Patterns of Elevation Gradients in Plant Composition and Diversity of Pinus pumila Communities in Zalinkur Mountain

Abstract

(1) Studying changes in plant composition and diversity of Pinus pumila communities along the elevation is significant for biodiversity conservation; (2) In this study, we systematically analysed Pinus pumila communities’ species composition and diversity characteristics on different altitudinal gradients in the Zalinkur Mountains, aiming to reveal their distribution patterns along the altitudinal gradient. (3) ① 37 plant species were recorded, including 9 trees, 12 shrubs, and 16 herbs. The species richness was in the order of herbs > shrubs > trees. ② The α-diversity of the tree layer decreased with elevation. The diversity of the shrub and herb layers decreased and then increased with elevation; (4) Elevation is an important ecological factor affecting the plant diversity of Pinus pumila communities in Zalinkur Mountain, playing a dominant role in the formation and maintenance of plant diversity on Zalinkur Mountain.

1. Introduction

Plant species diversity reflects the abundance and uniformity of species distribution in a given area and is an important indicator of community structure, developmental stage, stability, and habitat differences [1,2]. The study of plant diversity helps to understand species composition, community dynamics, and succession trends and is of key significance for maintaining ecological balance and conserving biological resources [3,4]. Its distribution pattern is influenced by multiple factors such as stand density, latitude and longitude, elevation, and anthropogenic activities, among which elevation, as a key factor, significantly affects the diversity pattern through vertical differentiation of hydrothermal conditions [5,6,7,8,9,10]. Changes in plant diversity along elevation reflect the results of long-term adaptation of species to the environment and also provide a window to reveal the response of communities to climate change [11,12]. Different life-types of plants respond differently to elevation changes due to differences in growth characteristics and resource utilization strategies, which provide important clues for understanding the composition and function of mountain ecosystems [13,14]. Therefore, the study of plant diversity along the elevation gradient is of great value in revealing the distribution pattern and mechanism of species [15,16,17].

Pinus pumila is an evergreen shrubby conifer of the genus Pinus, family Pinaceae. Widely distributed in China’s Daxinganling, Xiaoxinganling, and Changbai Mountain and other high-elevation areas, it is the key species of the forest-tundra transition zone, with cold-resistant and infertile characteristics, often forming pure forests or mixing with shrubs and herbs [18,19,20]. This community not only has the important functions of soil conservation and water conservation but also provides habitat for many rare plants and animals, with remarkable ecological value. Zalinkur Mountain is located in the northern section of Daxinganling, Tahe County. The elevation gradients are obvious, and the Pinus pumila community is well preserved; it is an ideal place to study the diversity of elevation patterns. Existing studies have focused on the seed kernel characteristics, growth physiology, or biomass of Pinus pumila sylvestris [21,22,23] or analyzed the elevation pattern of diversity at the level of the whole community [24], but they lacked the ability to explore the differential response of different layers (trees, shrubs, herbs) from the perspective of functional grouping of life types, which makes it difficult to reveal the mechanism of environmental filtering and its impact on the construction of the community. The results of regional studies are also difficult to generalize directly to this region due to geographic and climatic differences.

Therefore, in this study, we set up sample plots at 900 m, 1100 m, and 1300 m to compare the changes in plant composition and diversity along elevation for the first time based on the stratification of life types, with the aim of breaking through the limitations of the previous overall study, revealing the differential response of different functional groups to elevation, providing a new basis for understanding the environmental adaptation mechanism of cold-temperate alpine communities, and enriching the regional case of the elevation gradient theory of mountain plant diversity. The specific objectives of this study are as follows: (1) to reveal the changes in species composition of the Pinus pumila community as a whole and of different life types (trees, shrubs, and herbs) along the elevation gradient; (2) to analyze the α-diversity indices (including the Margalef richness index, Shannon-Wiener index, Simpson index, and Pielou evenness index) of the different plant life types along the elevation gradient. evenness index) distribution patterns along the elevation gradient. Based on the above objectives, we proposed the following research hypotheses: (1) Elevation differentially filters the species composition of different life types through environmental filtering, and herbaceous plants consistently maintain the highest species richness due to their greater ecotope widths and microenvironmental adaptations; (2) the diversity of the tree layer shows a monotonically decreasing trend with elevation, while the diversity of the shrub and herb layers shows a nonlinear response, and the diversity indices are sensitive to changes in elevation. The sensitivity of each diversity index to changes in elevation is significantly different; (3) elevation is the dominant factor driving changes in community structure and species diversity of Pinus pumila sylvestris.

2. Materials and Methods

2.1. Overview of the Study Area

Zalinkur Mountain, which is an Oroqen language, which means “the mountain of Pinus pumila”, is located in Heilongjiang province, Daxinganling region, Tahe county territory, in the northern foothills of the Yilhuli mountain range, and is located at 52.59233749° N, 123.50433499° E. Its elevation is 1352.6 m, with a total area of 22,027 hm². As shown in Figure 1, the map of Heilongjiang Province and distribution of the study area and sample plots. Forest coverage of up to 97%. It is a part of Zalinkur National Forest Park. Zalinkur Mountain has a cold-temperate continental climate. The average annual temperature is between about −2 °C and −4 °C. The average temperature in the hottest month (July) is around 20 °C. The average temperature in the coldest month (February) ranges from −20 °C to −25 °C, with extremely low temperatures occasionally dropping to −45.8 °C. Temperatures decrease as the terrain increases. Summers, on the other hand, are relatively warm, but precipitation is mainly concentrated in July and August, with an annual precipitation of about 463.2 millimetres [25]. The relative humidity in the Zalinkur Mountain is affected by seasonal and weather changes. Generally, the relative humidity is low in winter, typically ranging from 30% to 50%, while it is high in summer, usually ranging from 60% to 80%. In addition, the region has an average annual frost-free period of 98 days, and the number of hours of sunshine ranges from 2201 to 2865 h, indicating adequate sunshine conditions. The most distributed area in the Zalinkur Mountain is characterised by greyish and brown yellows, with the soil-forming parent material mainly derived from rock weathering and residual deposits. Due to its location in the north temperate zone, the climate of the Zalin Kul Mountains is strongly influenced by the alternation of continental and oceanic high and low pressures, as well as monsoon winds, resulting in noticeable seasonal changes. The vegetation community is a “continuous transition dominated by local node differentiation” with vertical distribution and no obvious band boundary and can be divided into three transition sections: 800–1000 m low-elevation mixed coniferous and broad forest transition section; 1000–1200 m in the middle-elevation Pinus pumila—coniferous forest mixed transition section; 1200–1352.6 m high-elevation Pinus pumila scrub transition section. Transition section. In summary, the vegetation of Zalinkur Mountain is not “all short-leafed pine community”, but with the elevation of the “mixed coniferous and broad-brush forests → Pinus pumila—coniferous forests mixed → pure Pinus pumila scrub” transition, the transition is “mixed coniferous and broad-brush forests → Pinus pumila—coniferous forests mixed → pure Pinus pumila scrub”. As shown in Figure 2, the habitat photographs of the Zalinkur Mountain Pinus pumila community at the three studied elevation gradients.

2.2. Research Methods

2.2.1. Sample Site Setup and Survey

A typical sample method was used in May 2024, with reference to the CTFS (Center for Tropical Forest Science) technical specifications for large sample plot construction and monitoring (Condit, 1998) and the unified standards of the China Forest Biodiversity Monitoring Network (CForBio) [26]. In Zalinkur Mountain, we selected the area with a complete community structure and no human interference (e.g., logging, grazing) within the elevation range of 800~1352 m and set up three fixed sample plots of Pinus pumila sylvestris with an area of 1 hm² (100 × 100 m) at three different elevations, namely, 900 m, 1100 m, and 1300 m. The sample plots were all located at the southwest corner of the mountain, with the southwest corner as the coordinates. Each sample plot was divided into 25 20 m × 20 m sample squares using a total station, and then each 20 m × 20 m sample square was further subdivided into 16 5 m × 5 m small sample squares. In the calibrated sample plots, all individual woody plants with a diameter at breast height (DBH) of ≥1 cm were recorded. Their main trunks and branches were painted and labelled, and the species name, diameter at breast height (DBH), coordinate position, and growth condition of the plants were recorded. Five 5 m × 5 m shrub sample plots were randomly set up in each 1 ha² large sample plot. Herbaceous sample squares of 1 m × 1 m were set up at four corners within each shrub sample square. The tree, shrub, and herb layers, as well as the interlayer plants, were investigated sequentially. The species name, diameter at breast height, tree height, and crown width were recorded in the tree layer. In contrast, the species name, density, average height, and average cover were recorded in both the shrub layer and the herb layer. Because asexually propagated herbaceous plants often form “clonal segments” through tillers, stolons, rhizomes, or tubers, the traditional definition of an individual based on a “separate plant of seed origin” no longer applies. In this study, the following definitions were used: “independent tiller clumps” for tillers, “independent rooted plants at stolon nodes” for stolons/rootstocks, and “individual tubers/bulbs” for tubers/bulbs. For stoloniferous/rhizomatous grasses, “individual rooted plants formed on stolon nodes” is used as a single functional individual; for tuberous/bulbous herbs, “individual tubers/bulbs and their sprouted above-ground parts” are used as a single functional individual. For species with doubtful morphological characteristics or difficult to recognise on-site, representative plant specimens were collected, pressed, dried, and initially identified according to standard specimen production methods, and confirmed with reference to the Flora of China and relevant local floras. The voucher specimens were kept in the herbarium for review and subsequent research. The α-diversity (Margalef richness index, Simpson index, Shannon-Wiener index, and Pielou evenness index) was selected to measure plant species diversity. The importance values of different life types and α-diversity indices were calculated using the method of Hao Lei et al. [27].

2.2.2. Calculation of Importance Value:

Plant dominance was indexed using the Importance Value (IV).

For the tree-layer, IV = (Relative Density + Relative Frequency + Relative Significance)/3.

IV = (Relative Density + Relative Frequency + Relative Coverage)/3 for the shrub and herb layers.

In the formula: Relative Density = (Density of a particular plant species/Total Density of all plant species) × 100%; Relative frequency = (frequency of a specific plant/total frequency of all plant species) × 100%; Relative significance = (sum of the cross-sectional area at breast height of a specific plant/sum of the cross-sectional area at breast height of all plant individuals in the sample) × 100%; Relative cover = (cover of a specific plant/sum of the cover of all plant species) × 100%.

2.2.3. Alpha Diversity Index (ADI)

Margalef richness index (MDI):

R = (S − 1)/lnN

(1)

Simpson index:

$$D=1-{\displaystyle \sum }_{i=1}^s{P}_i^2$$

(2)

Shannon-Wiener index:

$$H^{\prime }=-{\displaystyle \sum }_{i=1}^s{p}_i\cdot ln{p}_i$$

(3)

Pielou evenness index:

$$J_{SW}={\displaystyle \frac{H^{\prime }}{lnS}}$$

(4)

where S is the total number of species in the sample plot, H′ is the Shanno-Wiener diversity index, N is the total number of individuals of all species in the community, and p_i is the ratio of the number of individuals of the ith species to the total number of individuals of plants in the community.

2.2.4. Data Processing

Excel 2019 software was used for data statistics, and R 4.4.3 software was used for Kruskal-Wallis nonparametric tests with Dunn’s test for post hoc tests (Bonferroni correction); one-way analysis of variance (ANOVA) was used, and post hoc tests were used to analyze the differences with Tukey HSD multiple comparisons using a Robust Linear Regression Robust Linear Regression combined with HC3 Heteroscedasticity-Consistent Standard Errors was used to analyze the correlation between diversity indices and elevation gradients. The results were plotted using Origin 2024 software.

3. Results

3.1. Characteristics of the Composition of Pinus pumila Communities at Different Elevations

The plant species richness of Zalinkur Mountain is low, with 37 species recorded in 23 families and 32 genera across the three fixed sample plots of the Pinus pumila communities. Among them are tree species totalling 4 families, 7 genera, and 9 species. Specifically, this includes 1 family, 2 genera, and 3 species of gymnosperms and 3 families, 5 genera, and 6 species of angiosperms. Shrubs comprising a total of 6 families, 9 genera, and 12 species. Specifically, there are 1 family, 1 genus, and 1 species of gymnosperms, and 5 families, 8 genera, and 11 species of angiosperms. Herbaceous plants total 13 families, 16 genera, and 16 species, of which 1 family, 1 genus, and 1 species are ferns and 12 families, 15 genera, and 15 species are angiosperms. In terms of family statistics, Ericaceae (5 genera and 7 species), Rosaceae (3 genera and 4 species), and Betulaceae (2 genera and 4 species) occupied a larger proportion in the study area, accounting for 13.04%, 31.25%, and 40.54% of the total families, genera, and species, respectively. According to the elevation distribution characteristics, the 900 m Pinus pumila community sample site had a total of 13 families, 19 genera, and 22 species of plants (Figure 3a); the 1100 m Pinus pumila community sample site had a total of 7 families, 11 genera, and 15 species of plants (Figure 3b); and the 1300 m Pinus pumila community had 11 families, 16 genera, and 18 species of plants (Figure 3c).

The total species richness of the community exhibited a “V-shape” pattern, decreasing and then increasing with elevation. The species richness of different life forms also differed significantly among elevations, with the species richness of trees gradually decreasing with elevation and the species richness of shrubs decreasing slightly and then stabilising with elevation. The species richness of herbs exhibits a strong “V-shaped” pattern, characterised by a significant decrease followed by a significant increase with elevation. The total species richness showed that herbs > shrubs > trees.

3.2. Tree Plants

3.2.1. Importance Value of Tree Plants

Importance value reflects the relative importance of a species in the community and its relationship with the environment. It can be used to measure the dominant position of plants in the community. The importance values of tree plants in the Pinus pumila community on Zalinkur Mountain varied significantly with the elevation gradient. There were 17 importance values for 9 species of tree plants in the 3 fixed sample plots of the Pinus pumila community (Figure 4). There were differences in the importance values of tree plants, with the variation in the importance values ranging from 0.002 to 0.913, and the minimum importance value of the Salix raddeana was 0.002 in the sample site of Pinus pumila community No. 2 at 1100 m; the maximum importance value of the Pinus pumila was 0.913 in the sample site of Pinus pumila community No. 3 at 1300 m. Pinus pumila was relatively dominant in the community across the three elevation gradients, and its importance value increased gradually with increasing elevation. At 900 m, it was accompanied by the dominant tree species, Larix gmelinii var. principis-rupprechtii, and the secondary tree species, Betula platyphylla. As the elevation increased to 1100 m, the dominance of Larix gmelinii var. principis-rupprechtii and Betula platyphylla was gradually replaced by Pinus pumila and Alnus mandshurica. Until 1300 m, it evolved into a single dominant species community of Pinus pumila. Pinaceae had high importance values at the sample site, whereas other occasional and associated species had relatively low importance values.

3.2.2. Changes in Tree Alpha Diversity Indices Across the Elevation Gradient

Differences in the alpha diversity of the tree layer within the Pinus pumila fixed sample plots along the elevation gradients were observed by visual assessment from the Margalef richness, Simpson, Shannon-Winner, and Pielou evenness indices, which declined with the increase in elevation (Figure 5). The maximum values of the Margalef richness, Simpson, Shannon-Winner, and Pielou evenness indices were 0.94, 0.70, 1.40, and 0.67, respectively, in the Pinus pumila community sample site at an elevation of 900 m. The Margalef richness, Simpson, and Pielou evenness indices were 0.94, 0.70, 1.40, and 0.67 in the Pinus pumila community sample site at an elevation of 1300 m. The Margalef richness, Simpson, Shannon-Wiener, and Pielou evenness indices were all at a minimum, with values of 0.19, 0.15, 0.28, and 0.41, respectively.

3.3. Shrub Plants

3.3.1. Importance Value of Shrub Plants

Considerable changes in shrub-plant importance values with elevation gradient in the Pimus pumila community of Zalinkur Mountain, different species ecological amplitude is different, in the elevation of 900 m, 1100 m, 1300 m on the 15 shrub plant sample plots (No. 1–5 sample sit elevation of 900 m; No. 6–10 sample site elevation of 1100 m; No. 11–15 sample site elevation of 1300 m) There were 69 importance values for 12 shrub plant species (Figure 6), and there were differences in the importance values of the shrub plants, with a range of 0.06 to 0.62, with the minimum importance value of Vaccinium uliginosum was 0.06 in shrub sample plot 14 at 1300 m, and the most significant value of 0.62 for Vaccinium vitis-idaea in shrub sample plot No. 3 at 900 m. In an elevation gradient of 900 m, 1100 m, and 1300 m above sea level, the importance value of Vaccinium vitis-idaea in the shrub layer was greater, and Vaccinium vitis-idaea was the dominant species in the community. The importance of Vaccinium vitis-idaea in the community showed a decreasing trend with elevation, dominating the shrub layer at low elevations (900–1100 m), while its dominance at high elevations (1300 m) weakened. At 900 m, it was accompanied by a secondary dominant species, the Linnaea borealis, whose importance value was second only to Vaccinium vitis-idaea. As the elevation increased to 1100 m, the dominance of Vaccinium vitis-idaea decreased slightly but still dominated the community, while the dominance of low-elevation dominant species, such as Linnaea borealis and Juniperus communis var. saxatilis, decreased significantly. The dominance of high-elevation-adapted species (e.g., Empetrum nigrum subsp. asiaticum and Betula middendorfii) began to increase. At 1300 m, the dominance of lingonberries declined significantly, and they dominated the community with the Empetrum nigrum subsp. asiaticum. The dominance of cold-tolerant species, such as the Empetrum nigrum subsp. asiaticum and the Arctous alpinus, increased, with the dominant species of the overall community shifting from the low-elevation, widely adapted species to the high-elevation, specially adapted species. Overall, the importance value of Ericaceae at the sample site was high, and the importance value of other occasional and companion species was relatively low. companion species was relatively low.

3.3.2. Differences in Shrub α-Diversity Across Elevation Gradients

The Margalef richness index, Shannon-Wiener index, and Simpson index of 900 m dates did not conform to normal distribution; the sample size was small (n = 5); the Kruskal-Wallis nonparametric test was used; and Dunn’s test was used for the post hoc test (Bonferroni’s correction). The Pielou evenness index of all elevation dates conformed to normal distribution, and the variance was uniform; one-way analysis of variance (ANOVA) was used; and Tukey HSD multiple comparisons were used for the post hoc test. Index: All elevation dates conformed to normal distribution, and chi-square, one-way analysis of variance (ANOVA) was used, and Tukey HSD multiple comparisons were used for post hoc tests. As shown in the box line plot (Figure 7), there was no significant difference (p ≥ 0.05) in the Margalef richness index along the elevation gradient. There was no significant difference (p ≥ 0.05) between the Simpson index, the Shannon-Wiener index, and the date at elevations of 900 m and 1100 m. The Simpson and Shannon-Wiener indexes at 1300 m were significantly different (0.01 ≤ p < 0.05) from those at 900 m and 1100 m. There was no significant difference (p ≥ 0.05) between Pielou’s uniformity index at elevation 1100 m and both dates at elevations 900 m and 1300 m, and the difference between Pielou’s uniformity index at elevations 900 m and 1300 m was significant (0.01 ≤ p < 0.05). Overall, the α-diversity of shrubs in the Zalinkur Mountain changed significantly with the rise of the elevation gradient, with most measures of richness, dominance, diversity, and evenness at high elevation (1300 m) being significantly higher than those at low and middle elevations (1100 m, 900 m). The differences in some indices among low elevations were not significant, reflecting the influence of the elevation gradient on the α-diversity of shrub communities. The details of the changes reflected by different indices varied.

3.3.3. Changes in Shrub Alpha Diversity Indices Across the Elevation Gradient

The alpha diversity within the 15 shrub plant sample plots differed at elevations of 900 m, 1100 m, and 1300 m, and their alpha diversity indices all showed a “decreasing and then increasing” trend with increasing elevation from the elevation gradient: the 13th sample plot at 1300 m had the largest Margalef (0.67), Simpson Margalef (0.67), Simpson (0.69), and Shannon-Wiener (1.31) indices at 1300 m, and the maximum value of the Pielou index (0.73) appeared at Sample Plots 13 and 15 at 1300 m; the minimum values were found at Sample Plot 2 (Margalef 0.29) and 3 (Simpson 0.07) at 900 m and Sample Plot 6 (Shannon-Wiener 0.07) at 1100 m, respectively. Shannon-Wiener 0.20, Pielou 0.15). Robust Linear Regression (RLR) combined with Heteroscedasticity-Consistent Standard Errors (HC3) to analyse the correlation between the shrub plant diversity index and the three elevation gradients (Figure 8) showed that the correlation between the shrub plant diversity index and the three elevation gradients was as follows: The Margalef (p = 0.0221) and Shannon-Wiener (p = 0.0443) indices were significantly correlated with elevation, indicating that the number of species and the overall diversity of shrubs were significantly affected by elevation, whereas the Simpson (p = 0.1713) and Pielou (p = 0.313) indices were not significantly correlated with elevation, indicating that the dominant species status and the uniformity of species distribution in the community were relatively stable.

3.4. Herbaceous Plants

3.4.1. Importance Values of Herbaceous Plants

The field survey found that due to environmental constraints, such as low temperatures, strong winds, and poor soil nutrients at high elevations, some herbaceous plants did not occur in herbaceous plant sample plots at 1100 and 1300 m above sea level. According to the statistical principle of “only retaining effective sample sites with herbaceous plant growth”, 38 herbaceous plant sample sites were finally selected (No. 1-(Sample sites 1–20 are located at 900 m above sea level; Sample sites 21–27 are located at 1100 m above sea level; Sample sites 28–38 are located at 1300 m above sea level). The plant importance values of herbaceous communities in Zalinkur Mountain varied significantly with the elevation gradient, with a total of 87 importance values for 16 herbaceous species in 38 herbaceous sample plots at three elevations: 900 m, 1100 m, and 1300 m (Figure 9). The plant importance values of the herbaceous communities in Zalinkur Mountain varied significantly with the elevation gradient. The variation in importance values was from 0.08 to 1. The least important value was 0.08 for Clematis sibirica var. ochotensis in sample plot No. 1 at 900 m; the important value of Deyeuxia purpurea in sample plot No. 13 at 900 m; importance values for Deyeuxia purpurea in Herbaceous Plot 13 at 900 m; Koenigia divaricata in Herbaceous Plots 21, 22, 23, 24, 26, and 27 at 1100 m; and a maximum of 1 in Herbaceous Plot 28 for Ligusticum ajanense, in Herbaceous Plot 33 for Koenigia divaricata, and in Herbaceous Plots 36, 37, and 38 for Hedysarum branthii at 1300 m. At 900 m, the community was dominated by the Deyeuxia purpurea and the Maianthemum bifolium. As the elevation increased to 1100 m, the importance value of Koenigia divaricata was the largest. It was dominant in the community, while the low-elevation dominant species of Deyeuxia purpurea and Maianthemum bifolium were replaced by Koenigia divaricata. The dominance of high-elevation adaptive species (such as Koenigia divaricata and Saxifraga bronchialis) began to increase. At 1300 m by 1300 m, the dominance of Koenigia divaricate decreased, and the dominance of cold-tolerant species such as Ligusticum ajanense and Hedysarum branthii increased, resulting in an overall community shift from low-elevation, widely adapted species to high-elevation, specially adapted species. Overall, the importance values of Poaceae and Asparagaceae were higher in the low-elevation samples; Polygonaceae and Leguminosae were higher in the middle- and high-elevation samples, and the importance values of other occasional and companion species were relatively low.

3.4.2. Differences in Herb α-Diversity Across the Elevation Gradient

The absence or scarcity of herbaceous plants at 1100 and 1300 m elevations resulted in smaller sample sizes, leading to a higher proportion of zeros in the data. The Margalef richness index, Shannon-Wiener index, Simpson index, and Pielou evenness index did not conform to the normal distribution and were tested using the Kruskal-Wallis nonparametric test. The Dunn’s test (with Bonferroni’s correction) was used for the post hoc test. The Kruskal-Wallis nonparametric test was used, and the Dunn test (Bonferroni correction) was used for the post hoc test. As can be seen from the box line plot (Figure 10), the Margalef richness index, Simpson index, Shannon-Wiener index, and Pielou evenness index were significantly different between the dates of 1100 m elevation and 900 m and 1300 m elevation (0.01 ≤ p < 0.05). The Margalef richness index, Simpson index, Shannon-Wiener index, and Pielou evenness index at elevation 900 m were not significantly different from those at elevation 1300 m (p ≥ 0.05). The Margalef richness index showed that the herbaceous species richness was significantly lower at elevation 1100 m than at 900 m and 1300 m. The Simpson index and Shannon-Wiener index showed the same trend, and the indexes at 900 m and 1300 m were significantly higher than those at 1100 m, indicating that the dominant species of the herbaceous community were concentrated. The species diversity was low in the herbaceous community at 1100 m, and the species diversity of the herbaceous communities at 900 m and 1300 m was also low. Herbaceous communities of 1300 m have a more balanced species composition and higher diversity. The Pielou evenness index further verified that the species evenness of the herbaceous community was low at the 1100 m elevation; it was higher at both the 900 m and 1300 m elevations, indicating a more stable community structure. In summary, the α-diversity of herbaceous plants in the Pinus pumila community on Zalinkur Mountain exhibited non-monotonic changes along the elevation gradient, with relatively low herbaceous plant diversity at an elevation of 1100 m and richer herbaceous plant diversity at elevations of 900 m and 1300 m.

3.4.3. Changes in Herb Alpha Diversity Indices over the Elevation Gradient

Herbaceous plant α-diversity varied among the 38 sample sites (900 m: Nos. 1–20; 1100 m: Nos. 21–27; 1300 m: Nos. 28–38). Diversity indices exhibited a decreasing trend followed by an increasing trend with elevation, with maximum values of the Margalef richness index (1.53), Shannon’s index (1.64), Simpson index (0.90), and Pielou evenness index (1.00) observed at the 900 m sample site. In contrast, multiple sample sites (900 m—13, 1100 m—21/22/23/24/26/27, 1300 m—28/33/36/37/38) had minimum values of 0. Robust linear regression, combined with HC3 Heteroscedasticity-Consistent Standard Errors, analysed the relationship between herbaceous plant diversity indices and the three elevation levels (Figure 11). The results showed that elevation changes did not significantly affect the species diversity, dominant species pattern, and individual distribution evenness of herbaceous plants in the Pinus pumila community of Mt. Zarincourt within the studied elevation range.

4. Discussion

4.1. The Effect of Elevation Gradient on Community Species Composition and Importance Value

The species composition of plant communities is the result of long-term interactions between plants and their environment, and there are differences in plant composition depending on the community’s habitat conditions. In this study, a total of 37 plant species were recorded in the three fixed sample plots of the Pinus pumila community, including 9 tree species, 12 shrubs, and 16 herbaceous plants. The species richness showed that herbaceous plants > shrubs > trees. This is consistent with previous studies, which concluded that the herb layer contributes the most to community species richness [28,29,30,31]. Species richness is one of the most objective and realistic indicators of species diversity [32]. In this study, the total species richness exhibited a V-shaped pattern, initially decreasing and then increasing with elevation. Among the plants of different life types, tree species richness decreased monotonically with elevation; shrub species richness showed a decreasing and then increasing trend; and herb species richness showed an evident “V-shaped” pattern of change, which reflected the law of “life type determines the threshold of elevation adaptation”, which was consistent with the results of the previous studies on the distribution of species diversity at different elevation gradients in mountainous areas [33,34,35]. However, it is different from Liang Yuanwen et al. [36]. In this study, the “first decline and then rise” of the shrub layer may be related to the special topography of Zalinkur Mountain—the competition for light due to the dense tree layer at low elevation and the release of light after the reduction in trees at high elevation jointly shaped the non-monotonous pattern of the shrub layer. The low species richness of the Pinus pumila community in the Zarincourt Mountains is closely related to the limiting factors of low temperatures, strong winds, and infertile soils in high-elevation habitats. It has been demonstrated that precipitation is a key factor influencing species diversity in low-elevation areas. At the same time, low temperatures are a key factor limiting community diversity in high-elevation areas [37]. Zalinkur Mountain belongs to the cold-temperate zone, with high elevation forming the “alpine tundra” and “cold-temperate scrub” habitats: with the increase in elevation, the temperature decreases, the wind enhances, the soil is poorly developed, and the low temperature inhibits microbial activity, resulting in barren gravelly soil, which can only support shallow root systems. Only shallow-rooted, cold-tolerant species are supported, and resource constraints reduce the number of species. The combination of water, heat, and light is more effective at low elevations, as photosynthetic efficiency and community productivity are high, and the soil is rich in nutrients, which can accommodate more thermophilic or broadly thermophilic species. Consequently, the overall species richness decreases with increasing elevation.

Importance value is a key indicator of the degree of species dominance and ecological adaptation in a community [38]. The present study revealed significant differences in the composition of dominant species across different elevation gradients and life types of plants in the Pinus pumila community of Zalinkur Mountain. This result reflects the general influence of environmental filtration and ecological niche differentiation on community construction. The dominant species turnover in the tree layer along the elevation was characterised by the dominance of Larix gmelinii var. principis-rupprechtii and Betula platyphylla at low elevation (900 m) and the shift to a single dominant species of Pinus pumila at middle and high elevations (1100 and 1300 m), with the importance value increasing continuously with the increase in elevation. This change is in line with the typical pattern of global alpine ecosystems; that is, with the increase in elevation, the coniferous species with cold-resistant, wind-resistant, and barrenness-resistant traits gradually replace the broad-leaved species as the community-building species. The dominance of Pinus pumila sylvestris at high elevations reflects its high degree of adaptation to harsh habitats and confirms the general pattern that Pinus pumila sylvestris often serves as the dominant species in late-successional or top-end communities in cold-temperate and subalpine regions. In the shrub layer, Vaccinium vitis-idaea maintained its dominance at all three elevations. However, its importance value gradually decreased with increasing elevation, while a pattern of co-dominance with Empetrum nigrum subsp. asiaticum appeared at high elevation (1300 m). The widespread nature of Vaccinium myrtillus suggests that it has a broad ecological range and can adapt to varying resource conditions at different elevations. The decreasing importance values may be related to the limitation of total resources at high elevations. The fact that the Empetrum nigrum subsp. Asiaticum became the dominant species at high elevations, on the other hand, reflected the functional adaptation of the species to special habitats (e.g., low temperatures, strong winds, and thin soils), and a similar specialisation phenomenon has been reported in various alpine scrub studies. The dominant species in the herb layer underwent significant turnover along the elevation, from low-elevation moisture-loving and shade-tolerant species (Maianthemum bifolium, Deyeuxia purpurea) to mid-elevation transitional types (Koenigia divaricata) and then to high-elevation typical alpine herbs (Saxifraga bronchialis, Ligusticum ajanense, etc.). This pattern of successive replacement of dominant species is consistent with the study of Hao Lei et al. [26] in Fanjingshan, indicating that the herb layer is sensitive to changes in hydrothermal conditions and light, and its species composition shows substantial turnover along the elevation gradient, which is also a common feature of most mountain vegetation systems.

Overall, the elevation pattern of dominant species of different life-types of plants in this study reflects the ecological transition from the dominance of resource competition at low elevation to the screening of environmental stress at high elevation, which is in line with the classical theory of ecological succession in mountains, and provides a regional case study and generalised cognition for the understanding of the mechanism of the construction of mountain communities in the cold-temperate zone. As the elevation increases, the successional stages may be different, and the community succession process gradually transitions from the dominance of negative herbs to the predominance of positive herbs. This is consistent with the results of Hao Lei et al. [30] on the composition and diversity pattern of understory herbs in the forest communities of Fanjing Mountain.

4.2. Change Pattern of α-Diversity in Pinus pumila Communities at Different Elevations

Species diversity is a crucial indicator that reflects community structure and function, serving as the basis for maintaining ecosystem stability and providing the fundamental conditions for the operation and maintenance of ecosystem functions [39]. A large number of studies have demonstrated that environmental pressure gradients (e.g., elevation changes) have a significant impact on species diversity, which is often expressed in various patterns, including a single peak, a linear trend, or no discernible pattern. For example, species diversity may peak under moderately disturbed or resource-rich conditions and generally decline in extreme environments. In addition, there is a close relationship between diversity and ecosystem functions (e.g., productivity, stability), and high diversity tends to enhance the community’s ability to resist environmental fluctuations through complementary and selective effects. He Jinsheng et al. [40] synthesised the findings from previous studies. He found that there are five trends in plant diversity with elevation, i.e., plant diversity is positively correlated with elevation, negatively correlated, non-significantly correlated, and the diversity is most incredible or lowest at intermediate elevations, which is mainly due to the scale of the elevation gradient, anthropogenic disturbances, and differences in habitats in the study area [41]. In this study, we focused on the Pinus pumila community in Zalinkur Mountain, Daxinganling, and aimed to investigate the pattern of α-diversity of different life types (trees, shrubs, and herbs) along the elevation gradient and analyse its association with habitat factors (e.g., hydrothermal conditions, soil thickness, and anthropogenic disturbances) in order to fill the gap in the relationship between diversity and the environmental gradient in this region.

The study found that the α-diversity index of the tree layer gradually decreased with elevation; the α-diversity index of the shrub layer showed a pattern of change of decreasing and then increasing with elevation; the Margalef richness index and the Shannon-Wiener index of the shrub plants were significantly correlated with the elevation; the α-diversity index of the herb layer showed a pattern of change of decreasing and then increasing with elevation, and the indexes were weakly negatively correlated with elevation.

This finding is consistent with the research results of Lin Yang et al. [42] and Zhao Shuqing et al. [43] on the pattern of change in tree and herb layers with elevation, whereas the pattern of change in the shrub layer with elevation differs. This may be related to the differences in resource conditions, such as water and heat, at different elevation gradients in the Daxinganling region [44]. This investigation found that the lower elevation areas of Zalinkur Mountain have closer water sources.

In comparison, higher elevation areas have fewer water sources, scarce nutrients, and thinner soil, as well as lower temperatures and stronger winds. This may be because the hydrothermal and nutrient conditions in the lower elevations are more favourable for trees and herbs than in the higher elevations. Shrubs are shorter and less affected by low temperatures and strong winds at higher elevations than trees, and the light needs of shrubs are met by abundant light at higher elevations. It was also found that the α-diversity of the shrub layer and herb layer of the Pinus pumila community decreased to a valley at 1100 m, which may be due to the occurrence of more Pinus pumila forests at 1100 m, and the shrub layer and herb layer were in a disadvantageous position in terms of resources such as light and water. Additionally, residents are collecting Pinus pumila cones at elevations of 1100 m and 1300 m above sea level, which disrupts the forest’s continuity and is one of the key factors affecting the α-diversity index [45]. Overall, the α-diversity indices of the tree, shrub, and herb layers of the Pinus pumila community varied across the elevation gradient. This study demonstrated that elevation was the primary factor influencing the diversity of the Pinus pumila community structure and species composition. However, the mechanism of its influence was significantly stratum-specific. The elevation gradient directly acts as a powerful environmental filter, monotonically limiting the diversity of the tree layer; at the same time, it significantly regulates the diversity pattern of the shrub layer by altering the hydrothermal and light conditions, whereas for the herbaceous layer, elevation may exert its indirect influence mainly by altering the biotic environments, such as canopy closure, rather than directly through climate. This differential response pattern underscores the need for life-type-specific studies when evaluating the impact of environmental gradients on community assembly.

4.3. Limitations and Next Research Direction

This study analyses the effects of elevation on the plant composition and diversity of the Pinus pumila community in Zalinkur Mountain, providing an important addition to the study of community responses to environmental gradients in this region. The community is characterised by a combination of cold temperatures, alpine conditions, and anthropogenic disturbance, which is of outstanding value for understanding species coexistence and diversity-stability relationships in extreme habitats under climate change.

There are some limitations in this study: soil factors (nutrients, physical properties, etc.) were not systematically measured, which makes it difficult to comprehensively analyse the mechanisms of plant diversity differentiation and soil-climate interactions among different life types; meanwhile, the elevation gradient and sample size are limited, which may affect the accurate capture of ecological processes in the transition zone.

Suggestions for follow-up include setting up additional transition elevations and replicated sample plots, increasing quantitative measurements of soil indicators, and analysing spatial heterogeneity by combining remote sensing and topographic data. It is proposed to quantify the contribution of environmental variables through multi-factor, synchronised monitoring, using RDA and SEM, and to predict community changes in combination with climate scenarios, thereby providing a basis for regional ecological protection. In the future, time series data can be introduced to analyse the lagged response of vegetation dynamics to climate fluctuations and long-term change patterns.

5. Conclusions

In the three fixed sample plots in the elevation range of 900–1300 m in Zalinkur Mountain, a total of 23 families, 32 genera, and 37 species of plants were found, including 4 families, 7 genera, and 9 species of trees; 6 families, 9 genera, and 12 species of shrubs; and 13 families, 16 genera, and 16 species of herbaceous plants. The total species richness was Herbs > Shrubs > Trees.

The elevation gradient of 900 m, 1100 m, and 1300 m basically covers the key ecological transition zones in the distribution of the Pinus pumila community: 900 m represents the transition zone between the community and the low mountain forest; 1100 m is the middle part of the core distribution, where environmental pressures and disturbances have jointly shaped the special adaptive strategies of the shrub layer; and 1300 m is close to the upper limit of the distribution, where a strong filtering effect of the environment has led to the significant reduction in the diversity of the tree layer. The strong environmental filtering effect significantly reduced the diversity of the tree layer. There are significant differences in the change rule of α-diversity of different life-types along the elevation: it was found that the α-diversity index of the tree layer gradually decreased with the increase in elevation; the α-diversity index of the shrub layer decreased firstly and then increased with the increase in elevation, and the relationship between the Margalef richness index and the Shannon-Wiener index of the shrub plants and the elevation was significant; the α-diversity index of the herb layer decreased firstly and then increased with the increase in elevation, and the relationship between each index and the elevation was significant; the α-diversity index of the herb layer showed a decrease firstly and then increased firstly.

Elevation is a key environmental factor influencing the spatial pattern of species diversity in the Pinus pumila sylvestris community, which significantly regulates the distribution of diversity among different life forms of plants through the combined effects of hydrothermal conditions, soil nutrients, and anthropogenic disturbances. The results of this study emphasise the important role of elevation gradient in community construction and the maintenance of multi-level diversity in cold-temperate mountain ecosystems and also provide a scientific basis for future monitoring and protection of the dynamics of subalpine forest communities under climate change.