4.2. The effects of energy, climatic seasonality, and water availability on conifers richness
Effects of water availability and climatic seasonality, particularly precipitation, were relatively weak compared to environmental energy, and this may be due to the differences among conifer species in the demand of water (Liu et al. 2018b). This finding supports the hypothesis that environmental energy limits regional species richness (Hawkins et al. 2003) showing significant negative relationship, particularly with the endemic-threatened conifers. The energy variables (PET) explained the richness pattern better than water variables (AI), and this agrees with Panda et al. (2017), who studied the plant species richness in western Himalaya. PET was found to decrease along the elevation gradient; but, its low level had a positive correlation with species richness and this opposite to our results might due to the different elevations and taxa where higher plants had different response to environment compared to gymnosperms (Rueda et al. 2018) and also the regional effect is driven by topographic and edaphic factors (Zellweger et al. 2015). Moreover, PET saturates at an elevation above 900 mm with no significant variations, and this indicates the indirect influence of elevation on the pattern of species richness, which supports our finding of topographic heterogeneity. In other words, in Eastern Himalaya and Alxa Plateau of China, species richness showed a significant negative relationship with temperature (Sharma et al.2019; Wang et al. 2019) and this supporting our findings particularly MAT and MTCQ.
On the contrary, positive relationships were reported in China and explained the pattern of species richness of Gesneriaceae (Liu et al. 2017), and this may be because Gesneriaceae species comparatively prefer warmer habitats; so, high elevations with cool climate are probably not favourable for their distribution, while favourable for conifers distribution (Fang et al. 2011; Liuet al. 2017). Moreover, the negative effect of energy on conifer richness indicates that endemic-threatened conifers may face high risk of extinction in the future due to global warming (Ye et al.2015) , and this indication supports the finding of conifer forests range-shift under climate change (Dakhil et al. 2019) and the suggestion of Rosenblad et al. (2019) who implied that species currently restricted to environments, e.g. alpine or endemic island species, are at risk of extinction due to climate change. Furthermore, the relationship between species richness and climate can be influenced by soil fertility in forest ecosystems (Zhang et al. 2014).
In contrast, Wang et al. (2010) reported that water availability explained the pattern of woody species richness more than energy, climatic seasonality, and topographic factors and this might be due to variations among woody species, and the definite community type (i.e. conifer forests) which may have characteristic response, as recommended recently we focused on conifer community type at regional scale (Zhanget al. 2014; D’amen et al. 2017). The environmental drivers of energy, water, and seasonality showed variation across the different four richness groups, and this may be due to the microtopographic heterogeneity supporting the endemic or threatened conifers (Hofer et al. 2011; Zellweger et al. 2015). Furthermore, the different combined effect of these drivers supports the suggestion that relative effect of drivers varies among taxa with different levels of vulnerability and endemism (Shrestha et al.2018; Liu et al. 2018b).