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Article

The Fine Root Distribution and Morphology of Mature White Poplar in Natural Temperate Riverside Forests under Periodically Flooded or Dry Hydrological Conditions

by
Anna Frymark-Szymkowiak
* and
Barbara Kieliszewska-Rokicka
Department of Environmental Biology, Kazimierz Wielki University, Ossolińskich Al. 12, 85-093 Bydgoszcz, Poland
*
Author to whom correspondence should be addressed.
Forests 2023, 14(2), 223; https://doi.org/10.3390/f14020223
Submission received: 18 November 2022 / Revised: 2 January 2023 / Accepted: 19 January 2023 / Published: 24 January 2023
(This article belongs to the Section Forest Hydrology)
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Abstract

:
Fine roots are a key component of carbon turnover in the terrestrial environment. Therefore, their distribution allows for the estimation of areas of carbon in the soil. The vertical distribution of roots is the result of both the tree species and various environmental factors. Research on the architecture of root systems most often includes seedlings and young trees growing under experimental conditions; however, little is known about trees in their natural habitats. The aim of this study is to analyze the fine root distribution of mature white poplar trees in natural riverside temperate zone forests of Central Europe (Poland) periodically flooded and in dry hydrological conditions. The length, diameter, and area of the fine roots, as well as the specific root length (SRL) and specific root area (SRA) of white poplar were measured in three layers of the soil, 0–10 cm, 10–20 cm, and 20–30 cm depths, in three forest sites. Two of the sites experience periodic floods, and one has been without flooding for 80 years, due to the construction of a flood embankment. The highest values of the lengths and surface areas of the poplar fine roots were observed at a depth of 0–10 cm at all sites. Soil moisture was positively correlated with the analyzed root parameters. The presence of understory plant roots contributed to the reduction in the fine root length of poplar in the subsurface layer, compared to the site that was not affected by the presence of plants other than poplar. The distribution of fine roots, the most dynamic part of the plant root system, reflects the most active areas in the soil profile. The presented research will allow for a better understanding of the functioning of natural riverside ecosystems, as well as show the great adaptability of white poplar fine roots to various conditions in the soil.

1. Introduction

Roots play a crucial role in the growth of plants, directly supplying water and nutrients [1]. They participate in many important processes of the forest ecosystem, such as nutrient cycling and belowground carbon allocation. By convention, the roots are divided into long-lived coarse roots (≥5 mm in diameter) that mostly perform mechanical and transport functions, and fine roots (≤2 mm in diameter) with very fast turnovers, which are mostly responsible for the absorption of water and nutrients [2,3]. The first-order fine roots are characterized by the highest absorption capacity and respiration activity, and colonization by mycorrhizal fungi, as well as the shortest life span [4,5,6]. The soil environment is one of the largest pools of carbon in terrestrial ecosystems. Mycorrhizal fungi and plant roots, especially the most distal fine roots, play a crucial role in underground carbon turnover [7]. The highest amounts of carbon produced by photosynthesis are transported directly to these roots [8]. On the other hand, due to root exudation, a short vitality, and a high decomposition rate, fine roots release carbon into the soil over a relatively short period of time. [9,10,11]. This highly dynamic nature of fine roots determines the amount and rate of carbon input into the soil [6,12,13,14]; therefore, the assessment of fine root biomasses in different ecosystems is an important element that contributes to a better understanding of the global carbon cycle.
The number of first-order roots in the pool of all fine roots differs between herbaceous (60%–100%) and woody (10%–58%) plants, as well as between species [15,16]. Among trees, conifers are characterized by a lower fine root biomass than deciduous trees [1,17]. The distribution, biomass, and functional diversity of plant roots in the soil can be modified in a number of ways, which is the result of their high plasticity in relation to various environmental conditions [18], the availability of nutrients [19], or the age of the plants [20]. Very thin roots react to stress factors much faster than leaves and thicker roots, so they can serve as early indicators of tree responses to environmental changes [21,22,23].
Poplars are trees resistant to periodic flooding, which allows them to grow in riverside areas with very dynamic water conditions. Different species and varieties are characterized by various tolerance to water stress, which translates into morphological features of leaves and roots [24,25,26].
Most research on the fine roots of poplars has investigated their adaptability to local ecological conditions [27,28,29,30,31] or the availability of nutrients [32,33,34]. However, these studies have mainly focused on seedlings or young trees growing in agricultural plantations or under controlled conditions, which is related to the worldwide use of various species and varieties of poplar for industrial purposes. Relatively little is known about the biomass, distribution, and morphology of poplar roots in riparian environments, especially for mature poplar trees. Such studies most often concern plantings in riparian buffer zones [35,36,37], while reports on the morphology of fine roots in natural floodplain sites are sparse [38,39].
We investigated the horizontal distribution, morphology, and biomass of the fine roots of white poplar (Populus alba L.) in the upper 30 cm of soil. White poplar is one of the main species of trees that comprises natural riverside forests of the temperate zone that experience seasonal or irregular flooding [40]. Riparian forests are distinguished by a high level of biodiversity, and they play an extremely important role in the ecosystem, forming, along with rivers, migration corridors that encourage the spread of organisms [41]. They also protect against floods and create a buffer that protects against the flow of nutrients into the river channel [42]. Mature, properly functioning floodplain trees, through an extensive root system and the mechanical properties of the roots, stabilize and strengthen riverbanks [43]. The widespread agricultural use of rich alluvial soils and the regulation of rivers has almost completely destroyed these various ecosystems all over Europe; therefore, the rare riparian forests still existing in the valleys of large unregulated rivers are valuable research areas.
In this study, we examine the vertical distribution and morphology of fine roots of mature white poplar trees in three natural riverside forests with different hydrological conditions, located in the temperate zone of Central Europe (Poland). Two forest stands were subject to periodic floods, and one has been cut off from seasonal flooding for 80 years via a flood embankment. The absence of temporary floods, a key process that is characteristic of riparian forests, offers up the opportunity to study how changing hydrological conditions affect the structure and distribution of fine roots in a natural site. Both the direct features of the roots, such as the fine root length (FRL; m m−2), fine root diameter (FRD; mm), fine root area (FRA; m2 m−2 of soil), and vertical distribution in the soil profile, and the coefficients related to root parameters, such as specific root length (SRL; m g−1) and specific root area (SRA; cm2 g−1), were analyzed. SRL and SRA are the two key indicators of the morphological and functional plasticities of fine roots in response to changes in environmental factors, such as water content and nutrient availability [20,23]. The growth parameters of fine roots can provide important information on belowground biomass allocation in the forest, depending on conditions in the soil environment.
The objectives of the study are to investigate the vertical distribution and morphology of fine roots of Populus alba in natural alluvial forests differing in the presence of periodic floods. This information will help with an understanding of the functioning of valuable and rare natural riverside forests within semi-arid environments.

2. Material and Methods

2.1. Study Sites

The study was conducted in the Valley of the Lower Vistula River (53°15′–53°34′ N; 18°19′–18°38′ E) in the Kuyavian-Pomeranian Voivodeship, Central Poland. The Vistula is the second-largest river flowing into the catchment area of the Baltic Sea. In contrast to the other great rivers in Europe, it is neither fully regulated nor transformed, which has allowed riverside forest habitats to be preserved. The mean annual precipitation in the study region is 500 mm, and the mean January, July, and annual temperatures are −2 °C, 18 °C, and 8 °C (1951–2015), respectively [44]. The cyclical floods of the river in the study area are mainly related to spring thaws. They usually appear in mid-March, and water levels above the warning level persist, on average, for two weeks. Summer floods are associated with persistent and increased rainfall, and they occur mainly from June to August. They last for much a shorter time than those occurring in the spring season—on average, four days (1921–2007). However, there have also been years where there were no floods [45].
The study area included riverside deciduous forests located on the right bank of the Vistula River. We chose three forest stands on the floodplain with naturally occurring mature white poplar trees, located 300 to 400 m from the river bank: (1) the nature reserve Wielka Kępa (WK); (2) the forest Starogród (STA), which is characterized by the presence of seasonal floods of the river; and (3) the nature reserve Ostrów Panieński (OP), which is cut off from periodical flooding, following the construction of a flood embankment in 1935. WK includes willow–poplar and elm–ash alluvial forests, and is characterized by the presence of numerous monumental, nearly 200-year-old white (Populus alba L.) and black poplar (Populus nigra L.) trees, and approximately 150-year-old common oak (Quercus robur L.), European field elm (Ulmus minor Mill.), European ash (Fraxinus excelsior L.), and willow (Salix L.) trees. The understory layer of vegetation is well represented. The STA site is dominated by white poplar trees of different ages, with individuals reaching an age of over 100 years (own observation). This study area is situated only three meters above the river level; therefore, during floods, the duration of the flooding is longer than in WK, where the height difference between the site and river is 13 m. The understory vegetation is poor, mainly formed by young poplar trees. The third studied forest, OP, represents a transition community between the elm–ash alluvial forest and oak–hornbeam forest, and it mainly consists of oak, elm, and ash, with some specimens of field maple (Acer campestre L.), European white elm (U. laevis Pall.), and white poplar. The understory is well developed and varied. All study sites are characterized by analyzed trees of similar average ages, which range from 99 in OP to 107 years in WK. The soils are typical brown alluvial soils (fluvic cambisols—WRB), with a silty loam texture in the STA and OP sites, and a loamy texture in WK in the top 20 cm, and sandy loam at a greater depth (Table 1).

2.2. Sampling Procedure

Soil–root samples were taken in late spring, after the spring floods had subsided (May 2011 and 2012), and in the fall (October 2011 and 2013), using a core sampler with a diameter of 5.5 cm and a length of 30 cm. In the spring of 2012, due to extremely difficult weather conditions, no samples were taken in WK. Instead, samples were taken in the spring of 2014 in order to equalize the number of spring seasons for all sites. On each of the three study sites (STA, WK, OP), three adult white poplar trees (with at least 6 m distance between trees) were randomly selected at each sampling date. Root samples were collected from the soil (1–2 m from the stem) at three depths (0–10 cm, 10–20 cm, and 20–30 cm), but due to the local soil conditions, it was not possible to collect all of the assumed samples every time; therefore, the amounts of the collected soil samples were different for each site. The total numbers of samples were as follows: STA—107 samples (soil layer 0–10 cm—35, spring—17, autumn—18; 10–20 cm—36, spring—18, autumn—18; 20–30 cm—36, spring—18, autumn—18), WK—81 samples (soil layer 0–10 cm—29, spring—12, autumn—17; 10–20 cm—27, spring—10, autumn—17; 20–30 cm—25, spring—10, autumn—15), and OP—103 samples (soil layer 0–10 cm—35, spring—17, autumn—18; 10–20 cm—35, spring—17, autumn—18; 20–30 cm—33, spring—17, autumn—16). Then, the collected soil–root samples were placed into plastic bags and stored at −20 °C until further analyses.

2.3. Fine Root Measurements and Physicochemical Properties of Soil

After first being carefully cleaned and washed in distilled water, all of the fine roots (diameter less than 2 mm) were manually separated using a vernier caliper and then they were differentiated into poplar and other plants roots (trees, shrubs, and herbaceous plants). The fine roots of Populus alba were distinguished based on their color, texture, and smell. Living and dead roots were counted together. Then, the selected roots were placed in a polycarbonate cuvette filled with distilled water and scanned. The obtained images were analyzed using the radixNova 1.0.1483 program by Cortex Nova, to determine the total length of all fine roots in the sample (FRL, m m−3), fine root area (FRA, cm2 cm−2), fine root diameter (mm), and root length in thickness classes (ø ≤ 0.5 mm, ø 0.5–1.0 mm, and ø1.0–2.0 mm). After scanning, the roots were placed in envelopes and dried at 50 °C for 48 h to obtain their dry matter. A parametric evaluation of the fine roots was performed using the following coefficients, specific root length (SRL, m g−1) and specific root area (SRA, cm2 g−1), describing the length and area, respectively, of the roots in relation to their dry mass and fine root biomass (FRB, g m−2). The soil moisture contents were determined using the standard drying technique [46]. Soil physicochemical properties of the study sites were characterized in our previous research (Frymark-Szymkowiak and Karliński [47]), which showed higher concentrations of total and organic C, total N, ammonium (NH4), and bioavailable orthophosphate (PO4-P) at the forest site regularly flooded by the river (Starogród), than at the two non-flooded sites.

2.4. Statistical Analysis

Statistical analyses were performed using the software package Statistica 12 StatSoft. Fine root morphological traits (root length, surface area, fine root diameter, specific root length, specific root area, and fine root biomass) and soil moisture were compared between individual study sites or soil levels at a significance level of p < 0.05 using one-way ANOVA. Site, depth, and season were subjected to a three-way ANOVA. The mean values of all characters were separated using Tukey’s honest significance test for unequal n. A Pearson’s correlation analysis was performed to analyze the relationships between root traits and soil moisture.

3. Results

3.1. Soil Water Content

The soil moisture at a depth of 0–30 cm was significantly higher in the sites with periodic flooding than in the forest separated from the river. The average water contents in the soil were 25% in STA and WK, and 20% in the OP site. However, such significant differences were only observed at a soil level of 0–10 cm. At sites with seasonal flooding, the water content also differed between the soil levels. A significantly higher soil moisture level was observed in the uppermost layer (0–10 cm) than in the deeper soil levels (10–20 and 20–30 cm). At the study site cut off from periodic flooding (OP), all soil layers were characterized by similar moisture values (Table 2).
Pearson’s correlation showed a significantly positive relationship between the mean values of fine root features and soil moisture, except for the fine root diameter, which was negatively correlated. However, when the fine roots’ morphologies at each soil depth were calculated, we observed a significant positive relationship between soil moisture and fine root length (r2 = 0.30, p < 0.01), fine root surface area (r2 = 0.32, p < 0.01), and fine root biomass (r2 = 0.21, p < 0.05) only in the 10–20 cm soil layer; between soil moisture and SRA (r2 = 0.20, p < 0.05) at the 20–30 cm soil depth, and a negative relationship between fine root diameter and soil water content in the shallow layer of 0–10 cm (r2 = −0.30, p < 0.01) (Table 3).

3.2. Share of Poplar Roots in the Pool of All Fine Roots

The share of the total fine root length (FRL) of poplars and other plants in the samples is shown in Figure 1.
At the study site STA, there was a complete lack of roots other than poplar on the dates for when the samples were taken, while in other places, such roots were present at each of the analyzed depths. The largest share of the roots of the understory vegetation was observed in the WK and OP sites (41% and 27%, respectively), in the surface layer of the soil (0–10 cm). In WK, roots that did not belong to poplars accounted for 19% and 26%, at the levels of 10–20 cm and 20–30 cm, respectively, while in OP, the deeper horizons were characterized by a 17% share of this group of roots.

3.3. Fine Roots Morphology

The study sites significantly differed in the analyzed root parameters at a soil depth of 0–30 cm, with the exception of SRA. The highest mean values of FRL, FRA, and FRB were observed in STA (556 m m−1, 134 m m−1, and 60 g m−2, respectively) and they were at least twice as high as compared to the other two sites, WK and OP. On the other hand, the WK forest significantly differed from the others, with a smaller diameter of fine roots. In total, the lowest average length of fine roots was observed at the site without seasonal flooding (OP), but these values did not significantly differ from site WK (Table 4).
The majority of poplar fine roots in all of the analyzed sites was located in the upper 10 cm of the soil, and its frequency tended to decrease with the depth. The root lengths at this level accounted for 59% and 56% of the total root length, respectively (Figure 2).
Both of the direct root growth parameters, such as fine root length and fine root surface area, as well as the biomass of fine roots, were determined by the study site (F = 44.38, p < 0.001; F = 51.04, p < 0.001; F = 19.68, p < 0.001, respectively) and the depth of the soil (F = 46.57, p < 0.001; F = 2.54, p < 0.001; F = 18.75, p < 0.001, respectively). The study site also had significant effects on the fine root diameter (F = 7.86, p < 0.001) (Figure 2). The sampling season (spring or fall) had no effect on FRL, FRA, FRD, or FRB, but it had a weak effect on SRL (F = 4.93, p < 0.05) and SRA (F = 4.46, p < 0.05) (Table 4).
The highest values of FRL and FRA were observed in the STA in each of the analyzed soil levels; however, the greatest differences concerned the depth of 0–10 cm, where this forest site significantly differed from the other two study sites. In the remaining analyzed soil levels (10–20 and 20–30 cm), STA significantly differed only from OP. Similarly, STA was characterized by the highest values of fine root biomass, while significant differences were observed both for soil depths of 0–10 and 10–20 cm. In the deepest soil layer tested, all sites had similar root biomass values. The mean fine root diameter varied between 0.84 and 1.49 mm, with no influence of depth, but a significant influence from the study site. The lowest values of this parameter were observed in WK regardless of the soil depth, although significant differences between the sites were observed at the soil levels of 0–10 and 10–20 cm. WK was also characterized by the highest mean values of SRL and SRA. When these functional fine root traits were analyzed at individual soil depths, significant differences were observed between the two flooded sites (WK and STA) only at the level of 10–20 cm (Figure 2). Most of the fine roots of white poplars in the three floodplain forests were classified as very thin (less than 1.0 mm in diameter). They covered from 61% in OP to 78% in WK of the total length of fine roots (with a diameter of less than 2 mm). There were no significant differences in the share of the individual classes of root thickness between the sites (averaged values), as well as in the distribution of each thickness class (0.0–0.5, 0.5–1.0, and 1.0–2.0 mm) in the soil profile at each study site, where the highest share was observed in the subsurface layer (Table 5).

4. Discussion

Water and the availability of nutrients, as well as the species and age of the tree, play an essential role in the spatial distribution of the roots and their morphologies [10,19,20,48]. All of the analyzed study sites were characterized by the highest values of woody fine roots, as well as understory roots, in the subsurface layer of the soil (0–10 cm), and showed a decrease with depth (Figure 1 and Figure 2). Similar observations were made many times for various types of ecosystems (e.g., [49,50,51]), including periodically flooded riparian forests [35,52]. The high availability of easily digestible organic matter in the subsurface layers of the soil directly affects the abundance of roots, especially small ones, which are responsible for the nutrition of trees. The decrease in the root biomass in the deeper layers of the soil could be related to the release of nutrients, or variations in soil structure, temperature, and moisture. The largest share of roots in the upper part of the soil profile also reflects the structure of the poplar root system. Steele et al. (1997) reported the longest aspen root length of 0–20 and 5–10 cm in the southern and northern regions of Canada’s boreal forest, respectively [53]. Similar results have also been shown in a study on five poplar clones planted on an experimental field in Belgium, where most of the fine root biomasses of all the analyzed clones was distributed in the 0–5 cm layer [54]. However, Karliński et al. observed the most root tips at a depth of 10–20 cm, compared to at the topsoil horizon, in different poplar genotypes in contaminated soil, although individual genotypes differed in root distribution [55].
White poplar is a fast-growing tree, characterized by a smaller average root diameter, and a larger root area, and SRL coefficient, as well as an increased number of root tips, compared to slow-growing trees [56]. Therefore, distinguishing the general pool of roots by convention classified as fine roots seems insufficient. Among roots with diameters of less than 2 mm, the highest activity and turnover rate are characterized by very fine roots (up to 1 mm in diameter), the dominant share of which was observed both for species of the genus Populus [54,57,58], as well as maple, oak, spruce, pine, and juniper [12]. Ostonen et al. (2005) reported a 66% share of roots with diameters of less than 1 mm in the spruce fine root pool in Estonia [59], and Claus and George (2005) reported a 63% share of the same for Turkey oak [52]. Our results confirm these previous reports of the dominant participation of very fine roots (Table 5). The share of individual classes of the thicknesses of fine roots at specific depths seems to be a species-specific feature. In the present study, only the roots of white poplar were analyzed, and this could explain the almost identical results observed at all sites. However, when five different poplar clones were examined, differences in the vertical distributions of the individual thickness classes were visible [54].
The fine root biomass values in the floodplain forest of the Valley of the Lower Vistula River (29.2–104.1 g m−2 at a soil depth of 0–10 cm) were similar to the values of 10–100 g m−2 reported for various hybrid poplar plantations [60]. Such a high degree of variation may be the result of both the influence of the tree genotype and specific environmental conditions. One of the most important climatic factors controlling the distribution and quantity of fine roots is the water content in the soil. Although numerous studies have focused on the relationship between root biomass and soil moisture, these results are ambiguous. Some reports indicate a higher rate of fine root growth in a dry environment [61,62]. However, a positive effect of soil water availability on fine root growth has also been described [63,64]. In the present study, the mean values of fine root traits (except for fine root diameter) in the soil layers analyzed were positively correlated with soil moisture. The highest values of the examined root morphological (FRL and FRA) and functional features (FRB) were observed in the forest site with the most humid soil (STA). At the same time, there were no differences between WK and OP, although the seasonally flooded site, Wielka Kępa (WK), was characterized by a significantly higher water content in the soil compared to OP. This indicates that the water level in the soil does not appear to be a major factor affecting these root parameters. Similar results were obtained by Imada et al. (2010) in an experiment with white poplar cuttings. Thus, the availability of nutrients, rather than soil moisture, seems to be more important for the architecture of the white poplar root system in these floodplain forests. The obtained results can be related to the carbon content (total and organic) in the soil determined at the studied sites, the level of which was clearly the highest (especially at a depth of 0–10 cm) at the STA site, while the other two sites (WK and OP) were characterized by quite similar values [47]. Such relationships, in particular with regard to the contents of P and N, have also been described for other forest tree species [1,65,66], although the availability of water also turned out to be the main limitation in the case of the Scots pine forest in the Alps [67]. It has also been shown that the response of poplar trees to a lower water content in the soil involves functional changes in the architecture of the root system [29] or limitations of shoot growth [68], rather than a reduction in root growth or length. However, we did not observe any clear differences in the morphological parameters of fine roots (SRL or SRA) between the examined sites, proving their adaptability to limited water conditions, although there are results showing a reduction in SRL in drought conditions in Populus euphratica seedlings [25].
In natural forests, the biomass and distribution of fine roots may also be conditioned by the presence of other plants, including undergrowth species. Research has shown a variety of directions for such changes in mixed forests compared to monospecies stands, which are associated with strategies of nutrient and water uptake from the soil used by different species [69,70]. In our study, significantly higher values of FRL, FRA, and FRB were observed only in the site without the participation of roots of other plants (STA). These results are consistent with the observations of Kalliokoski et al. (2010) in temperate mixed forests in Southern Finland, where, as a result of intense underground tree species competition, the biomass of fine roots was reduced [71]. A negative correlation between the fine root biomasses of trees and understory vegetation was also shown by Hishi et al. (2015) in deciduous forests, and for the cultivation of larch within the cool temperate climate zone of Japan [72]. There are also reports that the coexistence of plants in the undergrowth increases their functional traits, such as SRL, for a better penetration of soil aggregates [73]. Our research also confirms such a relationship in the first-two analyzed soil levels (0–10 and 10–20 cm), where most of the root biomass was located, although these differences were not statistically significant (Figure 2).
The time of sampling may have affected the root biomass, which is related to the seasonalities and developmental stages of the above-ground parts of plants, but it is also modified by specific site conditions, including the level of soil moisture. When Kiley and Schneider (2005) examined riverside forests in North America, they observed that the highest root biomass occurred in late summer (August) during the first year of the study, while in the following year, after the occurrence of summer floods, the root biomass did not show such a tendency [74]. In a study devoted to the roots of fine spruce from Estonia, Ostonen et al. (2005) reported that the highest biomass occurred for samples from August, and the lowest for samples from November [59]. The lack of clear differences between the spring and autumn seasons in the present work may be related to the stability of soil conditions during these two specific moments of the growing season: the beginning and end.

5. Conclusions

Under natural conditions, the relationship between the traits of fine roots and soil properties is still vague, as plants are affected by many interacting environmental factors. In this study, we observed that features of Populus alba fine roots that are most sensitive to environmental changes are fine root biomass (FRB), fine root length (FRL), fine root surface area (FRA), and fine root diameter (FRD). The root parameters can be used to evaluate the development of the fine root system that was observed to be the most abundant in the Starogród site characterized by regular annual floods, compared to those in the two non-flooded sites. Specific root length (SRL) and specific root area (SRA) significantly differed between seasons (spring, autumn).
Based on the results of this study and previously obtained data, we concluded that the highest FRB, FRL, and FRA values of white poplar in the Starogród (STA) forest site were most affected by nutrient concentrations in the soil (N, C, available P), than by soil moisture. The significant positive correlation between FRB, FRL, and FRA and soil moisture concerned only the soil layer at 10–20 cm; however, all soil layers investigated in the STA site contained higher concentrations of nutrients than the soil in the two other study sites.
Fine roots of Populus alba show a high plasticity in relation to changing hydrological conditions, which can be both the result of hydrotechnical works (limiting the number and duration of floods) and progressing climate change associated with long periods of drought.
White poplar plays an important role as a species stabilizing the floodplain forest of Central Europe and can be considered as a suitable species for the restoration of degraded riparian habitats.

Author Contributions

Conceptualization, A.F.-S. and B.K.-R.; methodology, A.F.-S.; validation, A.F.-S. and B.K.-R.; formal analysis, A.F.-S.; writing—original draft preparation, A.F.-S.; writing—review and editing, A.F.-S.; visualization, A.F.-S.; supervision, B.K.-R. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Polish Minister of Science and Higher Education within the program “Regional Initiative of Excellence” in 2019–2022 (grant no. 008/RID/2018/19).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Forests 14 00223 g001 550
Figure 1. The contribution of the total length of roots of poplars and other plants in the samples in the upper layer of the soil profile (0–10 cm, 10–20 cm, 20–30 cm) at three forest sites (STA—Starogród, WK—Wielka Kępa, OP—Ostrów Panieński).
Figure 1. The contribution of the total length of roots of poplars and other plants in the samples in the upper layer of the soil profile (0–10 cm, 10–20 cm, 20–30 cm) at three forest sites (STA—Starogród, WK—Wielka Kępa, OP—Ostrów Panieński).
Forests 14 00223 g001
Forests 14 00223 g002a 550Forests 14 00223 g002b 550Forests 14 00223 g002c 550
Figure 2. Vertical distribution of fine root morphological traits ((A)—root length, (B)—surface area, (C)—root diameter, (D)—specific root length, (E)—specific root area, (F)—fine root biomass) in the analyzed depths of soil profiles in floodplain forests (STA—Starogród, WK—Wielka Kępa, OP—Ostrów Panieński). Significant differences between the sites at specific depths are marked with different letters, small letters for depths, capital letters for locations (p < 0.05, Tukey’s test).
Figure 2. Vertical distribution of fine root morphological traits ((A)—root length, (B)—surface area, (C)—root diameter, (D)—specific root length, (E)—specific root area, (F)—fine root biomass) in the analyzed depths of soil profiles in floodplain forests (STA—Starogród, WK—Wielka Kępa, OP—Ostrów Panieński). Significant differences between the sites at specific depths are marked with different letters, small letters for depths, capital letters for locations (p < 0.05, Tukey’s test).
Forests 14 00223 g002aForests 14 00223 g002bForests 14 00223 g002c
Table
Table 1. Mean values ± standard deviation (n = 3) of soil granulometric composition in tree forests in the Vistula River floodplain (STA—Starogród, WK—Wielka Kępa, OP—Ostrów Panieński) in three soil layers (0–10 cm, 10–20 cm, 20–30 cm).
Table 1. Mean values ± standard deviation (n = 3) of soil granulometric composition in tree forests in the Vistula River floodplain (STA—Starogród, WK—Wielka Kępa, OP—Ostrów Panieński) in three soil layers (0–10 cm, 10–20 cm, 20–30 cm).
Depth (cm)(%)Texture
SandSiltClay
0–10
STA33.33 (±2.89)53.67 (±2.08)13.00 (±1.00)Silt loam
WK50.33 (±2.08)41.00 (±1.73)8.67 (±0.58)Loam
OP42.02 (±9.51)53.93 (±8.41)4.05 (±1.13)Silt loam
10–20
STA29.00 (±12.12)55.00 (±7.00)16.00 (±5.29)Silt loam
WK51.00 (±3.00)40.33 (±1.53)8.67 (0.58)Loam
OP37.95 (±5.63)57.64 (±5.03)4.41 (±0.61)Silt loam
20–30
STA17.00 (±2.65)61.67 (±0.58)21.33 (±2.52)Silt loam
WK54.33 (±5.86)37.33 (±4.73)8.33 (±1.15)Sandy loam
OP38.76 (±6.66)56.00 (±5.29)5.24 (±1.37)Silt loam
Table
Table 2. Values ± standard deviation of soil water content (%) in three riparian forests in the Vistula River floodplain (STA—Starogród, WK—Wielka Kępa, OP—Ostrów Panieński) in three soil layers (0–10 cm, 10–20 cm, 20–30 cm) and average values (0–30 cm). Different letters indicate significant differences, the small letter for depths, the capital letters for locations (p < 0.05, Tukey’s test).
Table 2. Values ± standard deviation of soil water content (%) in three riparian forests in the Vistula River floodplain (STA—Starogród, WK—Wielka Kępa, OP—Ostrów Panieński) in three soil layers (0–10 cm, 10–20 cm, 20–30 cm) and average values (0–30 cm). Different letters indicate significant differences, the small letter for depths, the capital letters for locations (p < 0.05, Tukey’s test).
Depth (cm)STAWKOP
0–10 27.3 ± 6.3 bB30.6 ± 8.9 bB20.3 ± 6.4 aA
10–2024.6 ± 5.3 abA24.5 ± 7.2 aA20.5 ± 8.3 aA
20–3023.8 ± 4.8 aA20.1 ± 8.0 aA20.0 ± 8.6 aA
Mean values
(0–30 cm)		25.2 ± 5.5 B25.3 ± 9.1 B20.3 ± 7.7 A
Table
Table 3. Correlation analysis results of the relationships between root traits and soil moisture. Significant correlations are marked by * (p < 0.05), ** (p < 0.01) or *** (p < 0.001). The number of samples from each soil depth was: 0–10 cm n = 99, 10–20 cm n = 98, 20–30 cm n = 94.
Table 3. Correlation analysis results of the relationships between root traits and soil moisture. Significant correlations are marked by * (p < 0.05), ** (p < 0.01) or *** (p < 0.001). The number of samples from each soil depth was: 0–10 cm n = 99, 10–20 cm n = 98, 20–30 cm n = 94.
Fine Root LengthFine Root Surface AreaFine Root DiameterSpecific Root LengthSpecific Root AreaFine Root Biomass
Moisture at specific soil depth:
0–10 cm0.0230.125−0.298 **0.0720.027−0.028
10–20 cm0.302 **0.317 **−0.0590.1610.1320.207 *
20–30 cm0.1330.197−0.1620.1760.204 *−0.162
Moisture
(mean values)		0.201 ***0.251 **−0.164 **0.141 *0.117 *0.115 *
Table
Table 4. Results of a three-way ANOVA (F and p-values) on the influence of three study sites (STA, WK, OP), three different soil depths, and two variable seasons (spring, autumn) on various fine root architecture traits in three riverside forests.
Table 4. Results of a three-way ANOVA (F and p-values) on the influence of three study sites (STA, WK, OP), three different soil depths, and two variable seasons (spring, autumn) on various fine root architecture traits in three riverside forests.
SiteDepthSeason
Fine root length44.38 (p < 0.001)46.57 (p < 0.001)0.87 (NS)
Fine root surface area51.04 (p < 0.001)52.54 (p < 0.001)3.56 (NS)
Fine root diameter7.86 (p < 0.001)0.47 (NS)1.37 (NS)
Specific root length2.57 (NS)1.48 (NS)4.93 (p < 0.05)
Specific root area1.10 (NS)1.13 (NS)4.46 (p < 0.05)
Fine root biomass19.68 (p < 0.001)18.75 (p < 0.001)0.41 (NS)
Table
Table 5. Fine root length (mean ±SD) for different root diameter classes and soil layers in three study sites: Starogród (STA), Wielka Kępa (WK), and Ostrów Panieński (OP). The means are for all sampling times. Within each root diameter class, different letters indicate significant differences among depths at the same study site (p < 0.05, Tukey’s test).
Table 5. Fine root length (mean ±SD) for different root diameter classes and soil layers in three study sites: Starogród (STA), Wielka Kępa (WK), and Ostrów Panieński (OP). The means are for all sampling times. Within each root diameter class, different letters indicate significant differences among depths at the same study site (p < 0.05, Tukey’s test).
Study SiteSoil LayerRoot Diameter Class
0.0–0.5 mm0.5–1.0 mm1.0–2.0 mm
cm cm−2%cm cm−2%cm cm−2%
STA0–10 cm467.56 ± 241.52 b 60196.85 ± 93.40 b58316.12 ± 145.65 c58
10–20 cm194.84 ± 167.52 a2590.34 ± 65.67 a26144.93 ± 99.13 b27
20–30 cm118.08 ± 90.63 a1554.64 ± 36.25 a1684.05 ± 47.24 a15
Mean values260.16 ± 230.7246113.94 ± 90.9821181.70 ± 142.7133
WK0–10 cm181.24 ± 145.02 b4369.46 ± 67.744494.02 ± 90.8543
10–20 cm156.41 ± 141.94 ab3752.74 ± 43.293476.62 ± 64.6535
20–30 cm87.14 ± 104.28 a2034.30 ± 38.832250.24 ± 62.7322
Mean values141.60 ± 138.815352.17 ± 54.191973.63 ± 76.7528
OP0–10 cm174.33 ± 139.82 b5165.03 ± 59.81 b5090.96 ± 95.75 b52
10–20 cm98.81 ± 97.66 a2941.23 ± 42.21 a3255.37 ± 61.69 a31
20–30 cm66.78 ± 45.33 a2022.94 ± 16.46 a1828.92 ± 29.61 a17
Mean values113.31 ± 115.525343.07 ± 48.822058.42 ± 76.8327
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Frymark-Szymkowiak, A.; Kieliszewska-Rokicka, B. The Fine Root Distribution and Morphology of Mature White Poplar in Natural Temperate Riverside Forests under Periodically Flooded or Dry Hydrological Conditions. Forests 2023, 14, 223. https://doi.org/10.3390/f14020223

AMA Style

Frymark-Szymkowiak A, Kieliszewska-Rokicka B. The Fine Root Distribution and Morphology of Mature White Poplar in Natural Temperate Riverside Forests under Periodically Flooded or Dry Hydrological Conditions. Forests. 2023; 14(2):223. https://doi.org/10.3390/f14020223

Chicago/Turabian Style

Frymark-Szymkowiak, Anna, and Barbara Kieliszewska-Rokicka. 2023. "The Fine Root Distribution and Morphology of Mature White Poplar in Natural Temperate Riverside Forests under Periodically Flooded or Dry Hydrological Conditions" Forests 14, no. 2: 223. https://doi.org/10.3390/f14020223

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