Beta Diversity Patterns and Determinants among Vertical Layers of Tropical Seasonal Rainforest in Karst Peak-Cluster Depressions

Understanding the mechanisms that underlie the formation, maintenance, and loss of biodiversity is central to community ecology, as they play a crucial role in promoting sustainable development and effective conservation of biodiversity [1,2]. Beta diversity, which reflects variation in species composition among communities along spatial, temporal, or environmental gradients, is an important aspect of biodiversity [3]. Species beta diversity, specifically, captures differences in community composition over larger distances and species turnover across small-scale environmental gradients [4]. Phylogenetic beta diversity, however, involves measuring evolutionary relationships among species and how they change along environmental gradients, thus influencing community assembly [5,6]. Loss of beta diversity can lead to biological homogenization and a decline in ecosystem functioning [7]. Therefore, there has been increasing interest in the analysis of community assembly processes that drive patterns of beta diversity [8,9].

Beta diversity, which comprises measurements of variation in species composition among communities, can be divided into two processes: species turnover (or replacement) and richness differences (or nestedness) [10]. These processes differ and influence the spatiotemporal distributions of species, resulting in complex patterns of community similarity [11]. However, previous studies have often paid little attention to the different processes contributing to species-level differences between communities [12]. Understanding the mechanisms driving these patterns depends crucially on differentiating the underlying processes that contribute to beta diversity [13].

Species turnover, reflecting the replacement of species between different locations, is influenced by mechanisms including habitat filtering, competition, and geographic isolation [14]. For instance, natural selection along an environmental gradient can lead to different species occurring in habitats that are suitable for their survival [14], whereas geographic isolation caused by mountain uplift can result in population isolation and the formation of allopatric species [15]. In contrast, differences in richness reflect variation in species composition between communities and can be caused by species loss or gain along an environmental gradient or across the entire study area. The mechanisms influencing such differences include the diversity of available ecological niches and ecological processes leading to nestedness [16,17]. To gain a comprehensive understanding of the processes driving beta diversity and to implement effective biodiversity conservation strategies, assessing the relative contributions of species turnover and richness differences to the overall patterns of beta diversity is important.

Traditionally, beta diversity quantifies community diversity based on species classification, abundance, and differences in species composition between ecological communities [18,19]. However, a limitation of this approach is that communities may contain species with redundant evolutionary relationships, and changes in species composition often overlook species-level phylogenetic relationships [20]. Phylogenetic beta diversity provides a complementary perspective by addressing differences in evolutionary relationships between communities and highlighting the impact of historical processes on community assembly [21]. Therefore, to accurately capture the ecological processes and mechanisms underlying beta diversity, understanding it from both taxonomic and phylogenetic perspectives is essential [20].

When considering the ecological processes influencing beta diversity, there are two main perspectives. Some researchers argue that stochastic processes such as random speciation, dispersal, and extinction are sufficient to explain beta diversity patterns [22,23], whereas others contend that differences in species’ ecological niches play crucial roles in shaping beta diversity patterns, with deterministic processes such as habitat filtering and competitive exclusion being important factors in aggregating species diversity into deterministic states [24]. Community assembly is thus driven by both deterministic and stochastic processes, although quantifying the relative importance of each type of process is challenging owing to potential variation with spatial scale and to the quality and quantity of environmental data available [25]. To a certain extent, understanding the spatial patterns of beta diversity in relation to geographic distance and environmental differences can provide insights into the relative importance of deterministic and stochastic processes, currently a hot topic in ecology [26]. Karst regions, comprising approximately 15% of the world’s total land area [27], have received limited attention in terms of vegetation science compared to other ecosystems [28]. Northern tropical karst seasonal rainforest is a notable forest vegetation type found in the karst regions along the northern boundary of the tropics [29]. This forest type possesses distinctive features, such as a diverse community structure, rich tree species composition, and a prominent presence of endemic elements, primarily because of its geochemical background characterized by high levels of calcium and alkalinity, its diverse habitat types, and the influence of the monsoon climate [25]. Previous research has identified an aggregated distribution pattern among numerous tree species in karst regions; this may be attributed to limited dispersal distances or a narrow ecological niche [30]. Furthermore, the composition of plant species within communities varies across different habitats, and most karst tree species exhibit a stronger association with a particular habitat [31]. The interaction between species and the environment drives substantial shifts in species assemblages among diverse habitats, consequently influencing alterations in beta diversity. By assessing plant species composition and distribution in biodiversity hotspots influenced by environmental gradients, we can considerably advance our understanding of the local plant community and the effects of environmental factors on these communities [32]. Nevertheless, the mechanisms driving the formation of karst forest communities, which are undergoing notable microhabitat changes, are poorly understood.