4.1 Resource availability plays a large role for shaping plant community species composition
Our findings highlight that species diversity and abundance were both significantly associated with resource availability in karst ecosystem (Fig. 2a, b, c, d), which completely supports the resource availability hypothesis. Similar results have proven that the availability of single variables (soil organic matter, phosphorus, nitrogen, etc.) was closely related to species diversity (Ou et al. 2014), but our study further focused on the effect of integrated environmental variables on species diversity. It is important to note that the total amount of resources in karst regions is very limited because of the slow and patchy development of nutrient-poor soil over exposed rock (Kavouri et al. 2011). Given these resource limitations, species diversity and abundance significantly increased with increasing resource availability in shrubland and the tree layer of woodland. Broadly speaking, karst woodland with better soil environmental conditions could provide enough nutrients in terms of shrub growth (Asensio et al. 2013), therefore the negative resource availability-species diversity relationship was most likely because dense overstory canopy intercept available light (Neufeld and Young 2014; Zhang et al. 2017).
Meanwhile, plant community abundance significantly increased with increasing resource heterogeneity in the shrub layer and tree layer of karst woodland, corroborating the findings in other ecosystems (Tilman 1982; Silvertown 2004; Stein et al. 2014). This is because karst woodland with low fragmentation degree have a relatively mild soil environment for plant survival and growth. Increasing heterogeneity provides enough nutrients and more niches to seed germination and plant growth, allowing for increased community abundance in woodland. Therefore, the significant relationship between resource heterogeneity and community abundance also favour the resource heterogeneity hypothesis. Given that plant community abundance in karst region was significantly associated with resource availability, as well as resource heterogeneity, which suggested that resource availability and heterogeneity as two important drivers of community abundance is not mutually exclusive in karst shrubland and woodland. Our variation partitioning further elucidate the relative importance of resource availability and heterogeneity in determining community abundance. It is clear that a large proportion of variation in community abundance was explained by unique resource availability, with only a small amount explained by unique resource heterogeneity (Fig. 4), demonstrating that resource availability had a much higher explanatory power for the variation in community abundance compared with resource heterogeneity.
Species diversities of tree and shrub layer of woodland were not related with resource heterogeneity fully confirmed that resource availability was more significant predictor of species diversity rather than resource heterogeneity. This finding also corresponds with those from other ecosystems, where species diversity appeared to be determined by resource availability rather than resource heterogeneity (Baer et al. 2004; Lundholm 2010). The degree of rock fragmentation in karst shrubland is far higher than that in karst woodland (Liu et al. 2019), thereby plants are likely to face resource deficiency in the highly heterogeneous karst shrubland. Additionally, plant individuals cannot survive when the soil patch size is relatively smaller than the plant root size (Lundholm 2009; Gazol et al. 2013; Tamme et al. 2016; Schuler et al, 2017). To the extent that the size of the root system of individual plants is more likely to exceed the soil patch size with increasing environmental heterogeneity in karst shrubland, so that plants located in unfavorable soil patches face an increased risk of mortality (Laanisto et al. 2013). As a consequence, species diversity and abundance significantly decreased with increasing resource heterogeneity mainly because resource availability limits survival and growth of plant individuals in the highly heterogeneous karst shrubland, while increasing resource heterogeneity does not work when soil and space resource is extreme shortage in karst shrubland.