4.4. The effects of soil type, fertility, and nutrient availability on conifers richness
Our findings support the assumption that site-specific habitat factors, e.g. topography and soil, determine the pattern of species richness besides climate at the regional scale. The direction and strength of the determinants vary among taxa indicating a functional relationship between site conditions and the respective plant community (Zellwegeret al. 2015). Climate and soil types have a significant role in shaping plant richness patterns along the elevation gradient in Eastern Himalaya (Sharma et al. 2019), and this supports our finding that soil variables were the second-strongest predictors after topographic heterogeneity explaining the variance in all richness groups (see Fig. 2). Furthermore, Sharma et al. (2019), found similar findings that soil type was the second strong predictor and explained 36.4% deviance in the species richness pattern. Soil texture type and nutrient availability explained significant contributions, and similar findings were found in boreal conifer forests in Europe (Germany) and North America (Canada) (Schmiedinger et al. 2012). In detail, among the soil-type predictors, explanatory sand variable showed the highest significant positive contribution to all richness groups and this finding explaining the importance of sandy soils in the presence of high carbon which leads to an increase of the water-holding capacity, so soil aggregate stability increased with a greater number of macropores. The improvement of macropores and soil stability will increase soil infiltration capacity and enhancement of root penetration and subsequent boosting the growth of seedlings of various species. Therefore, soil type is an important factor, and ecological tool affecting vegetation dynamics and contributing to the sustainable management of conifer forests (Steinet al. 2014; Quesada & Lloyd 2016; Yao et al. 2019). In comparison to climatic water predictors, soil water availability explained and contributed more (see Table 1), and this result supports the finding that variables of soil water balance performed better than that of climatic water in the tree species distribution modelling (Piedallu et al. 2013). Furthermore, previous studies highlighted the positive influence of water and the importance of soil moisture availability to habitat quality and plant communities; where the abundance of soil water causes more activation of the weathering processes and produces more nutrients for plants (Hjort et al.2015; Moeslund et al. 2013).
CEC is an ecological indicator of soil fertility, and highly explained the variation in plant species richness in China (Zhang et al.2016) supporting our results that soil-fertility indicators (carbon and CEC) are main relevant drivers of the conifer richness pattern, and this is because soil fertility has a direct and indirect effects on the plant species growth, potential distribution, and thereby species composition and richness dynamics (Mod et al. 2016; Quesada & Lloyd 2016; van der Sande et al. 2018; Ali et al. 2019). Furthermore, several recent studies of biodiversity-ecosystem functioning supported our finding of the significant positive contribution of the available nitrogen and phosphorus in the variation of conifer richness pattern and suggested that abundance of soil biota increases the decomposition due to high availability of soil nitrogen and phosphorus. The diverse forms of such nutrients may promote plant diversity through partitioning of resources when species differ in their preference for different forms of these nutrients and subsequent rapid forest regeneration (Lalibertéet al. 2014; van der Putten et al. 2016).
Available soil phosphorus didn’t have a significant correlation with other soil variables (see Table S4, Appendix A) indicating to the significant and independent role of phosphorus in the soil spatial heterogeneity, forest ecosystem functioning and thereby community dynamics (Laliberté et al. 2014; Xu et al. 2016). This finding strongly supports the suggestion that soil fertility, especially phosphorus, increases species richness and forest productivity (van der Sande et al. 2018). Furthermore, our findings supports our soil nutrient-fertility hypothesis which could be used as evidence for the mutual interaction between species richness and ecosystem functions linked to carbon and nitrogen cycling, which sustain soil fertility and carbon sequestration (Maestre et al. 2012; Laliberté et al. 2014; Quesada & Lloyd 2016; Liu et al. 2018a). Also, soil carbon and nitrogen stocks increases the recovery of conifer species richness of the secondary forests at high elevations, and this finding strongly support our significant positive explanatory power of carbon stock on the richness pattern implying that global warming might decrease the soil carbon and nitrogen stocks at high elevations and thus may negatively affect the recovery and dynamics of conifer richness (van der Putten et al. 2016; Tashi et al. 2016; Chen et al. 2018; Liu et al. 2019).