Research on Vegetation Removal Strategies for the Ming Guangwu Great Wall Based on Clearance Resistance Assessment

Abstract

The Great Wall of China, one of the nation’s most remarkable military defense structures, possesses a history spanning several millennia and is associated with numerous heritage sites. Today, it stands as a world-renowned cultural heritage asset. Vegetation growing on the ruins of the Great Wall can exert both detrimental and protective effects on the structure. Indiscriminate removal of such vegetation may lead to unintended damage to the heritage site. Drawing on the theory of “evaluative conservation”, this study integrates the analytic hierarchy process (AHP) and the Delphi method to develop a resistance assessment system for vegetation removal. A case study was conducted on 40 plant species or categories located along the Ming-era Guangwu section of the Great Wall, with spatial zoning analysis applied to inform removal strategies. The results reveal the structure, key factors, and classification criteria of the resistance evaluation system. Corresponding management recommendations are proposed, including strategies such as “preservation”, “partial preservation”, “removal”, and “subsequent removal and management”. This research provides a foundational reference for the conservation and restoration of the Great Wall heritage, and for the management of associated vegetation.

1. Introduction

The Great Wall of China stands as one of the most iconic cultural heritage sites globally, embodying millennia of history and representing the longest-standing military defense system in ancient human civilization [1,2]. Since its designation as World Heritage Site No. 438 in 1961 [3], China has progressively developed comprehensive conservation standards aimed at protecting the Wall’s physical integrity. The release of the Master Plan for the Conservation of the Great Wall in 2019 [4] further advanced systematic preservation efforts. In response, Shanxi Province has actively integrated surrounding cultural and historical resources and adopted a spatial development model—referred to as “one belt, three sections, six zones, and multiple nodes”—to establish the Great Wall National Cultural Park as both a symbol of national identity and a platform for transmitting Chinese civilization [5]. Although the Wall has lost its original military function [6], its cultural significance remains profound, serving as a vital medium for preserving historical memory and sustaining the spirit of the Chinese nation.

Scholars have conducted extensive research on various aspects of the Great Wall heritage. Studies have examined the spatial distribution characteristics of the Great Wall, exploring influencing factors from natural, historical, and institutional perspectives [2]. Analyses have also focused on the spatial morphology of auxiliary structures, such as watchtowers and passes, investigating the relationship between garrison deployment and physical geography [7]. From a planning perspective, implementable proposals have been put forward for the construction of spatial corridors along the Great Wall [8]. Other studies have advocated for the development of scenic resources surrounding the Wall to promote integrated ecological, regional, and tourism-based development [9], and for tracing the spatiotemporal evolution of the surrounding environment to construct narrative strategies grounded in historical timelines [10]. From an architectural viewpoint, damage assessments have informed recommendations for the restoration and preservation of the Wall’s masonry structures [11]. Additional research has addressed contemporary revitalization, protection measures, planning and design strategies, and the cultural significance of the Great Wall [12]. Geospatial technologies, such as ArcGIS, have been employed to analyze the spatial structure of military settlements and to develop corresponding conservation strategies [13]. Moreover, data fusion techniques have been applied to construct new relational frameworks for understanding and managing the heritage of the Great Wall [14].

Since the 20th century, growing attention has been directed toward the relationship between heritage sites and their surrounding environments, particularly the interactions between the ruins of the Great Wall and the associated vegetation [15]. Initially, the conservation of historic sites and the management or removal of adjacent vegetation were treated as separate endeavors, with limited consideration of their interdependence. This conceptual divide persisted until the Italian theorist, Cesare Brandi (1906–1988), introduced the notion of “critical restoration” (restauro critico), a methodology that views built heritage and vegetation as an integrated whole, emphasizing restoration that is simultaneously functional and aesthetic. Compared with other conservation paradigms, Brandi’s approach advocates for a holistic preservation of the landscape across the heritage corridor and its broader ecological setting. It aims to establish a context-sensitive restoration framework by analyzing the present condition of heritage sites and the natural systems in which they are embedded, thereby revealing the interrelationship between architectural structures and surrounding ecosystems, and informing adaptive, site-specific conservation strategies [16,17]. This approach has been applied to cases such as the Garden of Ninfa in Italy, where researchers documented the species, growth conditions, and degrees of integration of the plants growing on the historic walls. The plants were then categorized based on their level of physical damage and assigned risk levels accordingly [17]. Further classifications were conducted across different sections of the site, evaluating plant threats based on root structure, vitality, and ecological or aesthetic value systems. Similarly, the method has been applied to vegetation management at the Dazhuangke section of the Great Wall in Beijing, where the plants growing on the wall’s upper surface were classified into five levels of damage impact. These levels informed the development of differentiated vegetation removal strategies according to the observed patterns of deterioration [18].

Initially, vegetation was regarded solely as a detrimental factor to heritage structures, prompting widespread efforts to remove it entirely, and replace affected areas with modern repair materials. However, such interventions have frequently compromised the authenticity of heritage sites and, in some cases, resulted in irreversible secondary damage [19]. A notable shift occurred with the development of the “Clayton Wall” restoration method at Hadrian’s Wall in the United Kingdom [20,21], which emphasized the use of locally sourced materials and the strategic integration of vegetation into repair processes to balance structural preservation with ecological integrity. This approach later evolved into the widely recognized “soft capping” technique, formally validated in 1993 [22,23], and it continues to shape contemporary heritage conservation practices.

The Great Wall represents a distinctive category of cultural heritage, where ancient architecture coexists with archaeological remains [24]. Vegetation has played a continuous role in the site’s transformation over time, underscoring the need to adhere to principles such as the “authenticity of current condition” and “minimum intervention” in vegetation management [3]. Studies have shown that the plants growing on the top and flanks of the Wall can begin to affect the structure—either destructively or protectively—even in the early stages of growth. From the perspective of sustainable preservation, the removal of highly invasive species that have spread extensively and pose a substantial threat to the Wall’s structural integrity is essential [25,26]. By contrast, certain plant species—due to their size, growth stage, or spatial position—may not compromise the structure and could, in fact, provide ecological, aesthetic, or even conservation benefits [26,27,28]. The indiscriminate removal of such vegetation may inadvertently undermine the Wall’s stability [29]. Therefore, decisions regarding plant removal or retention should be informed by a multifactorial assessment that considers threat level, ecological function, and current site conditions, while also accounting for the inherent complexity and uniqueness of the Great Wall as a heritage site [30,31].

This study aims to address two primary research questions: (1) What species of vegetation are distributed within the Great Wall heritage zone? (2) Which species pose a threat to the structural integrity of the Wall and should therefore be removed, and which possess conservation value and should be preserved? To answer these questions, a vegetation removal resistance evaluation model is developed. Based on the model’s outcomes, targeted vegetation removal strategies are proposed, with the goal of providing a scientific foundation for the effective conservation and management of the Great Wall.

2. Background and Methods

2.1. Study Area

The Inner Great Wall in Shanxi Province traverses 11 counties and districts, including Pianguan, Shenchi, Shuocheng, Ningwu, Yuanping, Daixian, Shanyin, Yingxian, Hunyuan, Fanshi, and Lingqiu. It originates in the west at Baiyangling in Pianguan, where the Inner and Outer Great Walls converge, and passes through key sites, such as Laoying Fort, Limin Fort, Yangfangkou, Pandaoliang, Baicaokou, Hou’erling, Xin Guangwu, Lingyunkou, Pingxingguan, Niubangkou, and Langyakou, ultimately terminating at Qiaomaicha in Lingqiu [32]. The Ming Guangwu section is situated within Shanyin County, Shuozhou City, Shanxi Province. The region is characterized by a temperate continental monsoon climate, with an average annual temperature ranging from 3 °C to 14 °C and marked by pronounced diurnal temperature variation. Annual precipitation averages between 400 and 650 mm [1,4]. Under the influence of this climatic regime, local vegetation flourishes during the summer, exhibiting dense foliage and a rich diversity of species—primarily temperate deciduous broadleaf forests—which collectively form a vast and scenic landscape. By contrast, the vegetation undergoes significant withering and decline during the winter months [33].

The Ming Guangwu section of the Great Wall extends approximately 7.2 km and currently retains 38 watchtowers and 27 beacon towers [1,4]. It stretches from Xin Guangwu Village to Baicaokou Village in Daixian County and historically constituted a key component of the Yanmen Pass defense system. In particular, it safeguarded two of the “four passes and two gates” along the Yanmen defensive line—Baicaokou and Guangwukou—both of which served as critical front-line positions against incursions from beyond the northern frontier [34] (Figure 1). The segment selected for this study is located between the third section of the Wall, near Xin Guangwu, and the 17th watchtower (Xionggao Zhuanglu), encompassing watchtowers from Zhenjiong (No. 13) to Kong’e (No. 15). The total surveyed length is approximately 700 m, covering an estimated area of 2800 square meters (Figure 2).

Recent research indicates that the conservation of the Great Wall in Shanxi has long been marked by disorder and neglect. The ongoing deterioration of the Wall can be attributed to five primary factors: (1) natural weathering caused by wind and rain erosion; (2) damage stemming from local agricultural and residential activities, wherein villagers have excavated bricks and soil from the Wall for use in farming or house construction; (3) destruction linked to transportation infrastructure development, particularly the construction of low-grade roads that either intersect the Wall or utilize its structure as a roadbed; (4) tourism development and inappropriate restoration practices, which have significantly compromised the Wall’s original architectural integrity by transforming it into a commercialized tourist destination; and (5) illegal excavation and unregulated resource extraction, such as mining and quarrying, which have caused extensive damage to both the Wall and the surrounding ecological environment [35,36].

Survey Findings: (1) Wall Structure: The masonry sections exhibit extensive surface deterioration, with only limited remnants of original brick and stone remaining. The rammed earth segments display pronounced cracking, and numerous cavities have developed within the wall body. Additional forms of structural degradation, including partial fractures, collapses, hollowing, and erosion gullies, are evident throughout the site. (2) Wall Top: While much of the original brick and stone material on the top surface remains intact, localized collapses have occurred. The watchtowers and beacon towers are comparatively well preserved, maintaining a relatively high degree of structural integrity. (3) Vegetation: A wide variety of plant species were recorded, with Pinus tabuliformis identified as the dominant species. Vegetation is widely distributed along both sides of the Wall, its flanks, and the top surface. Numerous plants are in direct contact with the structure, leading to visible damage, such as cracking, the development of biological crusts, and chemical interactions that accelerate material degradation. (4) Fauna: Evidence of animal activity was observed throughout the survey area. Livestock waste was present, and nests of ants, rodents, and birds were found on both sides of the wall structure (Figure 3).

Significant differences were observed among the plant types with respect to scale, growth characteristics, destructive potential, and spatial distribution. Herbaceous species were predominantly perennial, exhibiting healthy growth while remaining relatively short in stature (mostly < 50 cm). These plants typically featured small canopies and slender stems, yet were widely distributed across the site, including both flanks, the lateral wall surfaces, and the top of the Wall. High-density clusters were recorded in multiple locations. Their root systems were shallow and fibrous, generally extending less than 30 cm in depth, and their overall impact on the structural integrity of the Wall was relatively limited. By contrast, shrub species exhibited greater root depth, plant height, and canopy spread compared to herbaceous species. These plants were primarily concentrated within one meter of either side of the Wall, although some individuals were found growing directly on the wall surface, with several physically embedded within the masonry. Characterized by deep taproot systems exceeding 30 cm in depth, these shrubs pose a considerable threat to the Wall’s stability due to their invasive root penetration. Arboreal (tree) species demonstrated the most vigorous growth, with extensive canopy coverage observed in certain areas. However, these species generally maintained a physical distance from the Wall and did not exhibit direct destructive interactions with the structure. On the contrary, they may provide ecological benefits by stabilizing the surrounding environment (Figure 4).

Overall, the Ming Guangwu section of the Great Wall maintains a relatively intact structural framework. Nevertheless, the lack of a comprehensive conservation and restoration strategy has led to multiple latent threats that could undermine its long-term preservation.

The field investigation revealed a high diversity of plant life forms within the surveyed section of the Great Wall. A total of 33 plant species were identified, including 6 species classified as nationally protected (Class II), 5 species under regional protection, and 22 species native to Shanxi Province. These species belong to 17 families and 29 genera. From a botanical perspective, the flora comprises 20 deep-rooted species and 13 shallow-rooted or fibrous-rooted species. Regarding vascular plant life forms, six major categories were recognized, with herbaceous and shrub species predominating. Tree and vine species were less abundant, representing a smaller proportion of the overall plant community.

2.2. Research Methods

This study integrates multiple methodologies, primarily utilizing field investigation, the Delphi method, and the analytic hierarchy process (AHP). The detailed research process is illustrated in Figure 5, aiming to accurately identify and analyze the vegetation characteristics of the Ming Guangwu Great Wall and to provide a reliable foundation for this study.

2.2.1. Field Survey

Between July 2023 and September 2024, multiple field investigations were carried out at the Ming Guangwu section of the Great Wall, specifically within the area spanning the 13th to 17th watchtowers of the third segment. Data collection during these surveys included on-site observations, photographic documentation, and the comprehensive recording of the plant species alongside their surrounding environmental conditions. These efforts produced both qualitative and quantitative data regarding plant presence, spatial distribution, and ecological interactions [37].

2.2.2. The Delphi Method

The Delphi method, also known as the expert consultation method, was first developed by the RAND Corporation in the United States in 1946. It is a structured communication technique that relies on anonymous, iterative feedback from experts. The general process involves soliciting expert opinions on a given subject, followed by the organization, summarization, and statistical analysis of their responses. These results are then anonymously fed back to the experts for further evaluation [37]. This iterative cycle continues until consensus is achieved. In this study, the Delphi method was employed to assign weights and evaluate factor values within the plant clearance resistance assessment system for the Ming Guangwu section of the Great Wall. This approach enhances the scientific rigor, comprehensiveness, and validity of the data underpinning the evaluation model.

2.2.3. The Analytic Hierarchy Process (AHP)

The analytic hierarchy process (AHP), developed by American operations researcher Thomas L. Saaty, is a multi-criteria decision-making method that integrates both qualitative and quantitative analyses [38]. By establishing a systematic evaluation index system, AHP facilitates a scientific and objective assessment of evaluation subjects and criteria. When combined with the theory of critical restoration, this method assists in simplifying the assessment of the vegetation’s “dual impact”—encompassing both its destructive and protective roles—while also improving the authenticity and reliability of the evaluation model and its outcomes.

The construction process of the evaluation system follows these steps (Figure 6): Selection of evaluation factors → Development of the AHP model → Construction of the judgment matrix → Assignment of values → Generation of weights → Consistency test → Formation of the evaluation model → Output of results [39,40,41].

(1)

Calculate the square root vector of the judgment matrix, $A_i$ [38,39]:

$$A_i=\sqrt[n]{{\displaystyle \prod _{i=1}^n{E}_{ij}}}\left(i=1,2,\dots ,n;j=1,2,\dots ,n\right)$$

(1)

$E_{ij}$: scale value of the relative importance of the i-th factor to the j-th factor (i = 1, 2,..., n; j = 1,2,..., n; n: number of evaluation indicators).

(2)

Calculate the single-layer ranking weight value of each standard layer evaluation index, $W_i$:

$$W_i=A_i{\displaystyle \frac{1}{{\displaystyle {\sum }_{i=1}^n{A}_i}}}\left(i=1,2,\dots n\right)$$

(2)

(3)

Calculate the maximum characteristic root, ${\lambda }_{\max }$ [41]:

$${\lambda }_{\max }={\displaystyle \frac{1}{n}}{\displaystyle \sum _{i=1}^n\frac{\left(A{w}_i\right)}{W_i}}$$

(3)

$A{W}_i$: denotes the sum of the normalized weights derived from the judgment matrix.

(4)

Calculate the comprehensive consistency index [41]:

$$CI={\displaystyle \frac{{\lambda }_{\max }-n}{n-1}}$$

(4)

(5)

Test the consistency [30]:

$$CR={\displaystyle \frac{CI}{RI}}$$

(5)

Note: $RI$ is the average random consistency index.

(6)

The formula for calculating the composite score value (I) is as follows:

$$I={\displaystyle \sum _{i=1}^n{W}_i{F}_i}$$

(6)

$W_i$: Comprehensive ranking weight value of each evaluation index.

$F_i$: Score value of the i-th plant.

2.3. Data Source

The data used in this article mainly come from the following sources: field investigation, plant data collection, and data resources.

(1)

Field Investigation

The research team conducted comprehensive fieldwork at the Ming Guangwu section of the Great Wall. Utilizing a combination of grid-based surveys, photographic documentation of current conditions, UAV aerial photography, and interviews with local residents, detailed data on the vegetation within the study area were collected. In addition, relevant information regarding evaluation indicators and factors, as well as insights into future conservation and development plans for the region, was obtained.

(2)

Plant Data Collection

Relevant plant-related data were collected from multiple sources, including online databases, CNKI (China National Knowledge Infrastructure), and the academic literature. These sources comprised scholarly articles, professional monographs, local county chronicles, official historical records of the Great Wall, and policies and regulations pertaining to vegetation management along the Great Wall. The classification and characteristics of the identified plant species were determined based on authoritative references, such as the Catalogue of Rare and Endangered Plants of China, the List of National Key Protected Wild Plants, the List of Key Protected Wild Plants of Shanxi Province, the Checklist of Wild Plants of Shanxi Province, and documents issued by the Shanxi Forestry and Grassland Administration.

(3)

Data Resources

The base map data utilized in this study were obtained from platforms including Google Earth, OpenCycleMap, and Baidu Maps. Satellite imagery was sourced from Bigemap GIS and Bigemap Pro. Weight assignments and evaluation values were derived through expert consensus, using average scores. Data analysis and visualization were conducted using SPSS 25.0, ArcGIS 10.8, and other relevant software tools.

3. Results and Analysis

3.1. Evaluation Index System and Weight Assignment for the Ming Guangwu Great Wall Vegetation Clearance Model

Based on previous studies [38,39,40,41], the AHP-based vegetation clearance resistance evaluation model was developed using the Delphi method [42,43,44]. Experts from disciplines including ecology, landscape architecture, historical architectural conservation, and restoration were invited to assign values to the pairwise comparison matrices. In accordance with the conservation objectives and current conditions of the Great Wall, these experts proposed key evaluation indicators pertinent to their respective fields. Consequently, a comprehensive evaluation index system and corresponding weights were established for the AHP model. The final structure of the evaluation system is as follows: the target layer (A) represents the overall resistance to vegetation clearance on the Ming Guangwu section of the Great Wall. This is supported by three intermediate layers (C)—Intrinsic Characteristics (C1), Destructive Impact (C2), and Applied Value (C3)—and 13 specific criteria under the program layer (P). The construction of judgment matrices includes the following:

(1)

A–C matrix: comparing intermediate layer indicators with respect to the target layer, yielding the weights of C1, C2, and C3;

(2)

C1–P matrix: comparing the 13 program indicators with respect to Intrinsic Characteristics;

(3)

C2–P matrix: comparing the 13 program indicators with respect to Destructive Impact;

(4)

C3–P matrix: comparing the 13 program indicators with respect to Applied Value.

All judgment matrices were constructed using the 1–9 Saaty scale. Consistency checks were subsequently conducted for each matrix, yielding the following results: CRA = 0.015, CR1 = 0.040, CR2 = 0.016, and CR3 = 0.021. As all consistency ratios fall below the acceptable threshold of 0.1, the matrices are deemed to be consistent and thus valid for further analysis.

Ultimately, the weights of each evaluation indicator were calculated and consolidated through expert input and interdisciplinary analysis. Among the intermediate-level indicators, the weight ranking is as follows: C2 (Destructive Impact) > C3 (Applied Value) > C1 (Intrinsic Characteristics), indicating that Destructive Impact constitutes the core component of the evaluation model. At the criterion (program) level, the indicators are ranked by weight as follows: P7 (Physical Impact) > P9 (Biological Impact) > P8 (Chemical Impact) > P5 (Root System Strength) > P10 (Conservation Value) > P11 (Ecological Value) > P6 (Coverage Level) > P4 (Growth Status) > P12 (Landscape Value) > P13 (Other Values) > P3 (Occupancy Area) > P1 (Plant Species) > P2 (Plant Height) (

Table 1. Weight results of the resistance index system for the plant cleaning in the Ming Guangwu Great Wall.

The Intermediate Layer (C)	Weights of the Intermediate-Layer Indicators	The Program Layer (P)	Weights of the Program Layer
C1 (Intrinsic Characteristics)	0.250	P1 (Plant Species)	0.016
		P2 (Plant Height)	0.013
		P3 (Occupancy Area)	0.020
		P4 (Growth Status)	0.046
		P5 (Root System Strength)	0.103
		P6 (Coverage Level)	0.052
C2 (Destructive Impact)	0.530	P7 (Physical Impact)	0.287
		P8 (Chemical Impact)	0.116
		P9 (Biological Impact)	0.127
C3 (Applied Value)	0.220	P10 (Conservation Value)	0.090
		P11 (Ecological Value)	0.061
		P12 (Landscape Value)	0.043
		P13 (Other Values)	0.026

).

According to the ranking results, the experts concluded that, among the resistance evaluation indicators for vegetation clearance, the extent of damage caused by vegetation to the Great Wall is the most influential factor, with physical impact representing the most critical component. This suggests that, within the evaluation and decision-making process, the score assigned to Physical Impact plays a decisive role in determining whether a plant species should be removed or retained. In essence, it directly influences the overall resistance to clearance.

The decision to remove or retain vegetation primarily hinges on the extent to which it compromises the structural integrity of the Great Wall. In the case of the Ming Guangwu section, certain plant species growing directly on the wall pose immediate threats due to their invasive root systems, which may undermine the stability of the structure, necessitating prompt mitigation measures. However, these same species have, over time, become integrated into the historical narrative and cultural landscape of the Wall. They contribute to the site’s visual continuity, environmental context, and heritage integrity, and thus warrant careful consideration within the evaluation framework.

3.2. AHP-Based Vegetation Clearance Resistance Evaluation System and Classification for the Ming Guangwu Great Wall

The comprehensive evaluation in this study covered a total of 33 plant species—13 herbaceous plants, 13 shrubs, 5 trees, and 2 vines—classified into 40 species/categories, including 7 that grow near the wall edge or on the wall top. Each plant was assessed against 13 criteria (P1–P13) defined within the AHP-based evaluation model. A scoring system was developed for each criterion using a tiered grading scale based on the physical and ecological conditions of the Great Wall. The scoring values were assigned using the deviation value method, with scores of 5, 3, 1, and 0 indicating descending levels of clearance resistance. In this system, a lower score (closer to 0) reflects lower resistance to clearance, suggesting that the plant holds minimal conservation value and is more likely to be removed. Conversely, a higher score (closer to 5) indicates a higher clearance resistance, implying that the plant has significant preservation value and should be considered for retention [41,42] (

Table 2. Evaluation and assignment of plant cleaning resistance.

The Intermediate Layer (C)	The Program Layer (P)	Index Scoring					Definition of Indicators
		Evaluation Grading	5	3	1	0
C1 Intrinsic Characteristics	P1 Plant Species	Herbs	✔				Different plant species contribute to ecological stability, with each plant species receiving an average of 5 points.
		Shrubs	✔
		Arbor	✔
		Vines	✔
	P2 Plant Height	Greater than 160 cm	✔				The taller the plant, the better its growth status, which means it is more difficult to clean and should be preserved. Therefore, using plant height as the evaluation criterion, the higher the height, the higher the score.
		80~160 cm		✔
		10~80 cm			✔
		Less than 10 cm				✔
	P3 Occupancy Area	Greater than 200 cm	✔				When the proportion of plant area is larger (the crown width is larger), the cleaning work is more difficult, otherwise it is easier to clean. Using the size of the plant’s footprint as the evaluation criterion, the larger the footprint, the higher the resistance score.
		100~200 cm		✔
		20~100 cm			✔
		Less than 20 cm				✔
	P4 Growth Status	Excellent	✔				The stronger the growth and development of the plants, the higher their ecological and landscape value. Based on the comprehensive level of plant growth, the better the growth status, the higher the score condition of the plant, the higher the clearing resistance value.
		Better		✔
		General			✔
		Poor				✔
	P5 Root System Strength	Poor	✔				The criteria for determining the strength of the plant root systems are based on the type and growth of the root system. The stronger the root system performance, the lower the score assigned.
		General		✔
		Stronger			✔
		Pole-strength				✔
	P6 Coverage Level	Not Gathering	✔				The higher the degree of plant aggregation, the stronger the ability to adapt to the environment (overgrowth). Using the growth status and aggregation degree of the plants as evaluation criteria, the higher the coverage, the lower the score.
		Small Gatherings		✔
		Gather			✔
		Massive Gathering				✔
C2 Destructive Impact	P7 Physical Impact	No Impact	✔				The distance from the wall and the strength of the root system of plants can reflect the degree of physical damage. Therefore, the distance from the wall and the condition of the root system are used as evaluation criteria. The closer to the wall, and the stronger the root system, the more severe the corresponding impact, and the lower the score.
		Lighter		✔
		More Serious			✔
		Serious				✔
	P8 Chemical Impact	No Impact	✔				Judging from the comprehensive chemical effects of plants on walls, such as microbial blocks, mosses, and biological crusts, the more severe the phenomenon, the greater the impact on the wall, and the lower the score.
		Lighter		✔
		More Serious			✔
		Serious				✔
	P9 Biological Impact	No Impact	✔				Judging by the number, size, and degree of small mammal, bird, reptile, and insect nests distributed around the plants, the higher the degree, the higher the impact, and the lower the score.
		Lighter		✔
		More Serious			✔
		Serious				✔
C3 Applied Value	P10 Conservation Value	National level	✔				According to the protection level of the plant, the higher the protection level, the higher the score.
		Local level		✔
		Ordinary			✔
		Invasion				✔
	P11 Ecological Value	Important Value	✔				Plants can play a role in windbreak, sand fixation, and soil stability. The coverage of the plants is used as a critical criterion, and the higher the coverage, the higher the score.
		A More Important Value		✔
		General Value			✔
		No Value				✔
	P12 Landscape Value	Important Value	✔				The landscape value of the plants is comprehensively judged based on the color rendering degree, area, and time of their flowers, fruits, leaves, branches, etc. The higher the landscape value, the higher the rating.
		A More Important Value		✔
		General Value			✔
		No Value				✔
	P13 Other Values	Important Value	✔				According to the quantity of plants with different values, the higher the quantity, the higher the score.
		A More Important Value		✔
		General Value			✔
		No Value				✔

“✔”: Represents different evaluation grades and their corresponding scores.

).

Based on the established scoring criteria, the research team collaborated with domain experts to assign evaluation scores to the 33 identified plant species or classified into 40 species/categories. The scores were aggregated and used to calculate a comprehensive evaluation value for each species. According to the final scores, the plants were classified into three resistance levels: (

Table 3. Process of clearing resistance value of needle grass.

Serial Number	Species	C1 Intrinsic Characteristics	C2 Destructive Impact	C3 Value and Utilization	Comprehensive Evaluation	Grade
1	Prunus humilis	0.667	2.750	0.707	4.124	I
2	Artemisia frigida	0.950	2.750	0.391	4.091	I
3	Olgaea lomonossowii	0.997	2.486	0.578	4.061	I
4	Berchemia lineata	1.099	2.486	0.291	3.876	I
5	Daphne genkwa	0.997	2.146	0.639	3.782	I
6	Kali collinum	1.079	2.486	0.196	3.761	I
7	Ulmus pumila	0.687	2.750	0.291	3.728	I
8	Clematis leschenaultiana	0.585	2.750	0.385	3.720	I
9	Klasea centauroides	0.609	2.750	0.356	3.715	I
10	Crepis crocea	0.829	2.750	0.087	3.666	I
11	Sophora davidii	0.584	2.486	0.553	3.623	I
12	Pinus tabuliformis	0.739	1.880	0.933	3.554	I
13	Flueggea suffruticosa	0.484	2.486	0.402	3.372	II
14	Populus simonii	0.480	2.090	0.633	3.203	II
15	Ulmus glaucescens	0.523	2.486	0.188	3.197	II
16	Nitraria sibirica	0.891	2.046	0.257	3.194	II
17	Astragalus melilotoides	0.314	2.486	0.385	3.185	II
18	Artemisia stechmanniana	0.797	2.222	0.150	3.169	II
19	Potaninia mongolica	0.482	2.146	0.498	3.126	II
20	Dasiphora fruticosa	0.464	2.248	0.310	3.022	II
21	Prunus mongolica	0.691	2.248	1.055	2.994	II
22	Aster tataricus	0.461	2.248	0.230	2.939	II
23	Hippophae rhamnoides	0.290	2.116	0.501	2.907	II
24	Artemisia giraldii	0.382	2.254	0.110	2.746	II
25	Ephedra equisetina	0.467	1.248	0.639	2.354	II
26	Elaeagnus angustifolia	0.580	1.254	0.379	2.213	II
27	Artemisia stechmanniana (near the side wall)	0.797	1.222	0.150	1.169	II
28	Artemisia lavandulifolia	0.691	1.302	0.110	2.103	II
29	Lycium chinense	0.585	1.278	0.150	2.013	II
30	Prunus mongolica (near the side wall)	0.691	0.248	1.055	1.994	III
31	Nitraria sibirica (near the side wall)	0.591	1.046	0.257	1.894	III
32	Caragana rosea	0.485	0.782	0.484	1.751	III
33	Stipa capillata	0.672	0.814	0.110	1.596	III
34	Ixeris polycephala	0.672	0.814	0.110	1.596	III
35	Rhamnus arguta	0.590	0.660	0.150	1.400	III
36	Eremogone juncea	0.475	0.814	0.110	1.399	III
37	Ephedra equisetina (near the side wall)	0.467	0.248	0.639	1.354	III
38	Dasiphora fruticose (near the side wall)	0.464	0.248	0.310	1.022	III
39	Aster tataricus (near the side wall)	0.461	0.248	0.230	0.939	III
40	Hippophae rhamnoides (near the side wall)	0.290	0.116	0.501	0.907	III

).

(1)

Level I (Score < 5.000 to ≥ 3.500): High clearance resistance

(2)

Level II (Score < 3.500 to ≥ 2.000): Moderate clearance resistance

(3)

Level III (Score < 2.000 to 0): Low clearance resistance

The analysis yielded the following results:

(1)

In the Level I category, twelve plant species were identified, comprising five perennial herbaceous species, two tree species, three shrubs, and two vines. This group exhibited relatively high scores in both C1 (Intrinsic Characteristics) and C2 (Destructive Impact). The herbaceous plants, characterized by short stature, weaker root systems, and lower biomass, received high C2 scores due to their minimal physical damage to the wall, with values ranging from 2.2 to 2.7. Furthermore, their healthy growth conditions and substantial ground coverage contributed to favorable C1 scores, ranging from 0.6 to 1.0. By contrast, while the tree species in this group received lower C2 scores due to reduced P9 (Biological Impact) values, they achieved notably higher C3 (Applied Value) scores, primarily driven by strong P11 (Ecological Value). This resulted in a higher overall clearance resistance, indicating that these species are suitable candidates for preservation within conservation and management plans.

(2)

In the Level II category, seventeen plant species were identified. Similar to the Level I species, these plants received relatively high scores in C1 (Intrinsic Characteristics), with values ranging from 0.4 to 0.7. However, their scores in C2 (Destructive Impact), particularly regarding physical damage, were generally lower. This discrepancy can be attributed primarily to differences in their spatial positions relative to the wall structure, which influence the degree of physical harm they cause. For example, species such as Artemisia vulgaris, Elaeagnus angustifolia, and Lycium barbarum exhibited relatively low C2 scores, indicating lower overall clearance resistance within the Level II group and suggesting a higher likelihood of requiring removal under certain conservation scenarios.

(3)

Level III includes eleven plant species, consisting predominantly of shrubs (seven species) and a smaller number of herbaceous plants (four species). These species received moderate scores in C1 (Intrinsic Characteristics). However, due to their close proximity to the wall structure, their P7 (Physical Impact) values were particularly low, with most scores ranging between 0 and 1. This significantly reduced their overall C2 (Destructive Impact) scores. Moreover, the majority of these species are native but possess limited ecological or cultural value, leading to correspondingly low scores in C3 (Applied Value). These combined factors reflect their low clearance resistance and highlight the necessity of removal in conservation practice.

4. Discussion

The quantitative and comprehensive analysis above leads to the following conclusions, which can inform the research-based restoration of the Great Wall. (The process of plant removal and retention is shown in Figure 7).

4.1. Plant Removal Strategies by Category

(1)

Level I Plants: This category primarily includes perennial herbaceous plants and low-growing shrubs that are generally small in size and possess shallow or fibrous root systems. As such, they pose minimal threat to the structural integrity of the Great Wall while offering significant conservation value. These species are typically in good physiological condition and grow densely, often becoming dominant on the top surfaces of the wall. They can serve as a form of “soft cover”, providing both physical protection and ecological benefits [22,45]. Certain shrub species within this category can further contribute to reducing weathering, mitigating rain-induced erosion, and limiting additional surface damage [46,47]. However, their preservation should be conditional—restricted to individuals that do not currently, and are unlikely to, compromise the wall’s structure. Tree species (formerly categorized as arborous) assigned to Level I offer considerable ecological protective functions and should be fully preserved. Where conditions permit, their numbers may even be increased to enhance site resilience. In addition, special attention should be given to the clearance of vegetation near drainage outlets to ensure unimpeded water flow and prevent associated structural degradation.

(2)

Level II Plants: Compared to the Level I species, the plants in this category are generally larger and exhibit more vigorous growth habits. They are commonly found in proximity to the wall and possess robust, often densely distributed root systems that can exert moderate pressure on the wall’s surface structures. In line with the principle of minimal intervention, a species located at a reasonable distance from the wall and not currently causing structural damage may be retained under close monitoring. However, regular oversight is essential, particularly for species with excessive growth that may detract from the site’s visual integrity or serve as habitats for large numbers of nesting birds. Should any signs of potential structural threat arise, these plants should be promptly removed. In the case of rare or legally protected species, conservation should take precedence, and tailored, long-term management plans should be implemented to balance heritage protection with ecological preservation. During restoration and conservation activities, it is recommended that Level II plants—especially those currently encroaching upon or likely to affect the wall structure—be selectively removed to prevent future damage.

(3)

Level III Plants: Level III plants primarily consist of herbaceous and shrubby species found adjacent to the sidewalls of the Great Wall. These species often exhibit vigorous growth and well-developed root systems, posing a substantial threat to both the horizontal and vertical structural stability of the wall. Their biological activity is strongly associated with various forms of deterioration, manifesting as leaning sidewalls, internal voids, and other structural deformations. Present-day conditions, such as wall collapse, bulging, and cracking along the flanks, are inextricably linked to the mechanical and hydrological impacts of these vegetation types [24].

4.2. Vegetation Removal Strategies by Area

(1)

Microorganisms on the External Wall Surface

In contrast to earlier approaches that indiscriminately removed microbial growth from wall surfaces—often overlooking the varying difficulties associated with different types of microorganisms and their potential protective effects on the masonry—current best practices advocate for a more systematic strategy [30,46,48]. The recommendations include the following:

1.

Species Identification and Damage Assessment: Microbial species targeted for removal should first be identified, and the degree of damage they inflict on the wall surface should be assessed and tested. This evaluation serves as the basis for determining appropriate removal methods.

2.

Context-Specific Removal Techniques: Based on site-specific conditions, different removal methods may be applied: Physical removal: Techniques such as manual scraping, low-pressure water jets, vacuum suction devices, and thermal inactivation may be employed. Chemical removal: Following localized compatibility testing, eco-friendly biocidal agents—such as quaternary ammonium compounds or hydrogen peroxide—can be applied, along with the use of protective surface coatings. Biological removal: Enzymatic treatments may be used to reduce microbial activity and neutralize biological agents, thereby minimizing long-term recurrence.

3.

Assessment of Large, Long-Term Deposits: For substantial accumulations formed over extended periods, removal decisions should be made based on whether the growth poses a structural risk to the wall [30,46,48].

(2)

Herbaceous Plants

The investigated section of the Great Wall is located in an area that has not yet undergone comprehensive conservation or restoration, resulting in widespread proliferation of wild herbaceous vegetation. Certain species, such as Celastrus orbiculatus, Salsola collina, and Daphne genkwa, have exhibited uncontrolled growth due to the absence of effective management. While herbaceous plants generally pose a lower risk to the structural integrity of the wall compared to woody vegetation, excessive or oversized growth can still endanger the stability of the masonry. Unlike previous indiscriminate removal practices, vegetation management at the Great Wall heritage site should be guided by factors such as actual damage levels, visual landscape impact, and the strategic importance of specific locations. In structurally vulnerable areas, the real-time monitoring of herbaceous plant growth is essential. Technical removal should be prioritized for species identified as potential safety hazards to prevent the escalation of structural threats. Certain critical structural components of the Great Wall—such as drainage outlets, watchtowers, beacon towers, and fortifications—are especially susceptible to damage from vegetation overgrowth. For instance, drainage outlets serve the crucial function of discharging water rapidly; blockages caused by dense and low-growing herbaceous plants may lead to structural collapse and erosion-induced foundation instability. Therefore, targeted removal strategies should be applied to herbaceous vegetation growing in and around these sensitive features [46,48].

Recommended removal methods for herbaceous plants include the following:

1.

Manual removal for low-lying vegetation in sensitive locations;

2.

Low-toxicity herbicide treatments for plants growing in structurally unstable zones;

3.

Mechanical or technical methods, such as mowing, laser weeding, or steam-based weed control, for managing large-scale or excessive growth.

(3)

Vegetation Near Wall Structures and on the Wall Top

Previous vegetation removal practices at heritage sites, particularly on the Great Wall, have often relied on rapid manual extraction. However, this method poses significant safety risks and lacks contextual sensitivity. Therefore, it is recommended that vegetation removal strategies be tailored to specific site conditions and structural integrity zones [19,46].

• In severely collapsed areas where plant roots are intricately intertwined with masonry—threatening the stability of the structure—rapid excavation is no longer appropriate. Instead, the use of supportive techniques is advised. These include root-targeted chemical injections or sprout-inhibiting agents to effectively inactivate vegetation and prevent regrowth. Once plant vitality is neutralized, structural restoration should be conducted promptly, followed by ongoing monitoring and repair of the affected wall sections.
• In better-preserved areas of the wall, mechanical removal methods may be safely applied, including in situ manual clearing and the use of supplementary tools, such as low-temperature flame weeding or biological herbicides, depending on site conditions.
• Regarding the wall-top areas, although many brick and stone features remain intact, certain species—such as Hippophae rhamnoides (sea buckthorn)—have caused notable disturbances. These plants have formed elevated soil mounds around their bases, leading to the destruction of the original brick layering. Such vegetation requires both targeted removal and growth control interventions.
• For overly dense yet non-destructive vegetation, selective thinning is recommended. For plants that have damaged structural layers or show signs of intrusive root development along both sides of the wall, inactivation using herbicidal agents is appropriate to suppress further harm. Additionally, species with low landscape or ecological value may be removed selectively, while retaining those with high ornamental or ecological significance [46].

4.3. Post-Removal Vegetation Management and Utilization Strategies

(1)

Management Measures for Late Stage Plants of the Ming Guangwu Great Wall

In earlier conservation efforts, the lack of systematic post-removal vegetation management often led to secondary regrowth, resulting in repeated damage to heritage structures. Therefore, continuous monitoring and management following plant removal are essential components of heritage protection [18,47]. Emergency restoration should first be carried out in areas where cracks or structural vulnerabilities have developed in the wall itself. Additionally, voids created by vegetation removal must be assessed for their structural impact and appropriately repaired. These repairs aim to restore the stability of the heritage site while preventing new vegetation from establishing in the exposed areas, thereby reducing future risks. Specific management measures should include the following: routine inspections conducted by trained personnel to monitor and document the condition of vegetation, particularly along the top surfaces and both sides of the wall; damage assessment related to potential threats posed by plant regrowth; ongoing observation of plant development to identify early signs of regrowth and to mitigate associated hazards, such as pest infestations, structural collapse, or other environmental risks. In addition, the integration of modern monitoring technologies (e.g., remote sensing, drones, and automated data systems) can significantly enhance the efficiency and precision of long-term vegetation management [47].

(2)

Vegetation Utilization Strategy for the Protection of the Great Wall

While vegetation has often been regarded as a threat to heritage preservation, it also possesses beneficial ecological functions that were previously overlooked. Early interventions focused solely on removal, neglecting the potential of vegetation to contribute positively to the protection and stabilization of the Great Wall. Therefore, a scientific and strategic approach to vegetation utilization is essential. Plant selection should prioritize native species and align with local climatic conditions to enhance ecological resilience and long-term effectiveness [18,29,49]. For windbreak and erosion control purposes, native tree species with strong ecological adaptability are recommended. Ulmus pumila and Pinus tabuliformis should serve as the primary choices, with Platycladus orientalis and other hardy conifers as complementary species. In sloped areas requiring soil stabilization, shrubs such as Sophora alopecuroides, Hippophae rhamnoides, Prunus humilis, and Amygdalus mongolica are suitable. To enhance landscape aesthetics while maintaining ecological functions, sun-facing slopes may be planted with fruit-bearing, climate-adapted species, such as apricot, pear, and persimmon trees. By contrast, shady mountain slopes should be planted with shade-tolerant species, like birch, spruce, and wild chrysanthemum.

From a spatial perspective, vegetation protection can be organized into three concentric zones:

• Zone 1 (~500 m from the wall on windward slopes): Establish forest belts to mitigate wind erosion;
• Zone 2 (from ~500 m to ~10 m from the wall): Employ mixed shrub and tree plantings to reduce surface runoff and water-induced erosion;
• Zone 3 (directly on or adjacent to the wall): Introduce original perennial herbaceous species and small native shrubs historically present along the Great Wall corridor to provide localized stabilization and to minimize mechanical damage [29,49].

4.4. Study Limitations

The construction of the Great Wall spans several millennia and extends across vast areas of northern China. It exhibits significant variations in building techniques and materials, depending on the historical period and geographic region. This study focuses specifically on the Ming Dynasty section of the Guangwu Great Wall in Shanxi Province. Therefore, the conclusions drawn are most applicable to regions that share similar climatic conditions and architectural characteristics. In exceptional cases—such as those involving unique plant species, particular growth behaviors, or distinctive spatial distributions—on-site decision-making by conservation and restoration experts becomes essential. These situations require context-specific assessments grounded in the local environmental and structural conditions, in accordance with the principle of site-specific adaptation.

5. Conclusions

Based on field investigations and the development of a resistance-based vegetation removal assessment model for the Guangwu section of the Ming Great Wall, this study proposes targeted strategies for vegetation management, including spatial classification and corresponding removal recommendations. The main conclusions are as follows:

(1)

Twelve species categorized as Level I exhibit minimal current or potential damage to the structural integrity of the Great Wall. These species should be fully retained and utilized as a form of “soft coverage”, offering both protective and aesthetic benefits to the site. Level II species, which have not yet posed significant structural threats due to their geographic distribution, should be partially retained and subjected to continuous monitoring and adaptive management. By contrast, all seven shrub species and four herbaceous species classified as Level III, along with their residual root systems, are either actively damaging or present a high risk to the Wall’s structural stability and should therefore be completely removed.

(2)

This study recommends a series of technical and preventive measures, including the removal of microbial colonies on the outer wall surfaces, the targeted clearance of herbaceous vegetation in densely vegetated zones and structurally vulnerable areas, and differentiated removal strategies for vegetation located near or directly on wall structures, based on site-specific risk assessments. These actions should be supplemented by ongoing vegetation management protocols and the development of guided ecological utilization strategies.

The establishment of a vegetation removal resistance evaluation system for the Guangwu Great Wall provides a scientific basis for informed vegetation management and represents a significant advancement in the conservation of this cultural heritage site. Furthermore, it supports regional industrial and economic revitalization while contributing to the long-term preservation and interpretation of the Great Wall’s cultural and historical legacy.