Effects of Anthropogenic Disturbance on the Structure, Competition, and Succession of Abies ziyuanensis Communities

Abstract

Attention to habitat dynamics in subtropical mid-mountain forest plant communities containing endangered vegetation is critical for understanding the responses of ecosystems to global climate change and for their effective conservation. This study examines the species composition, structure, and interspecies competition within endemic and endangered Abies ziyuanensis (Abies ziyuanensis L.K.Fu and S.L.Mo) communities in China, comparing undisturbed and anthropogenically disturbed conditions. The survey recorded a total of 71 plant species across 39 families and 60 genera. PERMANOVA analysis highlighted significant disparities in species composition between the two forest community conditions. Communities impacted by anthropogenic disturbances showed a higher diversity of shrub and herbaceous species compared to those that were undisturbed, coupled with a significant increase in the number of Abies ziyuanensis seedlings, suggesting a greater potential for self-renewal. Nonetheless, the distribution of diameter class structures in these two community conditions indicates a declining trend in population numbers. In undisturbed Abies ziyuanensis communities, the Weighted Hegyi Competition Index (WCI) for Abies ziyuanensis was 6.04, below the average WCI of 12.24 for all trees within these communities. In contrast, within communities affected by anthropogenic disturbances, the WCI for Abies ziyuanensis reached 7.76, higher than the average WCI of 7.43 for all trees, indicating that Abies ziyuanensis in disturbed communities face heightened competitive pressure compared to undisturbed settings. These findings underscore that previous anthropogenic disturbances have altered the community composition, competition dynamics, growth environment, and succession trends of Abies ziyuanensis communities. While these disturbances promote the regeneration of Abies ziyuanensis, they also reduce its current dominance as a target species.

1. Introduction

The subtropical mid-mountain forest ecosystems of China nurture a distinctive array of relict and endangered tree species, including Glyptostrobus pensilis (Staunton ex D. Don) K. Koch, Metasequoia glyptostroboides Hu & W. C. Cheng, Cathaya argyrophylla Chun & Kuang, and others, making a significant contribution to global biodiversity [1,2,3]. However, even within communities with the same endangered relict species, significant heterogeneity exists in community structure and species composition. This is because the succession and expansion of forest communities are not only influenced by the functional traits of each species, but also constrained by environmental filtering [4,5]. The forest communities of endemic relict species are considered particularly sensitive to climate changes [6]. As elevation increases, ecological niches may shift rapidly, which greatly affects the species composition within these communities [2,7,8]. Additionally, anthropogenic disturbances, such as land-use changes, forest management, and grazing, can significantly alter community structures [3,9,10,11]. These disturbances often affect forest biodiversity by creating new microhabitats, enhancing resource availability, and limiting the dominance of certain species [12]. The inhibition theory and the tolerance theory further elucidate how species influence changes in community structure through their adaptability to the growth environment [13]. Amid the uncertainty and complexity of current forest changes, closely monitoring and protecting relict and endangered forest communities is crucial for tracking global biodiversity and climate change trends, and it is a prerequisite for formulating effective species and habitat conservation management strategies.

Abies ziyuanensis (Abies ziyuanensis L.K.Fu and S.L.Mo), a critically endangered species within the Pinaceae family, is a prominent tree and a relic of the Quaternary glaciation [14]. It is found in subtropical mid-mountain regions above 1400 m in central and southwestern China, including parts of Guangxi, Hunan, and Jiangxi Provinces. The survival of Abies ziyuanensis is threatened by various factors, including climate change, deforestation, species invasion, and geographic isolation. These challenges lead to slow natural regeneration, difficulties in artificial propagation, and ongoing habitat fragmentation, which increases the trend toward habitat reduction and casts doubt on its future existence [14,15].

Research on the community attributes of Abies ziyuanensis began in the late 1970s, highlighting the fragile nature of its communities [15,16]. Current research on Abies ziyuanensis primarily focuses on the mechanisms of degradation and conservation strategies for the structural characteristics of its communities in specific habitats, growth dynamics, genetic diversity, and predictions of suitable habitats [15,16,17]. However, there is limited research on how Abies ziyuanensis communities respond to environmental changes and human activities, specifically in areas such as species trait diversity, variations in composition, and competition dynamics.

This research targets the endangered forest communities of Abies ziyuanensis situated within four distinct geographic regions: Erbaoding Mountain in Chengbu County, Shunhuang Mountain in Xinning County, Dayuan Farm in Yanling County, and Jingangshan in Ji’an City. The study thoroughly analyzes the differences in community dynamics between habitats affected by anthropogenic disturbance and those in an undisturbed state, with a focused examination on three pivotal questions: (1) Does the overall plant species composition of Abies ziyuanensis communities differ between undisturbed and anthropogenic disturbance? (2) Does the spatial competition of Abies ziyuanensis communities vary between undisturbed ones and those subjected to anthropogenic disturbances? (3) Does anthropogenic disturbance alter the growth environment and succession of Abies ziyuanensis communities? By exploring these questions, the goal is to provide empirical data and insights to guide the restoration of Abies ziyuanensis communities and contribute to conservation strategies for other endangered species.

2. Materials and Methods

2.1. Study Area

The study sites are located between Xinning–Chengbu–Ziyuan (110°28′24″ E—111°30′55″ E, 26°09′54″ N—26°58′13″ N) and Yanling–Jinggangshan (113°50′24″ E—114°30′55″ E, 26°30′54″ N—27°08′63″ N), closely aligned in latitude. The undisturbed Abies ziyuanensis communities in the subtropical mid-mountain regions of Erbaoding Mountain, Shunhuang Mountain, and Jingangshan Mountain are located in remote areas, requiring 4–5 h of trekking to reach. In contrast, the conservation communities of Abies ziyuanensis at Dayuan Farm and the lower regions of Erbaoding Mountain are situated near areas of human activity. Historically, these communities have experienced 2–3 anthropogenic disturbances per year, such as brush cutting, replanting, thinning, and occasional grazing. This setting provides an excellent opportunity to study the impacts of human activity and natural conditions on the ecological changes and adaptive mechanisms of Abies ziyuanensis habitats.

All sites are situated at altitudes ranging from 1400 to 1800 m, within a subtropical continental monsoon climate zone. Meteorological data sourced from the World Climate Database (https://worldclim.org (accessed on 10 October 2023)) indicate that the annual average temperatures in these regions range between 16 and 19 °C. The regions benefit from abundant sunlight, have significant evaporation rates, and receive annual precipitation of approximately 1500 to 2000 mm. The predominant soil type is dark earth, noted for its high organic matter content. The forests in the study area are primarily composed of evergreen broadleaf species that form the community structure. An overview of the research area and the distribution of study sites is illustrated in Figure 1.

2.2. Research Methods

2.2.1. Site Investigation and Data Processing

The study was conducted from August to December 2020. In our study, “undisturbed forests” are defined as Abies ziyuanensis forest communities where ecological processes are predominantly free from significant human interference, without systematic human activities such as logging, grazing, brush cutting, weeding, and fencing. Conversely, “disturbed forests” refer to Abies ziyuanensis forest communities affected by such systematic human activities. Based on management history records and evidence of human activities, and considering the community area and the number of Abies ziyuanensis individuals, we conducted a sampling survey of the communities. We investigated three disturbed forests and one undisturbed forest at Dayuan Farm, one disturbed and one undisturbed forest at Erbaoding Mountain, and one disturbed forest each at Shunhuang Mountain and Jingangshan Mountain, totaling eight Abies ziyuanensis communities (

Table 1. Community survey sample information of Abies ziyuanensis.

Sample Plot Number	Site	Longitude	Latitude	Elevation (m)	Slope (°)	Canopy Cover	Conditions of Communities
SHM1	Shunhuang Mountains	111°00.695′	26°22.266′	1713.03	38	0.70	Undisturbed
EDM2	Erbaoding Mountains	110°32.410′	26°16.602′	1783.12	24	0.80	Undisturbed
JGS3	Jinggang Mountains	114°06.689′	26°30.136′	1657.71	20	0.90	Undisturbed
DYF4	Dayuan Farm	114°01.793′	26°24.470′	1501.10	18	0.95	Undisturbed
DYF5	Dayuan Farm	114°01.727′	26°24.476′	1483.27	9	0.70	Disturbed
DYF6	Dayuan Farm	114°03.170′	26°26.240′	1467.00	18	0.75	Disturbed
DYF7	Dayuan Farm	114°02.824′	26°25.910′	1407.85	2	0.85	Disturbed
EDM8	Erbaoding Mountains	110°30.466′	26°18.233′	1490.34	35	0.65	Disturbed

). Each community had one plot measuring 20 m by 20 m. To mitigate edge effects, the plots were located at least 100 m away from the forest edge. In field ecology plot experiments, pseudoreplication is a common challenge [18]. To address this issue, we selected sites within the Abies ziyuanensis communities that were both densely populated and well-preserved for establishing plots. Due to the management objectives of the disturbed communities at Dayuan Farm and Erbaoding Mountain, which focus on the conservation of Abies ziyuanensis, the frequency and intensity of disturbance activities were relatively similar.

The plot survey was divided into three layers, each with specific contents and survey methods:

(1)

The canopy layer. We surveyed all trees and large shrubs with a diameter at breast height (DBH) of 4 cm or greater. We recorded factors including tree height, DBH, location, count, canopy width, and species attributes.

(2)

The understory layer. We established four 5 m × 5 m subplots at each plot corner. This layer included saplings taller than 10 cm and with DBH < 4 cm, as well as all shrubs not investigated in the canopy layer. The recorded factors were plant height, cover, count, and species attributes.

(3)

The ground layer. We placed five 1 m × 1 m subplots evenly at each plot’s corners and center. This layer targeted all herbaceous plants and tree seedlings less than 10 cm in height. The recorded factors included plant height, cover, count, and species attributes.

Measurements were facilitated by a tape measure for canopy width and shrub and herbaceous cover (line transect method), a diameter tape for DBH, a laser rangefinder for tree height, and Real-Time Kinematic (RTK) technology for plot and individual tree positioning. Additionally, ecological factors and coordinates such as altitude, aspect, slope, soil texture, and canopy closure were recorded for each site. In total, this setup resulted in 32 understory-layer subplots and 40 ground-layer subplots across all eight canopy layer plots. Post-survey, the species within the Abies ziyuanensis communities were categorized by family, genus, and species.

2.2.2. Analysis of Structural Characteristics of the Abies ziyuanensis Communities

The current statuses of the Abies ziyuanensis communities were delineated and summarized using community characteristic indices such as species size class structure, species importance values, and species diversity indices.

(1)

Diameter Class Structure Characteristics of Abies ziyuanensis Species

The diameter class structure is an important indicator reflecting the age structure and growth status of the forest, providing reliable data for studying forest succession trends and ecological balance [19,20,21,22]. Based on the survey data, this study utilized the upper limit exclusion method to categorize the diameter class structure of the Abies ziyuanensis communities into seven levels according to the following criteria: Level I saplings, with heights of <0.5 m; Level II young trees, with heights of 0.5–1.2 m and DBH < 2.5 cm; Level III small trees, with heights of >1.2 m and DBH of 2.5–5 cm; Level IV medium trees, with DBH of 5–10 cm; Level V large trees, with DBH of 10–25 cm; Level VI larger trees, with DBH of 25–45 cm; and Level VII largest trees, with DBH of >45 cm. The number of Abies ziyuanensis individuals in each diameter class was statistically analyzed for each plot. Subsequently, we analyzed the data using a Chi-square test to determine whether there were statistically significant differences in the diameter distribution between undisturbed and disturbed Abies ziyuanensis communities.

(2)

Species Importance Value Index

The species importance value index reflects, to some extent, the role and value of a species within a community and is commonly used to compare the significance of a species across different communities. The importance value index (IV) for all recorded species within a population is calculated using relative density (RD), relative cover (RC), relative frequency (RF), and relative prominence (RP) [23,24,25]. The formula for calculation is as follows:

$${IV}_1=(RD+RP+RF)/3$$

(1)

$${IV}_2=(RD+RC+RF)/3$$

(2)

$$RD=\left(\frac{D_i}{{\sum }_{i=1}^n{D}_i}\right)\times 100\%$$

(3)

$$RP=\left(\frac{P_i}{{\sum }_{i=1}^n{P}_i}\right)\times 100\%$$

(4)

$$RF=\left(\frac{F_i}{{\sum }_{i=1}^n{F}_i}\right)\times 100\%$$

(5)

$$RC=\left(\frac{C_i}{{\sum }_{i=1}^n{F}_i}\right)\times 100\%$$

(6)

In these formulas, IV₁ represents the importance value for tree species; IV₂ denotes the importance value for shrubs or herbaceous species; D_i is the ratio of the number of individuals of species i to the total number of individuals across all species; P_i is the ratio of the cross-sectional area at breast height of species i to the total cross-sectional area at breast height of all species; F_i is the ratio of the frequency of species i to the total frequency of all species; and C_i is the ratio of the total canopy cover of all individuals of species i to the total canopy cover area of all species.

(3)

Species Diversity Characteristic Indices

Employing the Shannon Diversity Index (SHANNON) [26], the Simpson Diversity Index (SIMPSON) [27], and Pielou’s Evenness Index (PIELOU) [28], we statistically analyzed and compared the biodiversity characteristics of each study site. The formulas for calculation were as follows:

$$\mathrm{SHANNON}=-{\sum }_{i=1}^s{P}_i\times Ln\left(P_i\right)$$

(7)

$$\mathrm{SIMPSON}=1-{\sum }_{i=1}^s{\left(P_i\right)}^2$$

(8)

$$\mathrm{P}\mathrm{I}\mathrm{E}\mathrm{L}\mathrm{O}\mathrm{U}=SW/ln\left(S\right)$$

(9)

where $P_i$ = $n_i$/N, ${\mathrm{n}}_{\mathrm{i}}$ denotes the number of individuals of the ith species within a plot, N represents the total number of individuals of all species within the plot, and S signifies the total number of species present in the plot.

2.2.3. Analysis of Species Composition Differences in Communities

To quantitatively assess the differences in species composition between disturbed and undisturbed forests, this study employed Permutational Multivariate Analysis of Variance (PERMANOVA) based on Bray–Curtis distance metrics [29]. This process was facilitated through the Adonis function, with the normalized relative density (RD) and relative cover (RC) of each species within each plot as the input. Based on the p-value and R from the PERMANOVA analysis, we drew the following conclusions:

(1)

Extreme difference: 0.50 $<$ R $\le$ 1.00 and p-value ≤ 0.05, indicating a vast difference in species composition between the two forest conditions;

(2)

Significant difference: $R \le$ 0.50 and p-value ≤ 0.05, showing a clear distinction between the two forest conditions;

(3)

Moderate difference: $R \le$ 0.50 and $0.05<$ p-value ≤ 0.075, where the two forest conditions partially overlap but are distinct to a certain degree;

(4)

Slight difference: $0.075<$ p-value ≤ 0.10, where, although the forest conditions are similar, some differences still exist;

(5)

Almost no difference: p-value $>0.10,$ where the two forest conditions can be considered extremely similar.

We used the similarity percentage analysis (SIMPER) method to quantify each species’ contribution to the inter-group Bray–Curtis distances based on the results of the PERMANOVA analysis computed through the Adonis function. Finally, to better visualize the differences in species composition between samples, we performed principal coordinate analysis (PCoA) and visualized the results.

All the aforementioned statistical analyses were conducted using R 4.2.0 software, utilizing the Vegan 2.5–4 package developed by Oksanen et al. [30].

2.2.4. Analysis of Individual Competition Differences in the Abies ziyuanensis Communities

The Hegyi competition index (CI), based on the spatial distances between trees, offers a nuanced reflection of the competitive pressure exerted on individual trees [31]. A higher competition index indicates greater competitive stress on a tree [32,33,34,35]. Studies have shown a negative correlation between competition indices and DBH, leading this research to employ the differential DBH relationships between target and competing trees to weight the competition index of the target tree [36,37]. The competitive unit is determined using Voronoi diagrams. A Voronoi diagram is defined within a two-dimensional Euclidean plane comprising a set of points X, where X = {X₁, X₂, …, X_i, X_j, …}, with 3 ≤ n < +∞ (i ≠ j, i, j∈n = {1, 2, …, n}), and each X_i (i = 1, 2, …, n) represents one of n points on the plane.

$$P_{\mathrm{n}} \left(X_i\right)={\bigcup }_{j\ne i}\left\{X|d\left(X_1{,X}_i\right)<d\left(X_1{,X}_j\right)\right\}$$

(10)

In the formula, $d\left(X_1{,X}_i\right)$ represents the distance from any given point X on the plane to $X_i$, and $d\left(X_1{,X}_i\right)$ signifies the distance from any given point X on the plane to $X_j$. $P_{\mathrm{n}}\left(X_i\right)$ is defined as the ratio of the distance from all points within a certain weighted Voronoi polygon to the centroid of that polygon versus the distance to the centroid of the adjacent polygon, which is less than the ratio of the weights of the two polygon centroids. Utilizing the R programming language, a Thiessen polygon program was developed to generate a Voronoi single-tree competition heatmap, and the weighted competition index, denoted as ${WCI}_i$, was calculated. The calculation formula was as follows:

$${CI}_i={\sum }_{j=1}^{n_i}\frac{D_j}{Di} \times \frac{1}{L_{ij}}$$

(11)

$${WCI}_i=\left\{\begin{array}{c}{CI}_i D_i\le D_j\\ {CI}_i+\raisebox{1ex}{${(D}_i-D_j)$}\!\left/ \!\raisebox{-1ex}{$2$}\right.\times 0.1 D_i> D_j\cup D_i<12 \\ {CI}_i+\raisebox{1ex}{${(D}_i-D_j)$}\!\left/ \!\raisebox{-1ex}{$4$}\right.\times 0.1 D_i> D_j\cup D_i\ge 12\end{array}\right.$$

(12)

wherein $n_i$ is the number of competing trees within the competition unit of the ith Abies ziyuanensis (i = 1, 2, …, N; N is the number of Abies ziyuanensis individuals); $D_i \mathrm{a}\mathrm{n}\mathrm{d} D_j$ are the diameters at breast height (DBH) of the target tree i and the surrounding competing tree j, respectively; and $L_{ij}$ is the distance between Abies ziyuanensis i and the surrounding competing tree j.

3. Results

3.1. Description of Plant Genus Information

Across all surveyed plots, a total of 71 species were recorded, encompassing 60 genera and 39 families. These included 1 species of fern and 70 species of seed plants, representing 59 genera and 38 families. Descriptions of the plant genus information are shown in

Table 2. Description of plant genus information in the Abies ziyuanensis communities.

Conditions of Communities	Vegetation Layer	Number of Family	Number of Genus	Number of Species
Undisturbed forests	Canopy	19	25	31
	Understory	14	16	16
	Ground	9	9	9
	Total	32	41	48
Disturbed forests	Canopy	14	17	17
	Understory	15	16	16
	Ground	11	14	14
	Total	29	39	40
All total		39	60	71

. The specific species information of the sample plot is presented in Table S1.

3.2. Structural Characteristics and Species Diversity

In undisturbed forests, there are significantly more trees compared to in disturbed forests. Statistical analysis (

Table 3. The number of Abies ziyuanensis trees with different DBH class structures under different community conditions.

Conditions of Communities	Level Ⅰ	Level Ⅱ	Level Ⅲ	Level Ⅳ	Level V	Level Ⅵ	Level Ⅶ
Undisturbed forests	0	0	0	5	9	1	4
Disturbed forests	603	1	2	19	23	12	1
Total	603	1	2	24	32	13	5

Note: Level I saplings, with heights of <0.5 m; Level II young trees, with heights of 0.5–1.2 m and DBH < 2.5 cm; Level III small trees, with heights of >1.2 m and DBH of 2.5–5 cm; Level IV medium trees, with DBH of 5–10 cm; Level V large trees, with DBH of 10–25 cm; Level VI larger trees, with DBH of 25–45 cm; Level VII largest trees, with DBH of >45 cm.

) revealed a decline in the DBH class structure for Abies ziyuanensis in undisturbed forests, characterized by a notable scarcity of saplings in the I and II diameter classes and a predominance of larger trees in the V diameter class. This trend contrasts with areas subjected to anthropogenic disturbance interventions, where there are noticeable increases in saplings of the I diameter class. However, this does not necessarily indicate successful natural regeneration, as larger specimens predominantly fall into the IV diameter class and above, and saplings in the II and III diameter classes remain scarce. Meanwhile, our calculated chi-square statistic is 232.65, which far exceeds the critical value at the 95% confidence level (Figure S1). This strong statistical evidence strongly indicates a significant difference in diameter distribution between undisturbed and disturbed forest conditions.

From Figure 2, it is evident that within the canopy layers of the Abies ziyuanensis communities affected by anthropogenic disturbance, Phyllostachys edulis (Phyllostachys edulis (Carrière) J. Houzeau) stands out with an importance value far exceeding that of other plant species, reaching 37%. Following it is Abies ziyuanensis, with an importance value of 27%. In contrast, within the canopy layers of undisturbed Abies ziyuanensis communities, Abies ziyuanensis holds the highest importance value at 19%, with other tree species exhibiting a more even distribution of importance values compared to the communities affected by anthropogenic disturbance. Regarding the understory layer of the community, under undisturbed conditions, Fargesia spathacea (Fargesia spathacea Franch.) achieves an importance value of 64% and a relative frequency of 27%, dominating the understory layer across almost all plots, which may limit space for other species and complicate the self-renewal of Abies ziyuanensis. However, under anthropogenic disturbance, the distribution of importance values among understory species becomes more even, with bamboo species no longer dominating. In the ground layer of the Abies ziyuanensis communities subjected to anthropogenic intervention, Carex (Carex L.) has the highest importance value at 28%, followed by Lophatherum gracile (Lophatherum gracile Brongn.) at 26%. The ground layers in undisturbed forest communities are primarily composed of Carex L. and Ophiopogon japonicus (Ophiopogon japonicus (L. f.) Ker Gawl). Compared to the undisturbed Abies ziyuanensis communities, anthropogenic disturbance improves the living space in the Abies ziyuanensis community’s understory layer, with visible sprouting plant saplings and a more even distribution of importance values among the understory layer.

The results from the Shannon diversity index (SHANNON) and Simpson diversity index (SIMPSO${\displaystyle \underset{﹉}{\mathrm{N}}}$) indicated significant differences across various vegetative layers and regeneration methods within the plots for the Abies ziyuanensis communities, as determined by t-tests (Figure 3). In undisturbed forests, the canopy layer exhibits the highest levels of species diversity and evenness (SHANNON index: 2.97, SIMPSO${\displaystyle \underset{﹉}{\mathrm{N}}}$ index: 0.93), which declines substantially in the understory and is moderately reduced in the ground layer. This gradient reflects a more uniform species distribution in the canopy compared to other layers. In contrast, disturbed forests show a different pattern: the canopy layer has lower diversity (SHANNON index: 1.68) and similar evenness (SHANNON index: 0.71) compared to undisturbed forests, while the understory and ground layers show increased diversity (SHANNON indices: 2.36 and 1.68, respectively). These layers in disturbed forests exhibit higher evenness (SHANNON indices: 0.33 and 0.17, respectively), suggesting that disturbances might enhance species distribution uniformity across layers.

3.3. Species Composition Differences in Abies ziyuanensis Communities

The PCoA plots (Figure 4A–D) distinctly showcase the differences in species composition between forests affected by anthropogenic disturbances and undisturbed forests across various vegetative strata of the Abies ziyuanensis communities. The PERMANOVA analyses revealed significant differences between disturbed and undisturbed forests in the entire forest, canopy layer, and understory layer. However, the ground layer showed only a moderate difference. Bamboo species contributed significantly to the compositional differences.

3.4. Single-Tree Competition in Abies ziyuanensis Communities

As presented in

Table 4. The competition index of forest trees in the communities under different conditions.

Community Conditions	Sample Plot Number	DBH of Abies ziyuanensis (cm)	DBH of all Tree (cm)	Number of Trees	Number of Abies ziyuanensis	$\mathit{W}\overline{\mathit{C}\mathit{I}}$ of Abies ziyuanensis	$\mathit{W}\overline{\mathit{C}\mathit{I}}$ of all Tree
Undisturbed forests	SHM1	44.8	18.3	53	4	5.33	16.12
	EDM2	8.2	14.5	32	2	9.40	8.87
	JGS3	20.0	9.1	69	1	4.05	14.77
	DYF4	19.0	12.1	38	12	5.38	9.21
Mean		23.00	13.5	48	5	6.04	12.24
Disturbed forests	DYF5	33.5	23.1	17	7	2.76	5.26
	DYF6	11.2	12.5	58	30	21.10	17.68
	DYF7	10.9	10.9	15	14	3.57	3.42
	EDM8	27.3	22.5	27	7	3.61	3.35
Mean		20.7	17.2	29	15	7.76	7.43

, irrespective of being located in undisturbed or disturbed forests, the competitive pressure faced by Abies ziyuanensis demonstrated a negative correlation with their average DBH. Notably, juvenile trees with smaller diameters (DBH of Abies ziyuanensis < 20 cm) experienced competition that exceeded the average intensity, demanding focused conservation efforts on communities with scarce and diminutive Abies ziyuanensis populations. For example, in disturbed forest plots such as DYF5 and DYF6, juvenile Abies ziyuanensis recorded $W\overline{CI}$ values of 2.76 and 21.10, respectively, significantly higher than the $W\overline{CI}$ of all trees in those plots (5.26 and 17.68, respectively). In contrast, mature Abies ziyuanensis specimens (DBH greater than 20 cm) exhibited lower WCIs than other trees within their communities, suggesting that their competition is relatively moderate and, thus, conducive to survival. Meanwhile, the $W\overline{CI}$ for Abies ziyuanensis within undisturbed forests showed less fluctuation, whereas in disturbed forests, it was higher, and more variable $W\overline{CI}$s indicated that anthropogenic disturbance might intensify changes in competitive ecological patterns.

A comparative analysis between the $\overline{CI}$ and $W\overline{CI}$ values across various communities elucidates that the WCI, considering the differential in DBH, provides a more refined adjustment for the competitive interactions between smaller and larger trees (Figure 5). In communities where Abies ziyuanensis had relatively lower DBH, such as in EDM2 and DYF6, the $W\overline{CI}$ values were significantly higher than the $\overline{CI}$, signifying that in disturbed environments, competition among larger trees and their diminutive counterparts is exacerbated—a dynamic traditionally underrepresented by the CI.

As illustrated in Figure 6, within the undisturbed forest sites (A, B, C, D), the distribution of the WCI for Abies ziyuanensis was relatively uniform, which may suggest that the intensity of competition is relatively stable in undisturbed environments. In disturbed forests (E, F, G, H), the WCI values for Abies ziyuanensis were more variable, with multiple notable areas of high competitive intensity in panels E and H. This indicates that in forest environments affected by anthropogenic disturbances, Abies ziyuanensis may face increased competitive pressure due to changes in the environment following the disturbance. Additionally, in some specific plots, such as D (undisturbed forest) and G (disturbed forest), Abies ziyuanensis appeared to experience little to no competition. This could be due to a lower number of canopy layer species within the community, abundant habitat resources, or other environmental factors that have a positive effect on the arboreal species of the entire community.

4. Discussion

4.1. Species Diversity of Abies ziyuanensis Communities

An examination of diversity and evenness indices reveals pronounced differences in the species diversity and structure between the Abies ziyuanensis mid-mountain communities in disturbed forests and undisturbed forests. The canopy layers of disturbed forests possessed higher species evenness and diversity. Concurrently, anthropogenic disturbances enhanced the species evenness and diversity in the understory and ground layers of the Abies ziyuanensis communities. In undisturbed forests, the predominance of Fargesia spathacea, which had an importance value of 64%, constrained the environmental conditions for the growth of the understory vegetation, thereby reducing species diversity. Anthropogenic disturbances create more growth space in the understory and ground layers, as reflected in changes in the abundance and frequency of dominant species, such as bamboo species. This effectively reduces the impacts of ecological niche changes, thereby incrementally enhancing the diversity and evenness of shrubs and herbaceous species within these layers. As highlighted by scholars [9,12,38,39], anthropogenic interventions typically result in the dominance of species with high adaptability, sidelining others, yet also improving survival and dissemination conditions for certain species—a conclusion corroborated by our research. Therefore, for individual Abies ziyuanensis, anthropogenic disturbances are likely to ameliorate conditions conducive to self-renewal, such as soil moisture and light availability.

4.2. Spatial Competition among Abies ziyuanensis Trees

Competition is a critical factor affecting the growth of individual trees [37,40,41]. In this study, we compared the WCI of Abies ziyuanensis within undisturbed and disturbed forest communities. Unlike the basic competition index (CI), WCI offers a nuanced depiction of competitive interactions between smaller and larger trees. Our analysis revealed a negative correlation between the competition pressure faced by Abies ziyuanensis and the average DBH across all populations, aligning with conclusions from previous research [36,37]. The significant impact of spatial competition among trees on individual growth conditions, particularly in terms of resource allocation and niche occupation [42], is underscored. In undisturbed forest communities, larger Abies ziyuanensis individuals exhibit relatively lower competition indices, possibly due to their more effective utilization of resources such as light, moisture, and nutrients [42]. These individuals, benefiting from early growth advantages, have already occupied favorable niches, thereby reducing direct competition with surrounding vegetation. Conversely, smaller individual trees face greater competition pressure due to less favorable positions or slower growth rates, making it challenging for them to secure sufficient resources [36]. Anthropogenic interventions, by altering forest structure and resource distribution, lead to changes in the ecological competition pattern among trees. For instance, disturbances may result in the reallocation of resources, rendering them more abundant in certain areas and thus reducing competition pressure [9,12,38,39]. Therefore, in formulating management and conservation strategies for endangered species, it is imperative for managers to consider inter-tree competition.

4.3. Growth Environment of Abies ziyuanensis Communities

In our surveyed plots, the underbrush in undisturbed forest communities was predominantly composed of bamboo, with a relative frequency of 74.48%, leading to a deteriorated growth environment for the understory vegetation in naturally regenerating Abies ziyuanensis communities and exacerbating the challenges of regeneration and propagation for Abies ziyuanensis. A prominent indicator of changes in vegetation ecosystem functionality and diversity is the expansion of shrubbery [43,44], potentially driven by climate change [45]. Our findings, to some extent, corroborate this habitat shift phenomenon, explaining the current state of fragmented and isolated habitats faced by Abies ziyuanensis communities. However, the study observed a sharp decline in the quantity and coverage of the competitive species Fargesia spathacea Franch within anthropogenically disturbed communities, alleviating the self-renewal difficulties of Abies ziyuanensis and suggesting that, to a certain degree, anthropogenic intervention can be beneficial [9,12,38,39]. Yet, in the long term, this may lead to further changes in ecological niches, such as the increasing dominance of Phyllostachys edulis in Abies ziyuanensis forest communities, which intensifies competition within the arboreal layer and, to some extent, is detrimental to the natural succession and healthy development of the ecosystem. In discussing the capacity of Abies ziyuanensis communities to adapt to changes in their growth environments, we must consider their vulnerability and potential adaptive strategies. In the context of global warming, Abies ziyuanensis communities need not only to mitigate the direct impacts of ecological niche changes, but also to implement a series of adaptive management measures, including breeding selection and habitat management, to enhance their adaptability to climate change, ensuring the long-term stability of biodiversity and ecosystems.

4.4. Disturbance Effects on Succession of Abies ziyuanensis Communities

The results of the Chi-square test indicate a significant difference in the diameter distribution of Abies ziyuanensis between disturbed and undisturbed forests. In undisturbed conditions, the absence of young Abies ziyuanensis in size classes I–III suggests that forests may become dominated by climax communities of late-successional species [46,47]. As the forest ages, the reduction in community resources and larger canopy width may negatively impact the reproduction of intolerant species [48], with the understory and ground layers gradually being dominated by slower-growing, shade-tolerant species. This natural extinction of Abies ziyuanensis could occur in the foreseeable future. Therefore, such undisturbed natural succession might be unfavorable for the development of the endangered Abies ziyuanensis population. In disturbed forests, the diameter distribution of Abies ziyuanensis exhibits a relatively healthy normal distribution, with young Abies ziyuanensis in size classes I–III visible in the understory. Anthropogenic disturbances lead to increased light and soil nutrient availability within the community [47], enhancing the diversity of understory species and improving the self-renewal conditions for Abies ziyuanensis. This mitigates the trend of Abies ziyuanensis being replaced by climax species to some extent. However, in our study, the importance value of bamboo in the plots now exceeds that of Abies ziyuanensis, indicating that in the later stages of disturbance, species initially thriving on ample light and nutrients may shift their competitive advantage away from Abies ziyuanensis. Consequently, forest management may be necessary, which is crucial for maintaining the dominance of target tree species or restoring forest habitats [49].

5. Conclusions

Significant differences have been identified between endangered Abies ziyuanensis communities subjected to anthropogenic disturbances and those that remain undisturbed, particularly in terms of community composition, species diversity, and spatial competition for tree growth. The undisturbed forest communities, particularly within the canopy layer, exhibit higher species diversity and face less individual competition despite poor understory growth conditions. Disturbed communities, on the other hand, boast a higher diversity of understory and ground layer plants, and Abies ziyuanensis in these areas displays moderate natural regeneration capabilities. Additionally, Abies ziyuanensis individuals in undisturbed forest communities are on a declining trajectory. While anthropogenic disturbances may slow these successional processes, they could potentially reduce the inherent advantages of Abies ziyuanensis as a target species. Given these observations, carefully crafted anthropogenic disturbances are deemed necessary. However, such interventions require vigilant oversight and shrewd management, which are crucial for the conservation of forest plant communities containing specific endangered species.