2007 [149].
b	P. trichocarpa × P. deltoides ‘TD3230’	intercropping	IC: soybean, barley, buckwheat, winter rye, winter wheat.	CA—Rivest et al., 2009 [126].
c	P. interamericana (P. deltoides × P. trichocarpa)	silvoarable, alley cropping	AC: wheat, barley, oilseed rape, red fescue, white clover, cocksfoot, timothy.	UK—Burgess et al., 2003 [145]; Kaske et al., 2021 [150].
d	P. nigra × P. maximowiczii ‘NM3729’; ‘Max 1’	intercropping, alley cropping	IC/AC: soybean, buckwheat, rye, oilseed rape, maize, barley, sorghum, lupine.	CA—Rivest et al., 2009 [126]; DE—Thiesmeier, 2024 [151].
f	P. × gansuensis	shelterbelts	FP: maize; R: water protection, roadsides.	CN—Chang et al., 2006 [152]; Ding & Su, 2010 [153]
g	P. × generosa	intercropping	IC: switchgrass, R: bioenergy	US—Collins et al., 2020 [154].
h	P. × generosa × P. nigra ‘Monviso’	alley cropping	AC: sorghum, soybean, switchgrass, cocksfoot, alfalfa, French honeysuckle.	IT—Pecchioni et al., 2018 [155].

Am.: America; AC: alley cropping; IC: intercropping; TS: taungya system; FP: field protection; SP: silvopastoral; SC: shade crops; R: role, service.

). These species belong to nearly all taxonomic sections of the genus—Populus, Aigeiros, Tacamahaca, Leucoides, Turanga, and Abaso—and are distributed across temperate, boreal, subtropical, and arid zones.

Among the sections, Aigeiros (black poplars) and Tacamahaca (balsam poplars) were most frequently reported in agroforestry contexts, particularly for P. deltoides, P. nigra, P. × canadensis, P. balsamifera, and their hybrids (e.g., P. × generosa, P. × interamericana, P. × beaupré). These species dominate intercropping, alley cropping, and silvopastoral systems in Europe, North America, and Asia [131,142,145,156,157].

The section Populus (aspens and white poplars) was also widely represented, including P. alba, P. tremula, P. tremuloides, and P. × canescens. These species were mainly employed in shelterbelts, windbreaks, wood-pasture, and alley-cropping systems for soil stabilization, microclimate regulation, and carbon sequestration [84,88,91,92].

Asian taxa such as P. davidiana, P. simonii, P. cathayana, and P. euphratica were especially important in shelterbelts and desert-oasis systems in China and Central Asia, supporting wind and sand control, habitat conservation, and phytoremediation [81,106]. These species were often combined with herbaceous crops, including soybean, amaranth, or cereals.

In North America, native poplars such as P. tremuloides, P. balsamifera, P. deltoides, and P. fremontii were integrated into silvopastoral and alley-cropping systems for purposes of soil erosion control, biodiversity enhancement, and bioenergy feedstock production [92,93].

Hybrid poplars (e.g., P. × canadensis, P. × generosa, P. × interamericana, P. × tomontosa, P. maximowiczii × P. trichocarpa) were widely used in intensive agroforestry systems due to their rapid growth, broad ecological amplitude, and high adaptability to intercropping and short-rotation coppice (SRC) management [116,117,150,158]. Clone I-214 and clone M-1 of the Populus x euramericana hybrid have often been used in Serbia for plantations, SRCs, and wind shelterbelts. Studies were also performed on Populus deltoides clone S_6–7 [159,160,161,162].

Modern poplar breeding increasingly focuses on four priority directions: (i) enhancing resilience to biotic and abiotic stresses—including drought, salinity, heat waves, and major pathogens such as Dothichiza and Cytospora spp.; (ii) improving productivity and wood quality for both timber and bioenergy markets; (iii) developing environmentally adapted, region-specific hybrid combinations to cope with climate-change-driven shifts in temperature and precipitation regimes; and (iv) optimizing traits that support agroforestry performance, such as stable crown architecture, reduced shading impact, rapid early growth, and efficient nutrient cycling. Recent advances in genomics and marker-assisted selection now enable breeders to target specific physiological traits—water-use efficiency, fine-root architecture, and hormonal signaling pathways—that enhance tree–crop compatibility and long-term system resilience [163,164,165,166].

The results show a global and multifunctional role of Populus species in agroforestry—ranging from production-oriented systems (biomass, fodder, timber) to ecological functions (erosion control, carbon sequestration, biodiversity maintenance, and landscape protection).

3.2.3. Growth, Biomass, and Carbon Stocks of Poplars in Agroforestry: Implications for Sustainability

Several studies have investigated the growth, biomass accumulation, and carbon sequestration potential of Populus deltoides in agroforestry systems, highlighting their relevance for sustainable land use. Allometric models have been widely applied to estimate above- and belowground biomass, demonstrating high predictive power. For instance, Das et al. [167] developed component-wise dry biomass equations with R² values ranging from 0.95 to 0.99. These models support sustainable management by enabling precise biomass estimation, facilitating economically optimal harvesting, and informing carbon sequestration assessments.

Growth patterns of P. deltoides indicate rapid early development. In plantations aged 1–11 years, aboveground biomass (AGB) increased steadily, reaching a maximum of 180.2 Mg ha⁻¹ at 11 years [168]. Carbon concentration in aboveground components ranged from 39.7% to 51.7%, and total carbon stocks—including soil—rose from 64.4 Mg ha⁻¹ at 1 year to 173.9 Mg ha⁻¹ at 11 years. Soil carbon stocks also increased with age, particularly in surface layers, emphasizing the role of poplar-based systems in enhancing soil carbon and supporting sustainable agricultural practices. Two plantations of P. euramericana cl. I-214 on a river alluvium in Serbia at the age of 31 reached volumes from 464.12 m³ ha⁻¹ to 582.65 m³ ha⁻¹, demonstrating productivity on optimal soils [160]. Stajić [162] notes that the increase in biomass over the second production period of poplar SRCs is higher than in the first and that that increase may be 35% or even 60% compared to the first rotation, so biomass yield increases and remains stable until the fourth cycle, after which it gradually declines.

The planting scheme strongly influenced biomass and carbon storage. Chavan et al. [169] reported aboveground biomass of 69.90–207.98 Mg ha⁻¹ and belowground biomass of 13.46–36.69 Mg ha⁻¹ in eight-year-old plantations, with total carbon stocks ranging from 38.84 to 112.48 Mg C ha⁻¹. Denser spacings (4 m × 5 m) showed the highest carbon sequestration rates (14.09 Mg C ha⁻¹ yr⁻¹), demonstrating how management practices can enhance sustainability outcomes. Results on the use of unrooted seedlings in establishing stands of selected white poplar clones were presented in the study by Kovačević et al. [170]. Results indicate that under favorable water conditions, planting unrooted seedlings at a depth of 2.5 m (deep planting) ensures a high survival rate (90–100%), as does planting rooted cuttings at the usual depth of 0.8 m.

Comparisons between agroforestry and monoculture systems reveal interesting trends. Per-tree biomass and carbon storage are higher in agroforestry systems (1223 kg tree⁻¹) compared to monocultures (1102 kg tree⁻¹), although per-hectare carbon storage may be lower due to reduced tree density (85 vs. 105 Mg ha⁻¹) [171]. This highlights the trade-offs between ecological and productive efficiency in sustainable agroforestry design.

Region-specific studies further confirm the potential of poplar plantations for climate mitigation. Rizvi et al. [172] reported carbon storage of 27–32 t ha⁻¹ in boundary plantations and 66–83 t ha⁻¹ in agrisilvicultural systems over a seven-year rotation. Pankaj et al. [173], using CO₂FIX modeling, showed that longer rotations favor carbon sequestration in some regions, while shorter rotations optimize productivity in others, indicating the need for context-specific sustainable management strategies.

Long-term adoption of poplar-based agroforestry also improves soil carbon pools. Sharma et al. [174] observed that recalcitrant carbon in surface soil increased from 3.14 Mg ha⁻¹ in the first cutting cycle to 6.41 Mg ha⁻¹ after four cycles, confirming the system’s capacity to act as a net carbon sink while improving soil quality and fertility.

3.2.4. Ecological Characteristics and Sustainability of Poplar in Agroforestry Systems

The physiological responses of nine commonly planted North American hybrid poplars to drought were evaluated to determine their relative drought resistance. Analysis of drought indicator genes revealed pronounced differences in expression between the most sensitive and most resistant clones, confirming variations in drought stress and highlighting potential mechanisms of drought tolerance. Further examination of genes typically up-regulated during drought stress showed significant differences in transcript abundance corresponding to the physiological status of the clones. Interestingly, several genes in the drought-tolerant clone were down-regulated rather than up-regulated. Notably, putative positive and negative regulators of abscisic acid (ABA) signaling exhibited expression patterns consistent with a potential role in conferring drought resistance [175].

Nutrient dynamics in poplar (Populus deltoides)-based agroforestry systems were assessed in Pusa, Bihar, India, to evaluate their contributions to system sustainability. Nutrient concentrations in the standing biomass of intercropped crops exceeded those in poplar trees, whereas total nutrient content was higher in trees than in crops. Soil, litter, and vegetation contributed 80.3–99.5%, 0.1–5.0%, and 0.4–14.7%, respectively, of the total nutrients within the system. Leaf senescence led to a substantial reduction (40–54%) in nutrient concentrations. These results indicate that poplar-based agroforestry systems can maintain sustainable soil nutrient status, supporting long-term productivity [176].

Sap flow dynamics of poplar in a poplar–maize agroforestry system in Western Liaoning were continuously monitored along with environmental factors, including air temperature, air humidity, net radiation, wind speed, soil temperature, and soil moisture. Canopy conductance, calculated using a simplified Penman–Monteith equation, displayed a single-peak diurnal pattern and a declining seasonal trend. A negative logarithmic relationship was observed between canopy conductance and vapor pressure deficit (VPD), with sensitivity decreasing from May to September. Canopy conductance correlated positively with solar radiation, although the strength of correlations with other environmental factors varied by month. Overall, VPD was identified as the most significant environmental driver of canopy conductance throughout the growing season [177].

Further investigation of sap flow velocity and its relationship with microclimate factors revealed temporal offsets between physiological responses and environmental cues. Using the dislocation contrast method, sap flow velocity was found to precede changes in air temperature, air humidity, and VPD, while lagging behind net radiation on sunny days. These findings highlight the complex regulation of water use by poplar in agroforestry systems, underscoring the importance of understanding temporal dynamics for sustainable management [178].

3.2.5. Effects on Biodiversity and Ecosystem Functions in Poplar-Based Agroforestry Systems

Hydrology and Nutrient Loss

A study conducted 1 km from a lakeshore evaluated the impacts of two poplar–wheat intercropping densities (AS1: 5 m × 2m; AS2: 15 m × 2m) on hydrological processes. Rainfall interception by poplar canopy occurred primarily from April to October, ranging from 8.6% to 44.5%. Canopy interception significantly reduced splash and moderate rainfall intensity (1.0 mm h⁻¹) by an average of 47.7%. The denser system (5 m × 2m) resulted in greater reductions in surface runoff, leaching, and nitrogen loss compared to the wider spacing [179].

Poplar buffer zones provided improved habitat for wildlife, improved biodiversity, and added value from recreational hunting and fishing. River poplar galleries improved the microclimate in arid areas, increased soil moisture, and ensured sand and soil erosion control, contributing to the greening of desert areas and the revitalization of the landscape [86,180,181,182].

Due to their fast growing rate, columnar port, and volatile compounds, poplars are frequently used to green urban areas by planting them on roadsides, near tall concrete constructions (industrial buildings or blocks of flats), bordering sports infrastructure, etc. [98,138,146].

Soil Nutrient Dynamics

Micronutrient availability and uptake were assessed in barley grown under poplar plantations versus sole cropping. Soils under poplar showed significantly higher DTPA-extractable Zn, Cu, Mn, and Fe across 0–15, 15–30, and 30–45 cm depths. However, micronutrient uptake by barley was higher in the sole-cropping system. Among cultivars, variety BH 946 demonstrated the highest micronutrient uptake, while BH 959 had the lowest [183].

In a semiarid temperate region of Northeast China, conversion of cropland to poplar-based agroforestry increased soil microbial biomass carbon (MBC) and the MBC:MBN ratio after 5 years, and reduced soil NO₃⁻–N concentration. However, changes in soil bulk density, pH, total organic C, total N, microbial biomass nitrogen (MBN), microbial metabolic quotient, and N mineralization were not significant relative to cropland [184].

Litterfall nutrient return in poplar systems with three planting densities (5 m × 3 m, 7 m × 3 m, and 8 m × 2.5 m, respectively) indicated positive contributions to soil nutrient cycling and fertility [185]. Similarly, poplar agroforestry in coastal eastern China significantly enhanced soil nutrient status and enzyme activities, with denser spacing showing stronger positive effects [186].

In time, in sites cultivated for extended periods with poplar monocultures, allelopathy contributes to poplar decline—the allelochemicals have phytotoxicity, changing soil microorganisms, enzymes, and nutrients [187]. The main natural antagonists of poplar in allelopathic or monoculture decline include the following: (i) soil-borne fungi such as Armillaria, Phytophthora, Cytospora, and Dothichiza species, which proliferate in stressed stands; (ii) competing understory species like Ailanthus altissima, Ambrosia artemisiifolia, and certain aggressive grasses that inhibit poplar regeneration; and (iii) microbial shifts toward polyphenol-tolerant bacteria that reduce nutrient mineralization. These antagonistic interactions are exacerbated in long-term monocultures where poplar litter phenolics accumulate, and microbial diversity becomes simplified. Intercropping systems contribute to soil enrichment/remediation and culture profitability. Including different types of plants (ecto- and endo-micorrhizal, bacterial symbionts) in agrosilvicultural systems improves the soil’s microorganisms [136,188].

Soil Fauna and Microorganisms

Earthworm abundance varied across tree species in a temperate intercropping system in Canada. Poplar rows hosted the highest earthworm density (182 m⁻²), with declines observed 2 m (117 m⁻²) and 6 m (95 m⁻²) from the tree rows, highlighting spatial gradients in soil faunal activity under agroforestry [189].

Poplar-based agroforestry increased soil bacterial abundance compared with monoculture croplands, though alpha diversity remained stable. Tree rows supported distinct bacterial communities, contributing to greater overall microbial diversity through the introduction of tree-associated taxa [190].

Across three poplar agroforestry systems, soil microbial metabolic capacity declined with depth and varied seasonally. Poultry overstocking reduced microbial activity, and plant composition influenced carbon source availability. Overall, agroforestry enhanced soil microbial functional diversity [191].

Aboveground Biodiversity

Arthropod diversity was significantly higher in tree canopies than in ground vegetation, and beneficial arthropods were more abundant in agroforestry plots than in monocultures. This indicates enhanced habitat suitability for beneficial arthropods under poplar-based systems [192].

In Mediterranean agroforestry landscapes, poplar plantations supported the Lesser Spotted Woodpecker (Dendrocopos minor L.), a forest specialist. Appropriate planning of poplar rotation cycles and integration with riparian forest recovery were suggested to support long-term woodpecker populations [193].

Nests of Swainson’s Hawks were most frequently constructed in mesquites (32%) in natural areas, and shelterbelt trees such as cottonwoods (Populus fremontii; 26%), elms (Ulmus spp.; 8%), and willows [94].

Hybrid poplar plantations are a suitable habitat for reintroduced forest herbs with conservation status [102].

Phytoremediation Capacity

Hybrid poplar clones irrigated with synthetic agricultural effluent showed differential accumulation of boron (B) and selenium (Se). Clone 49,177 (P. trichocarpa × P. deltoides) exhibited the highest tolerance and accumulation capacity, with the greatest phytoextraction potential at salinity < 7 dS m⁻¹ [194]. Various studies have demonstrated the effectiveness of poplars in phytoremediation of NO₃⁻, heavy metals, petroleum hydrocarbons, and pesticides. Because a poplar tree can transpire, depending on age, 18–111 L/day, and five-year-old poplars can transpire 100–200 L/day, poplars prevent leaching of NO₃⁻ from soil and remove significant amounts from contaminated groundwater [195].

3.2.6. Agroforestry Systems with Poplars Used

Table 5 summarizes the Populus species commonly employed in agrihortisilviculture and agrosilvopastoral systems, highlighting their applications and characteristics.

Poplars and Cereals

Across numerous agroforestry trials, wheat exhibited variable responses to poplar-based systems, influenced by genotype, planting density, and the age of the trees. In northeastern Italy, wheat grown in narrow (6 m) alleys within a four-year-old poplar plantation demonstrated high variability among cultivars under reduced irradiance, indicating considerable potential for selecting wheat genotypes adapted to shaded agroforestry conditions [196]. In India, wheat yields were generally higher in open fields, with the greatest yield observed for UP-2572 under open conditions. The greatest yield reduction under one-year-old poplar was observed for PBW 373, and the lowest for PBW 550. However, the agroforestry system enriched soil nutrients at 0–15 cm depth, demonstrating soil-health benefits [197].

Different poplar spacings in Iran showed that a 10 m × 3 m configuration maximized biomass with no significant reduction in crop production, indicating an optimal balance for wheat–canola systems in temperate climates [198]. Similarly, in China, wheat–corn intercropping under poplar showed significant variation in carbon stocks among biomass components, with configuration A providing the best economic and C-sequestration performance [199]. In Greece, poplar alley cropping improved pesticide and nutrient uptake efficiency, demonstrating potential for pollution mitigation by tree roots accessing nutrients beyond crop rooting zones [200].

Maize yields declined under poplar shading in northwestern China, with reductions of 27% (west side) and 22% (east side). Light-related microclimate variations strongly influenced transpiration and yield responses. In Turkey, alley cropping with maize, beans, and zucchini affected crop performance and reduced poplar growth, highlighting competitive trade-offs [192]. Over a 10-year poplar system in India, yield reductions increased as tree canopy expanded, though maize–wheat–turmeric and pigeon pea–turmeric rotations remained profitable, providing sustained income [201]. Nutrient competition was also observed in China, where maize yields and nutrient uptake decreased without fertilization, indicating the need for adequate nutrient supply in poplar–crop systems [202].

Many other cereals are used in poplar agroforestry systems, across the world: rice, paddy, barley, buckwheat, winter rye, maize, oat, sorghum, jowar, etc. (Table 5) [82,130,133,136,138,201,203].

Poplars and Vegetables

Intercropping (alley cropping, taungya systems) of poplars with vegetables (soybean, beans, peas, lentin, millets, cabbage, radish, potato, tomato, spinach, cilantro, cabbage, broccoli, Chinese cabbage, lettuce, turnip, sugar beet, cauliflower, brinjal, beans, peanuts, groundnuts, onion, garlic, carrot, radish, cucumber, alfalfa, chilies, amaranths, etc.) are especially used in undeveloped regions, where poverty and food security are important social problems (Table 5) [82,104].

Under poplar-based systems in India, onion performance varied with stand age and planting time. Higher yields occurred under two-year-old plantations compared to three-year-old ones, with a ~42% yield reduction under older stands compared to open fields. End-of-December planting and cultivar PWO 35 maximized yield across conditions [204]. Similarly, system evaluation showed that selecting appropriate onion varieties and planting time enhances profitability under poplar agroforestry [205].

Potato productivity under poplar systems is constrained by weed pressure and shade, though crop choice and scheduling can mitigate losses [205]. In India, appropriate cultivar selection and planting timing significantly improved potato yield under poplar intercropping, supporting long-term agroforestry sustainability [206]. Additional results showed severe yield reductions in soybean and sorghum near poplar rows, up to 74%, with sorghum being more shade-sensitive. Thus, crop rotation design must consider crop tolerance to light and soil variation [207].

Poplars and Aromatic Herbs and Condiments

Mint, mustard, coriander, bottle gourd, fenugreek, turmeric, ginger, colocasia, hot chilies, black pepper, cardamom, lemon grass, and many other aromatic herbs and condiments are suitable with poplar intercropping systems (Table 5) [174,192].

Optimizing sowing time and genotype selection significantly enhanced mustard productivity under poplar, demonstrating that agronomic adjustments can overcome shade-induced yield limitations in oilseed systems [208].

Mint (Mentha spp.) performance varied under poplar systems, with open farming showing maximum oil yield (CIM Kranti). However, poplar trees benefited from mint integration, showing improved growth (DBH, height, volume). Although profit was higher in open conditions, agroforestry offered strong returns and improved tree performance [209].

Not only poplars (buds, leaves, bark, etc.), but also some intercropped plants (i.e., lesser periwinkle; Table 5) have medicinal properties [210].

Poplars and Fruit Crops

Fruit species such as citrus, mango, guava, lychee, watermelon, apricot, apple, raspberry, strawberry, and others are commonly intercropped or sheltered under poplar stands within hortisilvicultural production systems (Table 5) [131,133].

Poplars and Multiannual Crops

Several significant multi-year crops, such as coffee, sugarcane, and Christmas trees, are occasionally reported within poplar-based agroecosystems [77,157].

Poplars and Oil Plants

Under poplar protection, industrial (oilseed rap) or edible (sunflower) oil cultures have good performances [210,211].

Poplar and Fodder Crops

Poplar agrosystems include fodder plants for hay and/or concentrated livestock food (oats, red clover, white clover, red fescue, tall fescue, orchardgrass, hairy vetch, reed canary grass, festulolium, fodder galega, cocksfoot, ryegrass, alfalfa, barley, triticale, silage maize, switchgrass, prairie cordgrass, intermediate wheatgrass, melilot, fallow land, sweet sorghum, lupine, perennial ryegrass, soft brome, barley grass, common timothy, and many others), including native tallgrass–forb–legume polycultures (Table 5) [67,81,130,143,147,153].

Intercropping fodder/forage crops with young poplars has yielded quantified agronomic and environmental benefits. For instance, in a Mediterranean rainfed trial with one-year-old Populus × euramericana I-214, the legume Hedysarum coronarium (sulla) produced significantly more above-ground dry biomass than ryegrass when intercropped with poplars: average AGB was 695.1 g DM m⁻² for sulla vs. 461.4 g DM m⁻² for ryegrass (≈+35% for sulla) [212]. Moreover, biological nitrogen fixation (BNF) by sulla was enhanced near the tree–crop interface: the amount of N fixed by sulla in the silvopastoral system (poplar + sulla) at 1 m from the tree row reached 17.89 g N m⁻², compared to 11.70 g N m⁻² in sole-sown (pasture) sulla—a ~+53% increase in N fixed near trees. This indicates that the presence of trees can stimulate BNF in adjacent legumes, potentially increasing N inputs to the system. Importantly, the same study observed a reduction in nitrate (NO₃⁻) leaching risk: even during the first year after planting, poplar trees in the silvopastoral system lowered the nitrate flux from the forage crop toward drainage ditches compared to systems without trees, suggesting a nitrate-scavenging (buffer) effect by the young trees. Further, in a classic agroforestry experiment from India under Cymbopogon flexuosus (‘CKP-25’)—transplanted under Populus deltoides G-3—a treatment with 45 cm × 45 cm spacing and a fertility regime of N₍₂₅₀₎P₍₁₀₀₎K₍₈₀₎ yielded 57.6 t/ha herbage in the first year and 68.0 t/ha in the second year, with essential oil yields of 260.9 and 309.4 kg/ha, respectively [213]. These results demonstrate that poplar-based agroforestry can support high-yielding herbaceous crops (lemongrass), not only in Mediterranean but also in (sub)tropical contexts. Together, these findings provide concrete evidence that poplar–forage intercropping can (i) significantly increase forage yield compared to grasses, (ii) enhance biological N-fixation in legumes, (iii) contribute substantial N inputs to the system, and (iv) reduce nitrate leaching—even during the first year of tree establishment. These quantified benefits strengthen the case for integrating understory forages in poplar-based agroforestry systems, rather than relying on qualitative or anecdotal benefits.

In the Pacific Northwest, poplar–switchgrass systems reduced greenhouse gas emissions and improved net climate benefits [214]. Poplar–alfalfa in China enhanced microclimate and soil quality, achieving a land equivalent ratio of 1.42 [215]. Poplar–turmeric systems optimized with 75% RDN + 25% FYM improved soil fertility and crop quality [214].

In India, common intercrops included wheat and sugarcane, followed by pulses and vegetables [216]. Jaswal et al. [217] reported that turmeric was more remunerative than ginger under wider poplar spacings; however, the study did not provide crop-specific economic metrics (e.g., net return or benefit–cost ratio), and thus the magnitude of the profitability advantage cannot be quantified from available data White clover showed temporary benefits for poplar growth and nitrogen supply in short-rotation coppice systems [153].

Across coastal soils, poplar–crop combinations improved soil properties and survival rates, particularly under desalinated conditions [218]. Boundary poplar plantations with berseem and sorghum proved more profitable than block plantations, offering higher benefit–cost ratios and internal rates of return [219].

Poplar and Forage Crops and Livestock

In New Zealand, poplars (P. deltoides × P. nigra ‘Dudley’) and willows (Salix matsudana × alba ‘Tangoio’) have an effect on pasture production and sheep grazing. Willow and poplar tree trimmings can increase fecundity in sheep and decrease lamb mortality compared with grazing drought pasture [220].

Silvopastoral systems with poplar (P. × canadensis) for cattle and multiple other uses (wood production) were used in the Paraná River Delta (Argentina) [132].

At the same time, fertilization with high-quality sewage sludge could be recommended for poplar silvopastoral systems [221]. Poplar diminished the nitrate leaching in an organic pig farm in Denmark [185]. Across global agroforestry trials, the crops that have shown the most stable and productive interactions with poplar are the following: (i) shade-tolerant cereals such as barley and certain wheat genotypes; (ii) legumes including alfalfa, clovers, and Hedysarum coronarium, which benefit from improved microclimate and contribute to nitrogen enrichment; (iii) root and rhizome crops (turmeric, ginger, colocasia) that maintain yields under partial shade; (iv) forage grasses such as tall fescue, orchardgrass, and ryegrass; and (v) several vegetables including onion, garlic, spinach, and cabbage in early rotation stages. These crop groups generally tolerate moderate shading, take advantage of cooler microclimates, and interact functionally with poplar roots, resulting in high combined land-equivalent ratios.

4. Discussion

4.1. Bibliometric Review

The bibliometric analysis highlights the growing scholarly interest in this research domain, as evidenced by the identification of 496 publications. The predominance of research articles (92%), similar to other topics [222,223,224,225], indicates that the field is largely driven by original empirical investigations, while the comparatively limited number of review articles and book chapters suggests that syntheses and conceptual consolidations remain underdeveloped. This scarcity of reviews may reflect both the relatively early stage of consolidation within poplar-based agroforestry research and the fragmented nature of existing studies, which are often dispersed across diverse themes such as silviculture, ecology, genetics, and socio-economic impacts. As a result, the field has not yet accumulated a sufficiently cohesive body of knowledge to support frequent, comprehensive reviews. This gap underscores the need for integrative assessments—such as the present study—to organize existing evidence, identify converging insights, and guide future research directions.

The steady increase in annual publications, with a marked acceleration after 2013, like in other cases [226,227,228], reflects the increasing recognition of its relevance to contemporary environmental and agricultural challenges. Notably, this post-2013 surge aligns with several global policy milestones that elevated the importance of climate-smart land-use systems. These include the launch of the European Union’s post-2013 Common Agricultural Policy (CAP), which introduced stronger incentives for agroforestry establishment; the expansion of EU and national subsidy schemes supporting multifunctional tree-based systems; and the international emphasis on greenhouse gas mitigation following the Paris Agreement in 2015. Collectively, these developments strengthened political and financial support for agroforestry and likely contributed to the observed rise in scientific output. The consistent yearly output of 30–40 publications over the past 14 years suggests a stable and engaged research community.

The distribution of publications across research areas further underscores the interdisciplinary nature of the topic. While Agriculture, Forestry, and Environmental Sciences/Ecology clearly dominate, their combined prevalence points to strong linkages between agricultural production, tree-based systems, and ecosystem processes. The prominence of French-speaking researchers illustrates the emergence of a geographically distinct and influential research nucleus, likely supported by long-term institutional programs and strong collaborative networks.

The global distribution of authors from 53 countries across five continents reveals a broad international engagement, although contributions remain unevenly distributed. India, Canada, China, and Germany emerged as the most productive countries, reflecting both their scientific capacity and the relevance of the topic to national research priorities. The clustering of countries into six collaborative groups suggests the existence of structured or regionally coherent research partnerships. Notably, the three major clusters, including multiple countries, reflect intercontinental ties, particularly across Europe, Africa, and North America. Such patterns may indicate shared agroecological conditions, comparable policy frameworks, or longstanding scientific exchanges.

The prominence of Agroforestry Systems as the leading journal demonstrates the centrality of agroforestry as a conceptual and practical framework guiding research in this field. The presence of other journals such as Forests and Agriculture and Ecosystems & Environment further confirms the dual agricultural and ecological orientation of the literature. Institutional contributions were similarly skewed, with Punjab Agricultural University, the Indian Council of Agricultural Research, and the Chinese Academy of Sciences emerging as the most productive institutions. Likewise, the predominance of publishers such as Springer Nature, Elsevier, and MDPI reflects the mainstream academic dissemination channels for this research area, like in other review articles [229,230,231].

Overall, the bibliometric findings illustrate a dynamic and expanding research landscape characterized by strong interdisciplinary connections, geographically diverse contributions, and increasingly complex thematic orientations. These trends suggest that the field is well-positioned to contribute to global discussions on sustainable land management, climate change mitigation, and bioresource production.

4.2. Insights and Implications of Poplar-Based Agroforestry Research

The global research trends on poplar-based agroforestry systems indicate a multidimensional interest spanning productivity, ecological functions, and economic viability. The prominence of studies in India reflects the strategic use of Populus deltoides in enhancing smallholder farm productivity and income [18,56]. Indian research has extensively documented both crop yield improvements and resource-use efficiencies, highlighting the role of poplar in sustainable intensification.

While these findings highlight clear trends, it is important to acknowledge that the strong concentration of publications from India—particularly on Populus deltoides grown in intensive, smallholder agroforestry systems—may influence the global patterns identified in this review. The predominance of research from Northwest India emphasizes themes such as crop yield enhancement, farm-level profitability, and resource-use efficiency under high-input, intercropped conditions. As a result, global research themes derived from bibliometric patterns may be somewhat skewed toward the production-oriented outcomes that are characteristic of these systems.

In contrast, studies from regions such as Central Asia, parts of Europe, and North America often prioritize different objectives, including ecological restoration, soil and water conservation, biodiversity enhancement, and long-term landscape resilience. These studies typically focus on species such as P. nigra, P. euphratica, and hybrid poplars, and are embedded in environmental or watershed-rehabilitation contexts rather than intensive agroforestry. Because these regions are underrepresented in the publication record, ecological or restoration-driven research themes may appear less prominent than they are in practice.

Recognizing these geographic and thematic imbalances is therefore critical for interpreting global trends. Future research syntheses should consider weighting regional representation or conducting region-specific sub-analyses to ensure that insights reflect the full diversity of poplar-based agroforestry systems. Addressing this imbalance will help align global conclusions with both production-focused and ecosystem-restoration paradigms, strengthening the broader applicability of research findings.

Economic assessments demonstrate that poplars can provide substantial financial benefits when integrated into short-rotation or intercropped systems [57,65]. However, regional differences in market demand, land tenure, and policy frameworks may influence adoption rates and profitability, suggesting the need for localized economic analyses.

Ecologically, poplar-based systems offer multiple ecosystem services. Improved nitrogen-use efficiency under elevated CO₂ [60], enhanced soil organic carbon [72], and optimized understory biodiversity [75] underscore the potential of poplars to contribute to sustainable land management and climate mitigation. Nonetheless, interactions between trees and crops, such as competition for light and water, remain critical considerations for system design [58,76].

Innovative applications, including silvopastoral systems, wetland integration, and biomass-oriented plantation models, demonstrate the flexibility of poplar in diverse agroforestry contexts [66,67,74].

Overall, the reviewed literature highlights a clear trend toward integrating poplars in multifunctional agroforestry systems that simultaneously address productivity, environmental sustainability, and economic resilience. Nevertheless, gaps remain in understanding socio-economic barriers, long-term carbon dynamics, and species interactions under changing climatic conditions, particularly in underrepresented regions such as Africa and South America.

4.3. Diversity, Distribution, and Functional Roles of Poplars in Agroforestry Systems

The diversity of Populus species recorded across agroforestry systems demonstrates the genus’s exceptional ecological plasticity and functional versatility. Poplars combine rapid growth, broad environmental tolerance, and compatibility with agricultural crops and pastures, making them among the most widely used tree genera in global agroforestry.

4.3.1. Geographic and Ecological Patterns

Distinct geographic patterns in species use are evident. In Europe and North America, agroforestry relies primarily on P. nigra, P. deltoides, P. × canadensis, and P. balsamifera, reflecting long histories of poplar breeding and hybridization for timber and bioenergy [137,232]. In Asia, native species such as P. simonii, P. cathayana, and P. euphratica are vital components of arid and semi-arid shelterbelt systems, mitigating desertification and wind erosion while improving microclimatic and hydrological stability [81]. Central Asian and Chinese research particularly highlights their ecosystem service functions, including soil carbon storage, phytoremediation, and wildlife habitat provision [84].

In tropical and subtropical regions, few native poplars occur, yet some taxa (P. ilicifolia, P. mexicana) are of conservation concern and may provide local benefits such as shade in coffee systems and riparian protection [157]. Their restricted use underlines both the climatic limits of the genus and the potential for targeted conservation and domestication efforts.

4.3.2. System Diversity and Functions

Populus species were integrated into a wide range of agroforestry systems, including intercropping (IC), alley cropping (AC), silvopastoral (SP), shelterbelt (SB), and taungya systems (TS). The species–system pairing varied according to climatic zone, soil conditions, and socioeconomic context. For instance, P. × canadensis and P. deltoides hybrids predominate in temperate alley cropping and intercropping with cereals (wheat, barley, maize), legumes (soybean, alfalfa), and forage species (tall fescue, clover) [145,157,161]. In contrast, P. euphratica and P. pruinosa fulfill crucial ecological functions in desert and riparian shelterbelts, sustaining biodiversity and cultural ecosystem services [109,110].

The multipurpose use of poplars—providing timber, bioenergy feedstock, fodder, and environmental protection—aligns with the core objectives of sustainable agroforestry [78,92]. In addition, the frequent mention of phytoremediation and carbon sequestration highlights the genus’s growing role in climate-smart agroforestry strategies.

4.3.3. Role of Hybridization and Breeding

A clear trend emerging from this review is the dominance of hybrid poplars in modern agroforestry. Interspecific hybrids (e.g., P. × canadensis, P. × generosa, P. × interamericana) combine vigorous growth, disease resistance, and site adaptability, making them ideal for intensive, short-rotation, or high-yield systems [136,180]. Moreover, their compatibility with diverse understory crops suggests strong potential for multifunctional land-use optimization [121,161]. As previously emphasized in the literature [233,234,235], deploying a broad range of climate-resilient varieties and provenances in afforestation efforts constitutes a critical strategy for enhancing the adaptive capacity of future forests.

When an appropriate hybrid or clone is selected, and essential agrotechnical measures are implemented, including land preparation to optimize growing conditions, weed control, fertilization, irrigation (potentially using wastewater), and coppicing, together with the application of suitable planting densities, it becomes possible to establish plantations specifically designed for energy production [159,161,162].

4.4. Poplar-Based Agroforestry Systems: Biomass, Carbon Sequestration, and Sustainability Considerations

The collective findings from various studies underscore that Populus deltoides agroforestry systems are effective for sustainable biomass production and carbon sequestration. Allometric models provide reliable tools for estimating tree and soil carbon stocks, which is essential for sustainable management, harvest planning, and carbon accounting [132,163].

The consistent increase in biomass and carbon stocks with age demonstrates the capacity of poplar-based systems to contribute to long-term climate mitigation. Aboveground biomass accumulation is complemented by significant soil carbon enhancement, particularly in surface layers, reinforcing the system’s dual role in carbon sequestration and soil fertility improvement [164,174].

Planting density and management practices are critical determinants of tree biomass and carbon sequestration efficiency [7,236]. Dense planting geometries achieve higher carbon storage per hectare, whereas lower-density systems optimize individual tree growth. Importantly, these patterns also carry economic implications: dense plantings tend to produce smaller-diameter wood suitable primarily for biomass or energy uses, which generally have lower market value, while lower-density systems favor the development of larger, higher-quality stems suitable for sawlogs and other high-value products. These findings highlight the importance of balancing productivity, ecological sustainability, and carbon management in agroforestry design [132,171].

Regional variations in growth and carbon accumulation emphasize the need for location-specific management strategies. Longer rotations favor maximum carbon sequestration in certain regions, while shorter rotations may improve productivity in others, illustrating the adaptability of poplar-based agroforestry systems for sustainable land-use planning [173].

In conclusion, the long-term enhancement of soil organic carbon underscores the broader sustainability benefits of poplar agroforestry. Beyond climate mitigation, increased soil carbon improves soil structure, fertility, and productivity, promoting resilient and sustainable agricultural landscapes [174]. Poplar-based agroforestry thus emerges as a model for integrating productive forestry, carbon management, and sustainable land-use practices.

4.5. Ecophysiological Implications of Poplar Integration in Agroforestry Systems

The results highlight multiple ecological and physiological traits of poplars that support their integration into agroforestry systems. The differential drought responses among hybrid poplar clones underscore the importance of genetic selection for resilience under water-limited conditions. The observed variation in ABA-related gene expression suggests that drought-tolerant clones may rely on nuanced regulation of both positive and negative signaling pathways, potentially optimizing water use efficiency while maintaining growth [175]. This aligns with broader evidence that hybrid poplars display plasticity in their stress-response mechanisms, enabling adaptation to diverse environmental conditions.

Sex reversal in poplars is genetically predisposed through the plasticity of the sex-determination region (SDR) located on chromosome 19 [237]. The SDR contains multiple regulatory genes—particularly ARR17, a known feminizing factor—whose expression can be epigenetically reprogrammed under stress [238]. Environmental triggers such as drought, nutrient imbalance, and hormonal fluctuations (especially cytokinins and gibberellins) can modify SDR methylation patterns, enabling phenotypic sex reversal in genetically susceptible genotypes. This phenomenon is rare but more frequently documented in hybrid poplars and dioecious species under environmental stress [239,240].

Nutrient cycling analyses indicate that poplar-based agroforestry systems can maintain sustainable soil fertility. While crops accumulated higher nutrient concentrations, poplars contributed substantially to total system nutrient content, particularly through biomass and litter. Leaf senescence was a key process for nutrient return to the soil, ensuring long-term nutrient availability. These findings confirm that the integration of poplars into agroforestry can balance crop production with soil conservation [172].

Sap flow studies demonstrate the sensitivity of poplar water use to microclimatic conditions, particularly VPD and solar radiation. The seasonal decline in canopy conductance suggests that water stress or phenological changes reduce transpiration over time. Temporal offsets between sap flow and environmental drivers, including advances and lags relative to VPD, temperature, humidity, and radiation, indicate a complex regulation of water transport that likely reflects interactions between stomatal control, hydraulic architecture, and local climate variability [177,178]. These findings have important implications for designing agroforestry systems, as they highlight the capacity of poplars to adjust water use dynamically in response to environmental cues, potentially reducing competition with understory crops.

Due to their fast growing rate, columnar port, and volatile compounds, poplars are frequently used to green urban areas, by planting them on roadsides, near tall concrete constructions (industrial buildings, blocks of flats), bordering sports infrastructure, etc. [98,101,107].

Overall, the integration of drought-tolerant poplar clones and understanding of nutrient and water dynamics can enhance the ecological sustainability and productivity of agroforestry systems. Selecting clones with favorable physiological traits and managing nutrient and water interactions will be key strategies for optimizing both tree and crop performance.

The most significant phytopathological challenge in poplar plantation cultivation is posed by opportunistic pathogens that induce bark cankers, such as Dothichiza populea and Cytospora spp., as well as by foliar parasites. Several studies indicate that the more humid and shaded microclimatic conditions typical of agroforestry settings can favor the establishment and progression of these pathogens. Reported findings emphasize the importance of early detection and a better understanding of environmental conditions that increase infection risk.

Poplar crowns often remain comparatively undamaged after lightning strikes due to their high moisture content, rapid vertical conduction through the straight bole, and the presence of well-developed lenticels that dissipate electrical energy along the stem. The sapwood’s high water content conducts current efficiently downward, reducing explosive bark detachment. Additionally, poplar bark contains fewer resin canals than conifers, lowering the risk of combustion or fragmentation. As a result, damage is typically limited to localized cambial streaking rather than full crown destruction [241,242,243].

4.6. Ecological and Functional Implications of Poplar-Based Agroforestry Systems

Poplar-based agroforestry systems demonstrated multiple ecosystem benefits, including improved hydrological regulation, soil quality enhancement, and biodiversity support. Hydrological findings highlight the significant role of canopy structure in reducing erosive forces and nutrient losses. Dense planting configurations (2 × 5 m) conferred superior interception and runoff reduction relative to wider spacing, aligning with general principles linking tree density to rainfall capture and soil protection [179].

Soil nutrient responses were positive but complex. Increased micronutrient availability under poplar [183] reflects enhanced nutrient cycling, likely driven by organic inputs and rhizosphere effects. Yet, reduced nutrient uptake by barley suggests belowground competition or altered microclimatic conditions affecting crop nutrient acquisition. This trade-off underscores the need to balance ecological benefits with crop productivity in agroforestry design.

Long-term soil studies indicate early improvements in microbial biomass and nutrient retention following agroforestry establishment [184]. Enhanced nutrient return through litterfall [154] and increased enzyme activity in densely planted systems [186] further confirm poplar’s role in improving soil biological functioning. Collectively, these results support agroforestry’s capacity to foster soil restoration, particularly in semiarid or nutrient-limited contexts.

Soil biota responses also confirmed ecological enhancement. Poplar rows acted as hotspots for earthworm distribution [189], while tree rows hosted distinct microbial assemblages, increasing overall system-level microbial diversity [190]. Functional diversity improvements [191] emphasize the role of diversified plant inputs in sustaining soil biological processes. However, management intensity (e.g., poultry over-stocking) can negate these advantages, implying the need for regulated integrated systems.

Aboveground biodiversity similarly benefited. Tree canopies and structural complexity favored beneficial arthropods [192], echoing findings from broader agroforestry biodiversity research. Poplar plantations also contributed to habitat provision for sensitive bird species like the Lesser Spotted Woodpecker, provided landscape-level planning is applied [193].

Although poplar seed “fluff” (pappus) has superior tensile strength and insulation capacity compared to bird down, industrial-scale processing facilities are still limited. Pilot initiatives exist in China, Russia, and parts of Eastern Europe where poplar fluff is used for biodegradable fillers, textile blends, acoustic insulation, and oil-adsorbing materials. However, commercialization remains small-scale due to seasonal availability, difficulties in mechanical separation from seeds, and lack of standardized harvesting equipment. Emerging innovations in pneumatic collection and fiber-cleaning technology may enable broader industrial use in the near future [244,245,246].

Finally, phytoremediation findings reinforce poplars’ applicability in saline or contaminated environments. Specific hybrid clones demonstrated high tolerance and uptake of B and Se [194], confirming poplars’ utility in bio-remediation strategies while cautioning against excessive salinity loads.

Overall, the reviewed studies demonstrate that poplar-based agroforestry systems enhance soil health, support below- and above-ground biodiversity, and improve hydrological regulation. However, interactions with crop performance and management intensity warrant careful consideration to optimize multifunctionality. Long-term monitoring and multi-species trials are needed to refine models of ecological and agronomic trade-offs in diverse climatic and soil conditions.

4.7. Performance and Management of Sustainable Poplar-Based Agroforestry Systems

Poplar-based agroforestry systems demonstrate multifaceted agronomic, ecological, and economic outcomes across diverse climatic and management contexts. Taken collectively, findings indicate a consistent trade-off between crop productivity and ecosystem services, yet the strength of supporting evidence varies considerably depending on crop species, tree age, and experimental design. Many studies remain short-term or site-specific, limiting broad generalization.

• Crop yield responses and shade tolerance

Yield reductions in shade-intolerant crops (e.g., sorghum, maize, mustard, mint) are well documented across replicated field trials and long-term observational studies [192,207,209]. However, variation in plot size, canopy measurement techniques, and crop management sometimes weakens cross-study comparability. In contrast, the evidence showing genotype-dependent shade resilience in wheat is stronger, drawing from controlled multiyear comparisons and physiological assessments [198,199].

Yet, contradictory results persist: some studies report minimal yield penalties under moderate shade, whereas others observe substantial declines even in early rotation stages. These discrepancies often stem from differences in tree age, stand density, and local resource stress. Notably, the temporal intensification of yield reductions as poplar canopies mature is one of the more robust patterns, supported by consistent reports across climatic zones [201,204]. Still, most studies evaluating mitigative strategies (e.g., altered sowing dates, spacing optimization) remain short-term or limited to single seasons [199,201,238], suggesting that claims about management-based yield mitigation should be interpreted cautiously.

• Nutrient cycling, soil health, and climate benefits

Evidence for soil fertility enhancement under poplar systems is generally strong but heterogeneous in methodology. Numerous trials report increases in soil nutrients, organic matter, and microbial activity [199,212,218], though the magnitude varies with tree age and sampling depth. While root-mediated nutrient and pesticide remediation is widely cited [202], only a few studies employ rigorous isotope or tracer-based methods to quantify nutrient flows [239], highlighting a need for more mechanistic research.

Carbon sequestration benefits—especially in forage-based systems—are among the most consistent findings, supported by both chronosequence approaches and carbon budget modeling [200,247]. However, estimates vary greatly due to inconsistent accounting of belowground biomass and litter contributions.

• Economic viability

Despite yield reductions, poplar agroforestry frequently remains economically profitable, particularly when high-value crops (turmeric, mint, fodder grasses) or diversified rotations are employed [209,214,217,219]. However, many profitability assessments rely on partial budgeting rather than full cost–benefit analyses, limiting conclusions about long-term system resilience. Evidence supporting boundary planting as more profitable than block planting is credible but regionally concentrated, largely from northern India [219,248]. Reports that flexible management—especially crop selection, planting dates, and fertilization—can maintain profitability are promising yet often based on short-term trials with limited replication [208,213,249,250].

• Implications for system design

Several design principles are repeatedly advocated, but their evidence strength varies:

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Spacing matters: the recommendation for wider row spacing or optimized alley widths is supported by replicated field trials and simulation studies [200,201], though the optimal spacing range differs markedly across environments.

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Crop selection aligned with light tolerance: compatibility of root/tuber crops and forage species is moderately supported, but many studies lack multi-year validation [204,206,215].

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Management intensity as a driver: while enhanced fertilization and cultivar choice clearly improve performance, most studies test management factors in isolation rather than in combination [202,209,213], limiting extrapolation to farm-scale decision-making.

Overall, the evidence base validates poplar agroforestry as a resilient and environmentally beneficial farming strategy. Still, the quality and consistency of evidence supporting mitigation of yield losses through management adjustments vary widely. Long-term, multi-site trials and mechanistic studies on light–water–nutrient interactions are needed to strengthen system-wide design recommendations.

Linking ecophysiological trade-offs to management strategies

The keyword co-occurrence analysis, particularly the cluster comprising competition, water, and nitrogen, highlights the central resource-use trade-offs that govern poplar–crop interactions. Our ecophysiological synthesis (Section 4.5) shows that poplars respond dynamically to water availability through stomatal regulation, sap-flow adjustments, and clone-specific drought tolerance. These traits influence the degree of belowground competition experienced by intercrops, especially during periods of high VPD or soil moisture deficit. Likewise, nutrient-cycling patterns indicate that poplar biomass and litter contribute substantially to system-level nitrogen pools, yet crops generally maintain higher concentrations of mobile nutrients, creating a spatial and temporal gradient in nutrient demand.

These mechanisms directly inform the management recommendations discussed in Section 4.7. Wider spacing, optimized alley widths, and strategic row orientation help minimize belowground competition for water while maintaining sufficient light availability. Crop selection also becomes a key lever: species with lower water demand or high shade tolerance (e.g., forage grasses, root and tuber crops) experience reduced competition and make more efficient use of nitrogen released through poplar litter inputs. Conversely, water-sensitive or nitrogen-demanding crops may require adjusted planting dates, supplemental fertilization, or placement in alleys with lower tree root density. Integrating drought-tolerant poplar clones and managing canopy development through pruning can further reduce seasonal competition peaks. Thus, the competition-focused keyword cluster aligns closely with the empirical patterns observed and underscores the importance of resource-based design principles when developing sustainable poplar-based agroforestry systems.

4.8. Research Gaps and Future Directions

Despite the growing body of literature on poplar-based agroforestry systems, several critical research gaps remain that limit the full realization of their ecological and economic potential:

Geographic and thematic imbalance: Most research has been concentrated in India, China, and parts of Europe and North America, whereas studies from Africa, South America, and Central Asia remain scarce. These underrepresented regions offer distinct climatic, edaphic, and socio-economic conditions that could inform the adaptability and resilience of poplar-based systems. Future studies should focus on expanding the geographical coverage of field trials and developing region-specific management models. In addition, identifying regions with high edaphoclimatic suitability—such as temperate and subtropical areas with adequate soil moisture, deep alluvial soils, and moderate temperature regimes—would help guide researchers and policymakers in targeting areas where poplar-based agroforestry could be most successful.

Underexplored socio-economic dimensions: While poplar-based systems have been shown to increase farmers’ income, few studies have examined adoption barriers, land tenure issues, gender participation, or market integration for poplar products. Social acceptance by farmers remains a major bottleneck, often influenced by perceived risks, labor availability, access to markets, and extension support. Interdisciplinary research combining biophysical and socio-economic data would support more effective policy and extension strategies. Strengthening rural extension services and conducting participatory studies would help identify constraints and improve adoption at scale.

Insufficient understanding of tree–crop interactions: Competition for light, water, and nutrients remains a major constraint to optimizing productivity in intercropping systems. Detailed modeling of belowground and aboveground interactions, using tools such as remote sensing, UAV monitoring, and process-based simulation models, would improve system design and management.

Climate resilience and genetic improvement: Although several Populus species and hybrids are used in agroforestry, the performance of many genotypes under drought, salinity, or temperature extremes is poorly understood. Future work should prioritize genetic selection and breeding for stress-tolerant clones adapted to diverse agroecological zones, integrating physiological and genomic approaches. Assessing genotype × environment interactions across a wider range of climatic regions—including newly identified suitable zones—will be essential for climate-resilient system design.

Ecosystem services and biodiversity assessment: Most studies have focused on carbon sequestration, while other ecosystem services—such as pollination, soil biota enhancement, water regulation, and habitat provision—are rarely quantified. Standardized indicators and long-term monitoring networks are needed to evaluate multifunctionality and trade-offs in poplar-based systems.

Integration into climate-smart and circular economies: Poplar-based agroforestry holds significant potential for bioenergy production, carbon credits, and ecosystem restoration. However, few studies link these systems to circular bioeconomy models or national climate adaptation frameworks. Future research should explore pathways for integrating poplar agroforestry into carbon markets, renewable energy systems, and sustainable value chains.

In summary, future research should move toward integrative, long-term, and regionally inclusive studies that combine ecological, agronomic, and socio-economic perspectives. Harnessing advances in genetics, remote sensing, and systems modeling will be essential for optimizing poplar-based agroforestry as a cornerstone of sustainable and climate-resilient land-use strategies.

5. Conclusions

Poplar-based agroforestry systems demonstrate substantial potential to enhance agricultural productivity, ecological integrity, and rural livelihoods across diverse climatic zones. The evidence reviewed reveals several overarching principles for advancing the sustainability and scalability of these systems.

(1)

Strategic genotype selection is foundational.

Matching poplar species, hybrids, and clones to specific environmental conditions and management objectives—whether for timber, biomass, fodder, soil restoration, or intercropping—is the most effective pathway to optimize system performance. Disease-resistant, drought-tolerant, and low-suckering genotypes are particularly important in water-limited, high-stress, or urban environments.

(2)

Integrating poplars with diversified cropping systems enhances multifunctionality.

Poplar-based alley-cropping, agrosilvopastoral, forest-farming, and multi-strata systems consistently provide provisioning services (food, fodder, fiber, timber, bioenergy feedstocks), while also supporting biodiversity, improving soil fertility, and increasing farm resilience. The capacity of poplars to shelter a wide range of agricultural, horticultural, forage, and medicinal crops makes them an exceptionally flexible agroforestry component.

(3)

Poplars play a critical role in landscape-level environmental regulation.

Across riparian corridors, shelterbelts, degraded lands, and dryland ecotones, poplars contribute to microclimate regulation, water and soil conservation, carbon sequestration, phytoremediation, erosion control, and habitat recovery. When well-managed, these systems can reverse land-degradation processes and contribute to climate change mitigation targets.

(4)

Urban and peri-urban applications are promising but require careful management.

Poplars can improve environmental quality in industrial zones, transport corridors, and urban green spaces; however, their deployment must consider root expansion, litter production, and disease susceptibility. Appropriate clone choice, site-species matching, and maintenance practices are essential to ensure long-term urban sustainability. Nevertheless, the suitability of poplars for urban greening must be evaluated in light of well-documented constraints, including vigorous root systems capable of damaging pavements and underground infrastructure, the production of seasonal litter (catkins, leaves, brittle twigs), and their susceptibility to certain fungal and bacterial diseases under high-stress urban conditions. The reviewed literature indicates that these limitations can be mitigated through genotype selection (e.g., less suckering hybrids, disease-resistant clones), appropriate spacing, root-barrier infrastructure, and targeted management. Thus, while poplars can contribute effectively to urban ecological restoration, their deployment requires carefully planned site–species matching and maintenance strategies

(5)

Policymakers, breeders, and farmers should prioritize system design that integrates ecological and socio-economic objectives.

Scaling poplar-based agroforestry will require breeding programs focused on multifunctional traits, incentives for farmers to adopt climate-smart diversification, and landscape planning that aligns poplar systems with watershed protection, biodiversity conservation, and rural development goals.

Collectively, these principles highlight that poplar-based agroforestry is most effective when treated not simply as a collection of practices, but as an integrated strategy that links genetic improvement, ecological processes, and farmer-centered design to advance sustainable agriculture and resilient landscapes.