Changes in species and phylogenetic diversity in tropical seasonal rainforests on steep karst hillslopes in southwestern China: implications for conservation

73 Changes in species and phylogenetic diversity in tropical seasonal rainforests on steep karst hillslopes in southwestern China: implications for conservation Qingling Pang1* , Cong Hu1* , Chaohao Xu1, Chaofang Zhong1, Zhonghua Zhang1,2,3 , Gang Hu1,2,3 1 Key Laboratory of Environment Change and Resources Use in Beibu Gulf, Ministry of Education, Nanning Normal University, Nanning 530100, China 2 Guangxi Key Laboratory of Earth Surface Processes and Intelligent Simulation, Nanning Normal University, Nanning 530100, China 3 Key Laboratory of Wildlife Evolution and Conservation in Mountain Ecosystem of Guangxi, Nanning Normal University, Nanning 530100, China Corresponding authors: Zhonghua Zhang ([email protected]); Gang Hu ([email protected]) Copyright: © Qingling Pang et al. This is an open access article distributed under terms of the Creative Commons Attribution License (Attribution 4.0 International – CC BY 4.0). Research Article Abstract Tropical karst seasonal rainforests (TKSR) in southwestern China have high levels of biodiversity and a significant number of endemic species. However, understanding the distribution of plant diversity and the mechanisms driving community assembly in these diverse and heterogeneous karst forests remains limited. This study examined the species and phylogenetic diversity of the herb, shrub, and tree layers in the TKSR along steep hillslopes within karst peak-cluster depressions in southwestern China. Species richness in the herb, shrub, and tree layers showed an inverted U-shape pattern, with richness lower in the middle and higher on the sides of the hillslope. The upper slope had a higher level of species richness than the depressions and the lower and middle slopes. The phylogenetic structures of the herb and tree layers tended to be clustered, whereas the shrub layer exhibited a divergent phylogenetic structure. These findings indicate that community assembly in the TKSR is influenced by habitat filtering and competitive exclusion. Soil total phosphorus (STP) and soil available phosphorus (SAP) were identified as significant factors affecting species diversity across the three layers, whereas the rock outcrop rate was a significant factor affecting the phylogenetic structure. In the shrub and tree layers, STP and SAP were important determinants of phylogenetic diversity. These results highlight the impact of environmental heterogeneity on species and phylogenetic diversity in the TKSR. Furthermore, depressions and upper slopes with high species and phylogenetic diversity were identified as priority areas for conservation in the karst peak-cluster depressions of southwestern China. Key words: Community assembly, karst forest, phylogenetic structure, plant diversity, slope position Introduction Changes in plant diversity along environmental gradients are important in the field of vegetation ecology (Liu 2017). Slope, as a topographical factor, plays a crucial role in controlling the spatial distribution of water, light, heat, and soil nutrients, and influences the abundance, distribution, and diversity of vegetation in mountainous Academic editor: Ilona Blinova Received: 19 June 2025 Accepted: 12 October 2025 Published: 4 November 2025 ZooBank: https://zoobank.org/ FDBA1FC3-45CC-4410-8568B24703AF42A6 Citation: Pang Q, Hu C, Xu C, Zhong C, Zhang Z, Hu G (2025) Changes in species and phylogenetic diversity in tropical seasonal rainforests on steep karst hillslopes in southwestern China: implications for conservation. Nature Conservation 60: 73–97. https://doi.org/10.3897/ natureconservation.60.162595 Nature Conservation 60: 73–97 (2025) DOI: 10.3897/natureconservation.60.162595 * These authors contributed equally to this work. 74 Nature Conservation 60: 73–97 (2025), DOI: 10.3897/natureconservation.60.162595 Qingling Pang et al.: Plant diversity on steep karst hillslopes and hilly areas (Zeng et al. 2014). Plant diversity patterns along slopes often exhibit complex and non-monotonic changes that are closely related to topography, water, and heat conditions, as well as various life forms, including trees, shrubs, and herbs (Deák et al. 2021; Sharma and Kala 2022). Many studies have investigated the impact of slope on vegetation in various ecosystem types (Zeng et al. 2014; Berry et al. 2016; Zhang et al. 2020; Dong et al. 2022; Liang et al. 2024). Understanding plant diversity patterns along slopes and the underlying mechanisms is crucial to elucidating the relationship between vegetation and the environment. Species diversity is a central aspect of biodiversity and ecology research, reflecting community structure, developmental stages, stability, and habitat differences, and can also reveal the organizational levels of plant communities (Li et al. 2019). Understanding the influence of environmental factors on species diversity is an important ecological question (Weigel et al. 2019). Plant communities are typically composed of species with various life histories that occupy distinct ecological niches (Nettesheim et al. 2018). As the community environment changes, the niches occupied by these species can change, leading to different patterns of heterogeneity in the community (Bagaria et al. 2019). Consequently, different communities often exhibit differences in species composition, structure, and functional traits; elucidating these differences is crucial to understanding how species diversity varies across different environments (Geng et al. 2022). Phylogenetic diversity can be used to assess the uniqueness of biological taxa in specific spatiotemporal contexts based on the evolutionary history of the species. It is also indicative of biodiversity conservation because it can be used to identify phylogenetic relationships and evolutionary information among different species (Chen et al. 2021). Phylogenetic structures complement phylogenetic diversity and can be used to infer the ecological processes affecting community assembly. When integrated, the composition, origin, and evolutionary processes of species can be explored from both evolutionary and ecological perspectives, thereby promoting the rational conservation of plant diversity (Ci and Li 2017). Phylogenetic diversity interprets a species’ evolutionary history from a phylogenetic relationship perspective. When combined with species diversity, it greatly enhances our understanding of population dynamics and ecological niche differentiation (Zeng et al. 2014), providing a new perspective for biodiversity conservation. Although previous studies have focused on species patterns and phylogenetic diversity in different forest types worldwide (Myers et al. 2013; Qian et al. 2014; Yang et al. 2014; Guo et al. 2018; Gastauer et al. 2020), understanding of vegetation distribution characteristics in special landforms, such as karst landscapes, remains limited. Karst landscapes are formed by the effects of rainfall and groundwater on carbonate bedrock (De Waele 2017). These landscapes are widespread, occurring on approximately 15%–20% of the Earth’s ice-free land surface, with the largest continuous area spanning roughly 0.51 million km2 in southwestern China (Jiang et al. 2014). Owing to their high sensitivity and vulnerability, the conservation of biodiversity in karst areas is of great importance (Sun et al. 2020). Karst areas in southwestern China are recognized as one of the 25 global biodiversity hotspots because of their unique biomes. The distinctive landforms they create consist of peak-cluster depressions, characterized as closed and flat depressions surrounded by a series of hills. Karst peak-cluster depressions contribute to the complexity and diversity of landforms in karst areas (Yang et 75 Nature Conservation 60: 73–97 (2025), DOI: 10.3897/natureconservation.60.162595 Qingling Pang et al.: Plant diversity on steep karst hillslopes al. 2014) and display rapidly changing topographical characteristics, transitioning from cliffs to depressions over short horizontal distances. In addition, the potential energy decreases dramatically from the top slope to the depression area, resulting in a significant potential gradient. These peak-cluster depressions provide the topographic and hydrological conditions necessary for the development of a catenary pattern of soil and vegetation (Meng et al. 2022). Karst peak-cluster depressions in southwestern China are generally at low elevations (between 200 and 500 m) but often exhibit steep slopes (greater than 30°). In these areas, elevation is not considered the primary factor influencing species diversity in communities, especially in lowand small-scale regions (Zeng et al. 2014). Slope position has been recognized as one of the major topographic factors that determine microclimates and ecological niches for species in karst mountain regions, as it influences various aspects such as sunlight, soil depth, soil moisture, and nutrient availability (Gong et al. 2008; Weintraub et al. 2015). Mountainous and hilly areas encompassing steep environmental gradients in small geographic areas are outstanding natural laboratories for biodiversity studies because numerous environmental factors, such as temperature (Wu et al. 2013), productivity (Ramírez-Bautista and Williams 2019), and anthropogenic disturbance (Santillán et al. 2020), can correlate with elevation and consequently exert effects on biological communities. Along slope position gradients, karst peak-cluster depressions exhibit highly heterogeneous microhabitats due to variations in multiple environmental factors, resulting in significant differences in biodiversity. For example, Peng et al. (2019) reported that slope was the major driver of the spatial distribution of soil microbial richness and diversity in a karst forest due to its effects on plant characteristics (i.e., tree Shannon diversity and tree density) and soil properties (i.e., soil pH and available phosphorus). Similarly, Sun et al. (2020) revealed spatial incongruence among the taxonomic, functional, and phylogenetic diversity of small mammals along elevation gradients in karst mountains. Other studies have focused on variations in soil physicochemical properties and plant–litter–soil ecological stoichiometry along slope position gradients in karst peak-cluster depressions (Zeng et al. 2015; Chen et al. 2019; Hu et al. 2020). However, it remains unclear how plant species and phylogenetic diversity change along slope position gradients in karst peak-cluster depressions and the driving factors behind these changes. The Nonggang National Nature Reserve in southwestern China is a representative of karst peak-cluster depression landforms worldwide. The tropical karst seasonal rainforests (TKSR) in this reserve are characterized by complex structures, rich species composition, and prominent endemic elements. This area is also one of the 14 key areas of terrestrial biodiversity of international significance in China (Huang et al. 2013). Thus, the TKSR is irreplaceable and significant for the restoration and protection of karst forest ecosystems (Zeng et al. 2023). Recent ecological studies on the TKSR have focused on species composition, spatial distribution, and diversity (Ma et al. 2023). Studies have found that the diversity of woody plant species in the TKSR is unimodal, but there is also a bimodal pattern that increases with elevation, and the diversity pattern is strongly influenced by topographic heterogeneity (Huang et al. 2016). Furthermore, Guo et al. (2017, 2018) investigated the effects of deterministic and stochastic processes on community assembly in the TKSR at a 15-ha plot scale and found that both processes play important roles. Although it is recognized that topography influences the spa- 76 Nature Conservation 60: 73–97 (2025), DOI: 10.3897/natureconservation.60.162595 Qingling Pang et al.: Plant diversity on steep karst hillslopes tial distribution of trees in the TKSR, the dominant biotic and abiotic factors that affect the distribution patterns of species and phylogenetic diversity remain unclear. In this study, plot surveys and environmental factor measurements were conducted in the TKSR of the Nonggang National Nature Reserve with a focus on different slopes (depression, lower slope, middle slope, and upper slope). Gradient changes in community species and phylogenetic diversity across three forest strata (herb, shrub, and tree layers) and the factors influencing these changes were examined. The objective of this study was to reveal (1) the spatial patterns of species diversity, phylogenetic diversity, and structure within slope gradients in the herb, shrub, and tree layers and (2) the primary environmental factors influencing these spatial patterns. The results provide a scientific basis for improving understanding of forest community structure, ecosystem functions, and ecological conservation in karst peak-cluster depressions. Materials and methods Study area The Nonggang National Nature Reserve, located in the southern Guangxi Zhuang Autonomous Region, southwestern China (22°13'56"–22°39'09"N, 106°42'28"– 107°04'54"E; Fig. 1), covers an area of 10,080 ha. The region has a tropical monsoon climate, with an average annual temperature of approximately 22 °C. The temperature ranges from above 13 °C in January, the coldest month, to above 28 °C in July, the hottest month. The annual accumulated temperature ranges from 7,400 to 7,800 °C. Annual average rainfall ranges from 1,200 to 1,500 mm and is concentrated between May and September. The bedrock of the area is primarily Figure 1. Geographical location of the study area, with photographs showing tropical karst seasonal rainforests along steep karst hillslopes. 77 Nature Conservation 60: 73–97 (2025), DOI: 10.3897/natureconservation.60.162595 Qingling Pang et al.: Plant diversity on steep karst hillslopes composed of limestone, and the main soil types are primitive calcareous, brown calcareous, and black calcareous. The topography of the region is characterized by typical karst peak-cluster depression landforms, with depressions formed by clustered peaks sharing a common base and a funnel-shaped landscape. Elevation ranges from 150 to 600 m. Unique landforms, such as peak clusters, forests, depressions, and funnels, result in significant variations in light availability, soil thickness, and moisture. Other micro-landform features formed by numerous rock outcrops—such as stone gullies, stone surfaces, and depressions—contribute to the heterogeneity of small-scale habitats. The reserve contains 1,752 recorded species of vascular plants belonging to 810 genera and 184 families (Huang et al. 2013). Among these, 33 species are rare and endangered in China, and 278 species are endemic to karst habitats (Wang et al. 2014). The dominant tree species in the TKSR include Excentrodendron tonkinense (A. Chev.) H.T. Chang & R.H. Miao, Camellia petelotii (Merr.) Sealy, Cephalomappa sinensis (Chun & F.C. How) Kosterm., Garcinia paucinervis Chun ex F.C. How, and Deutzianthus tonkinensis Gagnep. Data collection During the peak growing season from July to September 2020, forest plots were established at different slope positions based on elevation variations in the karst peak-cluster depressions in the study area (Fig. 1). There were 10 plots, each with an area of 20 × 20 m, at four slope positions—depression (DE), lower slope (LS), middle slope (MS), and upper slope (US)—with a spacing of more than 30 m between plots. A total of 40 plots were established along steep karst hillslopes in the study area. Tree-layer investigations included species names, diameters at breast height (DBH), and heights. Within each plot, two 5 × 5 m subplots were randomly established to investigate the shrub layer, including species names, average heights, coverages, and numbers. Additionally, four 1 × 1 m subplots were randomly established to investigate the herb layer, including species names, numbers, heights, and coverages. All vascular plants in each plot and subplot were inventoried, with species identifications verified by two plant taxonomy experts. Furthermore, latitude and longitude coordinates, elevation, aspect, slope position, and rock outcrop rate were recorded. Three experienced observers visually estimated the slope position and rock outcrop rate, and a handheld GPS (Garmin GPSMAP 60CSx) was used for all other measurements. Slope positions were represented by categorical variables 1, 2, 3, and 4, corresponding to depression, lower slope, middle slope, and upper slope, respectively (Qiu and Zhang 2000). The slope aspect was converted from a range of 0–360° azimuth to a value ranging from 0 to 1 using a previously documented formula (Yu et al. 2013). Within each plot, soil samples were collected at depths of 0–20 cm using the five-point sampling method (Lu et al. 2023). The collected soil samples were mixed and returned to the laboratory. After air-drying and sieving, the soil samples were subjected to chemical analyses to determine various soil parameters. The measured indicators included soil pH, exchangeable calcium (ECa), exchangeable magnesium (EMg), available phosphorus (AP), available potassium (AK), ammonium nitrogen (NH4 +-N), nitrate nitrogen (NO3 --N), organic matter (SOM), total nitrogen (TN), total phosphorus (TP), and total potassium (TK). Soil pH was determined in a suspension with a 1:2.5 soil-to-water ratio. ECa and EMg were measured using atomic absorption spectrophotometry (AA-7000, Shimadzu, 78 Nature Conservation 60: 73–97 (2025), DOI: 10.3897/natureconservation.60.162595 Qingling Pang et al.: Plant diversity on steep karst hillslopes Kyoto, Japan). NH4 +-N was quantified using the indophenol blue colorimetric method, and NO3 --N was measured using the cadmium reduction method followed by a sulfanilamide–NAD reaction. SOM content was determined using the traditional potassium dichromate oxidation method. TN was measured using the semi-micro Kjeldahl method. AP and TP levels were determined using the molybdenum–antimony colorimetric method. AK and TK levels were measured using the sodium hydroxide fusion flame photometry method. The surface soil water content (SWC) of each plot was measured using a portable three-parameter soil tester (WET-2, Delta-T Devices Ltd., United Kingdom) following the five-point sampling method (Lu et al. 2023). These methods for measuring soil properties were described in detail by Lu (1999) and Bao (2000). Environmental variables (mean ± SD) in four slope positions are presented in Suppl. material 1: table S1. Data analysis Species diversity The plant species names, along with their family and genus information within the study plots, were verified according to the APG III system using the plantlist package (Zhang 2018). Species richness (S), Simpson index (D), Shannon– Wiener index (H), and Pielou index (J) were calculated to assess species diversity in the plant communities (MacDonald et al. 2017). These indices were computed using the vegan package in R version 4.2.2 (Oksanen et al. 2019). The formulae used for the calculations are as follows: (1) (2) J = H / lnS (3) where S represents the species richness within the community plot and Pi represents the importance value of species i in the community. Phylogenetic structure and diversity A phylogenetic tree of vascular plants was constructed using the V.PhyloMaker package (Jin and Qian 2019), incorporating evolutionary branch lengths based on the “GBOTB.extended.tre” framework following the methodological approach of scenario S3. This scenario employs average distances to constrain phylogenetic tip positions, thereby reducing bias introduced by polytomies. The “GBOTB.extended.tre” tree integrates the evolutionary branches of seed plants from GBOTB (Smith and Brown 2018) and ferns from the updated, revised, and expanded phylogenetic system of Zanne et al. (2014). The Faith Diversity Index (PD) was used to measure phylogenetic diversity (Faith 1992). The net relatedness index (NRI) and nearest taxon index (NTI) were used to determine the phylogenetic structure. NRI and NTI represent the standardized values of the mean pairwise distance (MPD) and mean nearest taxon distance 79 Nature Conservation 60: 73–97 (2025), DOI: 10.3897/natureconservation.60.162595 Qingling Pang et al.: Plant diversity on steep karst hillslopes (MNTD), respectively. The NRI primarily describes the overall phylogenetic relationships among species in a community, whereas the NTI focuses on phylogenetic relationships at the terminal branches of the phylogenetic tree. Compared with NRI, NTI places greater emphasis on describing the phylogenetic distances between neighboring species (Webb et al. 2002). MPD and MNTD were calculated using the comdist and comdistnt functions in the picante package (Kembel et al. 2008). The actual values for each community were compared using an unconstrained null model (Kembel and Hubbell 2006), in which the terminal branches of the phylogenetic tree were randomly permuted 999 times for all species in the community. The differences were then calculated, and the NRI and NTI values for each community were obtained (Webb et al. 2002; Swenson et al. 2007). PD = ∑Lb (4) (5) (6) where Lb represents the branch length connecting the species on the tree. MPDobserved and MNTDobserved are the observed values of the average pairwise phylogenetic distance and mean nearest taxon distance, respectively, for each plot. MPDrandomized and MNTDrandomized are the average pairwise phylogenetic distance and mean nearest taxon distance, respectively, obtained by randomly generating null models for the community. sdMPDrandomized and sdMNTDrandomized are the standard deviations of the average pairwise phylogenetic distance and mean nearest taxon distance obtained from the randomly generated null models for the community, respectively. If the NRI (or NTI) > 0, the phylogenetic relationships among individuals in the community are closer than those expected from a randomly generated null model, suggesting phylogenetic clustering. Conversely, if the NRI (or NTI) < 0, it suggests phylogenetic overdispersion, indicating that the phylogenetic relationships among individuals in the community are more divergent than expected from the null model (Webb et al. 2002). If the NRI (or NTI) < −1.96, it indicates significant phylogenetic evenness compared with the null model, whereas an NRI (or NTI) > 1.96 indicates significant phylogenetic clustering compared with the null model (Vamosi et al. 2009). Statistical analysis Assuming that the data were normally distributed and satisfied the assumption of homogeneity of variances, one-way ANOVA was used to compare differences in environmental variables among the different slope positions. For multiple comparisons of plant diversity between slope positions, the least significant difference (LSD) method was applied if variances were equal; however, if variances were not equal, the Games–Howell test was used. Pearson correlation analysis was conducted to examine relationships between species, phylogenetic diversity, and environmental variables. Factors with r > 0.6 were removed. The remaining factors were then included in multiple stepwise regression analyses, along with diversity indices and phylogenetic structures, to identify the main 80 Nature Conservation 60: 73–97 (2025), DOI: 10.3897/natureconservation.60.162595 Qingling Pang et al.: Plant diversity on steep karst hillslopes factors influencing species and phylogenetic diversity. The stepwise regression procedure was based on the Akaike Information Criterion (AIC), in which variables were added or removed by selecting the model with the minimum AIC value. Residual analysis confirmed that the model satisfied the assumptions of normality, linearity, and homoscedasticity, thereby validating the use of a regression model based on the Gaussian distribution. The factors retained in the regression equations were quantified using the rdacca.hp package (Lai et al. 2022) in R 4.2.2 to determine their explanatory power for the diversity indices through hierarchical partitioning analysis. One-way ANOVA and Pearson correlation analysis were performed using SPSS 26.0 (SPSS Inc., Chicago, IL, USA). Results Species and phylogenetic diversity at different slopes Species diversity in the herb, shrub, and tree layers initially decreased and then increased with slope position (Fig. 2). In the herb layer, species richness in the middle slope was significantly lower than that in the lower and upper slopes (p < 0.05), and the Pielou index showed an increasing trend, with values for both the middle and upper slopes significantly higher than those in the lower slopes (p < 0.05). There were no significant differences in species diversity in the shrub layer among the different slope positions. In the tree layer, species richness initially decreased and then increased with slope position, whereas the other indices showed an overall increasing trend, with no significant differences among slope positions. Figure 2. Changes in species diversity at different slope positions. Different lowercase letters indicate a significant difference between communities at different slope positions (p < 0.05). DE, depression; LS, lower slope; MS, middle slope; US, upper slope. 81 Nature Conservation 60: 73–97 (2025), DOI: 10.3897/natureconservation.60.162595 Qingling Pang et al.: Plant diversity on steep karst hillslopes Phylogenetic diversity in the herb, shrub, and tree layers initially decreased and then increased with slope position (Fig. 3). Specifically, phylogenetic diversity in the lower and upper slopes was significantly higher than that in the middle slope in the herb layer (p < 0.05). In contrast, there were no significant differences in phylogenetic diversity among slope positions in the shrub and tree layers. Across the three layers, phylogenetic diversity was highest in the tree layer, followed by the herb and shrub layers. The NRI and NTI in the herb, shrub, and tree layers showed inconsistent patterns in community species phylogenetic relationships across the slope positions (Fig. 4). In the herb layer, both NRI and NTI indicated phylogenetic clustering across the four slope positions. In the shrub layer, both NRI and NTI exhibited phylogenetic divergence on the lower and upper slopes, whereas the middle and depression slopes showed a coexistence of phylogenetic divergence and clustering, with significant differences in NRI among the slope positions (p < 0.05). In the tree layer, both NRI and NTI exhibited an overall pattern of phylogenetic clustering on the lower and upper slopes. In contrast, the depression and middle slopes showed a coexistence of phylogenetic divergence and clustering. Overall, the herb and tree layers showed phylogenetic clustering, whereas the shrub layer exhibited phylogenetic divergence. Species and phylogenetic diversity in relation to environmental variables Correlation analysis revealed that species diversity in the herb layer was significantly and negatively correlated with NO3 --N, SOM, and SWC, and significantly and positively correlated with SP, SA, and SD (p < 0.05). The phylogenetic structure was significantly and positively correlated with AK, TP, and BA (p < 0.05). Phylogenetic diversity was significantly and negatively correlated with NO3 --N and SOM contents (p < 0.05; Fig. 5). In the shrub layer, species diversity was significantly and negatively correlated with TP and BA and significantly and positively correlated with SA and pH Figure 3. Phylogenetic diversity at different slope positions. Different lowercase letters indicate a significant difference between communities at the various slope positions (p < 0.05). DE, depression; LS, lower slope; MS, middle slope; US, upper slope. 88 Nature Conservation 60: 73–97 (2025), DOI: 10.3897/natureconservation.60.162595 Qingling Pang et al.: Plant diversity on steep karst hillslopes structural patterns within plant communities, which are influenced by multiple environmental factors at different levels. Specifically, phylogenetic aggregation in the herb layer was associated with increases in soil AK, TP, and BA. In the shrub layer, aggregation was driven by elevated soil NH4 +-N, NO3 --N, pH, and BA. Conversely, the tree layer responded to increases in soil ECa, EMg, TK, TN, TP, SOM, and ROR. However, an increase in SD promoted an overall aggregation trend in the phylogenetic relationships of tree layer species, whereas SOM had the opposite effect, increasing the phylogenetic distance between adjacent species and affecting the phylogenetic structure of the tree layer. The phylogenetic structure of plant communities is closely related to the response of plants to their environment, which in turn affects the ecological processes that dominate community assembly. The phylogenetic structures of the herb, shrub, and tree layers exhibited inconsistent patterns along the slope gradient. The herb layers across the different slopes exhibited aggregated phylogenetic structures. In addition to the influence of microhabitat conditions, the phylogenetic structure of the herb layer may be related to the life histories of herbaceous plants (Qian et al. 2014). Herbaceous plants have short life cycles and strong dispersal abilities, can rapidly occupy suitable habitats, exhibit large population fluctuations, and easily form aggregated phylogenetic structures (Niu et al. 2011). The dispersal of tree seeds may also influence the phylogenetic structure of the herb layer community. Herb layers include woody plant seedlings, and the limited dispersal of seeds from parent trees in the forest leads to a clustered pattern of seedlings, resulting in an aggregated phylogenetic structure in the herb layer community (Niu et al. 2011). The phylogenetic structure of the shrub layer exhibited divergence in the depression and upper slope and aggregation in the middle slope communities. In contrast, the lower slope communities showed inconsistent NRI and NTI values for the phylogenetic relationships of community species, with NRI indicating divergence and NTI indicating aggregation. In the tree-layer communities, the NRI and NTI of the depression and middle slopes exhibited inconsistent phylogenetic structures. In the depression, NRI showed divergence, whereas NTI showed aggregation. Conversely, the opposite pattern was observed in the middle slope communities, while the phylogenetic structures of the upper and lower slope communities were aggregated. In high-altitude areas, due to environmental stress, species within communities undergo convergent evolution, resulting in a trend toward aggregated phylogenetic structures (Kluge and Kessler 2011; You et al. 2013). Similarly, research has shown that the phylogenetic structures of evergreen broad-leaved forest communities exhibit aggregation in low-altitude areas and divergence in high-altitude areas (Huang et al. 2010). In this study, the harsh habitat conditions on the upper slope may have caused strong environmental stress, promoting convergent evolution in plants and resulting in aggregated phylogenetic structures in the tree-layer communities. Conversely, competition for limited resources among plants may have caused the phylogenetic structure of shrub layer plants to diverge. The inconsistent changes in community NRI and NTI values are related to their different emphases, indicating that distinct ecological processes influence the assembly of herb, shrub, and tree layer communities. The different ways in which communities respond to the environment affect the relative importance of ecological processes in community assembly. The results indicate that deterministic processes (environmental filtering and competitive exclusion) have significant impacts on com- 89 Nature Conservation 60: 73–97 (2025), DOI: 10.3897/natureconservation.60.162595 Qingling Pang et al.: Plant diversity on steep karst hillslopes munity assembly in the TKSR, while some communities may also be influenced by neutral processes (NRI or NTI < −1.96) (Vamosi et al. 2009). Conservation of karst forests Karst landscapes are crucial biodiversity reservoirs, particularly in Asia, hosting a wealth of endemic species (Clements et al. 2006). The steep karst regions of southwestern China are recognized as biodiversity hotspots, notable for their remarkable plant species richness and significant conservation value (Guo et al. 2018; Geekiyanage et al. 2019). Protecting biodiversity and rehabilitating degraded vegetation in these areas remain pivotal ecological priorities for the Chinese government and researchers (Chen et al. 2024). Conservation strategies should incorporate phylogenetic diversity into assessment frameworks, complementing traditional species diversity metrics to reflect evolutionary relationships (Tietje et al. 2023). Consequently, areas with high and unique phylogenetic diversity should be prioritized for conservation efforts (Zhang et al. 2024). Our study demonstrates that depressions and upper slopes exhibit greater species and phylogenetic diversity than other slope positions, highlighting their significant conservation value for plant diversity. Karst depressions, characterized by deep, fertile soils, are especially susceptible to anthropogenic conversion. In southwestern China, this has resulted in substantial natural forest loss and degradation, primarily due to their conversion into agricultural and plantation lands (Yuan et al. 2024). Therefore, conservation efforts for forests in karst depressions should be prioritized. Our findings provide critical insights for planning forest reserves and developing conservation strategies in karst regions. Conclusion The results show that species diversity in the herb and shrub layers initially decreases and then increases. In contrast, species richness in the tree layer initially increases and then decreases, while the remaining indices show an overall increasing trend in the TKSR along steep karst hillslopes. Stepwise regression analysis revealed that SA, SP, and SOM all have high explanatory power for species diversity in the herb layer. BA, SA, and soil pH explained much of the variation in species diversity in the shrub layer, whereas BA, CD, and SP had strong influences on species diversity in the tree layer. Phylogenetic diversity in the herb, shrub, and tree layers initially decreased and then increased as the slope gradient increased. Plant communities exhibited relatively high phylogenetic diversity in the depressions and on the upper slopes, suggesting longer evolutionary histories and greater evolutionary diversity and variation. In contrast, plant communities on the lower and middle slopes had relatively lower phylogenetic diversity. SA and SOM collectively had significant effects on the phylogenetic diversity of the herb layer; SA, NO3 --N, and BA significantly influenced the phylogenetic diversity of the shrub layer; and SP, ROR, BA, and CD had significant effects on the phylogenetic diversity of the tree layer. The herb and tree layers exhibited phylogenetic clustering, and habitat filtering was the primary ecological process influencing community assembly. The shrub layer exhibited phylogenetic overdispersion, and competitive exclusion was the primary ecological process influencing community assembly. BA, ROR, and soil pH were the primary factors influencing 90 Nature Conservation 60: 73–97 (2025), DOI: 10.3897/natureconservation.60.162595 Qingling Pang et al.: Plant diversity on steep karst hillslopes the phylogenetic structure of the herb layer; soil pH, NH4 +-N, and CD strongly influenced the phylogenetic structure of the shrub layer; and soil EMg, ECa, pH, and SP had high explanatory power for the phylogenetic structure of the tree layer. Overall, the results show that deterministic processes (habitat filtering and competitive exclusion) have significant impacts on community assembly in the TKSR; however, some communities are also influenced by neutral processes. Furthermore, depressions and upper slopes have higher conservation value because of their higher levels of species and phylogenetic diversity in southwestern China. Additional information Conflict of interest The authors have declared that no competing interests exist. Ethical statement No ethical statement was reported. Use of AI No use of AI was reported. Funding This work was supported by the Guangxi Natural Science Foundation (2021GXNSFFA196005, 2021GXNSFAA196024, 2022GXNSFBA035633, 2022GXNSFBA035461); the Special Funding for Guangxi Bagui Young Top Talents Program (to Zhonghua Zhang); and the National Natural Science Foundation of China (31960275, 31760128). Author contributions Qingling Pang: Conceptualization; data curation; methodology; writing—original draft. Cong Hu: Funding acquisition; supervision; writing—original draft. Chaohao Xu: supervision; writing—original draft. Chaofang Zhong: supervision; writing—original draft. Zhonghua Zhang: Conceptualization; funding acquisition; supervision; visualization; writing—review and editing. Gang Hu: Project administration; supervision; visualization; writing—review and editing. Author ORCIDs Qingling Pang https://orcid.org/0009-0002-1168-7402 Cong Hu https://orcid.org/0000-0001-7507-4520 Chaohao Xu https://orcid.org/0000-0002-0437-5390 Chaofang Zhong https://orcid.org/0000-0002-3509-3628 Zhonghua Zhang https://orcid.org/0000-0003-2094-698X Gang Hu https://orcid.org/0000-0002-0662-811X Data availability All data that support the findings of this study are available upon request. References Bagaria G, Rodà F, Pino J (2019) Extinction and colonisation of habitat specialists drive plant species replacement along a Mediterranean grassland‐forest succession. 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Supplementary material 1 Environmental variables along the slope position gradient in a karst peakcluster depression landform Authors: Qingling Pang, Cong Hu, Chaohao Xu, Chaofang Zhong, Zhonghua Zhang, Gang Hu Data type: docx Copyright notice: This dataset is made available under the Open Database License (http://opendatacommons.org/licenses/odbl/1.0/). The Open Database License (ODbL) is a license agreement intended to allow users to freely share, modify, and use this Dataset while maintaining this same freedom for others, provided that the original source and author(s) are credited. Link: https://doi.org/10.3897/natureconservation.60.162595.suppl1