4.2 Multiple direct and indirect processes of resource availability driving plant community species composition
Our SEMs further disentangled the multiple direct and indirect processes by which species diversity was influenced by the availability of each variable. Species diversity and abundance were mainly controlled by soil TN and soil moisture in shrubland (Fig. 5a, b, Fig. 6a, b, Table A2). Similar results have been found in karst region, where plant community structure was strongly influenced by soil TN during the early successional stages (Zhang et al. 2015; Li et al. 2017). In fragmented karst shrubland, a large amount of exposed bedrock and an extremely shallow soil depth lead to a shortage of soil resources, therefore, soil is highly TN deficient, which significant influences species diversity and abundance. Additionally, the soil TN is mainly originated from the slow release of soil organic matter (Jobbagy and Jackson 2000). However, the severe water loss along stone crevices and lots of water evaporation result in temporary droughts in karst soil patches, especially in shallow soil patches (Wang et al. 2003), which greatly affects the release processes of soil organic matter, further exacerbates the soil nutrient deficiency (Moyano et al. 2013). Thus, soil moisture had positive indirect effects on both shrub abundance and species diversity through affecting soil TN (Fig. 5a, b, Fig. 6a, b, Table A2). Furthermore, soil depth had a significant positive effect on species diversity due mostly to directly affecting plant survival space and soil resource level.
As a whole, the soil nutrient level of woodlands higher than that of shrublands because a large number of litters were accumulated in karst woodland (Zhang and Pan 2012; Islam et al. 2015), therefore woodland could provide enough nutrients in terms of shrub survival and growth, and soil nutrient level of woodlands did not limit the increase of shrub abundance. Soil depth was the significant predictor of shrub abundance in woodland, which was attributed to the fact that shallow soil always exposes a large amount of bedrock surface, but deeper soil provides more growing space for shrubs (Lundholm 2010). In contrast, species diversity was mainly positively influenced by soil TK, and negatively influenced by soil depth (Fig. 5d, Fig. 6d, Table A3), which demonstrated that plant species tend to occur and grow in shallow soils with high nutrient content. This pattern of shrub diversity in woodland was probably because other biological factors dominate the spatial distribution of different species, as well as was supported by the fact that shallow limestone soil is rich in organic matter, nutrients (N, P and K), and Ca (Zhang et al. 2010).
For the tree layer of woodland, soil depth and pH had significant positive and negative effect respectively on abundance and species diversity, which indicated that the deep soil with a high nutrient level and growing space, as well as low acidity, was favorable for the growth and survival of high-biomass wood species. Additionally, soil Mg had a significant negative influence on abundance, while the negative effect originated mostly from the indirect affecting soil pH. The result further suggested that soil pH is an important determinant for shaping tree abundance in the forest dominated by Pinus massoniana L., possibly because P. massoniana prefers slightly acidic soil rather than alkaline limestone soil. However, the species diversity was mainly negatively influenced by soil nutrients (TN and TK) was because of the cause that the significant negative relationship between soil nutrient content and soil depth in karst woodlands.
Conclusions
Our study provided novel insights to quantify and compare the effects of resource availability and heterogeneity on plant community characteristics in the most fragile karst landscapes in southwestern China. Resource availability and heterogeneity both played a certain role in determining karst plant community composition, while the importance of resource availability far exceeded resource heterogeneity, thereby tending to support the resource availability hypothesis. Thus, the resource availability of shrubland and woodland should be more important for protecting and restoring objects than resource heterogeneity, especially in highly fragmented shrubland. Our SEMs further demonstrated that the multiple direct and indirect processes of resource availability determined karst species diversity and abundance simultaneously, whereas the multiple pathways were different in different vegetation types, emphasizing that steering community restoration and reconstruction also have to take into account multiple pathways of resource availability for structuring different karst community types.
Supplementary material: Appendix A.
Acknowledgements
This study was funded by the National Key R&D Program of China (2016YFC0502304). We thank Xinjia He, Zhaojun Wei, Xiaoxian Lu and Lin Peng who helped with materials collecting and field work at Youyang, Yinjiang and Beibei.