4.3. The effect of topographic heterogeneity and human-influence factors on conifer richness
Our results supported the positive topographic heterogeneity-diversity relationship (see Table 1) which promotes the coexistence and diversification of species by offering niche spaces and this agrees with the findings of several studies (e.g. Stein et al. 2014; Lee & Chun 2016; Shrestha et al. 2018; Sharma et al. 2019; Wanget al. 2019). Topographic heterogeneity was the main strongest driver affecting conifers richness pattern, and this result has been reported in several studies for tree species of Rhododerndron in south-western China (Shrestha et al. 2018), rare woody plant species in Inner Asian drylands (Liu et al. 2018b), and conifer species in south-east Alaska (Bisbing et al. 2016). The high richness of endemic conifers in Qionglai-Minshan and Hengduan Mountains may be rooted to the complex topography which creates a wide range of habitats thereby providing more resources and supporting a high number of species (Bisbing et al. 2016; Shrestha et al. 2018). Furthermore, historically high rate of plant diversification in south-western China was due to topographical isolation based on a recent study of molecular phylogeny (Xing & Ree 2017; Shrestha et al. 2018). Hence, heterogeneity is thought to have a significant effect on the structure and dynamics of plant communities (Lee & Chun 2016; Moeslund et al. 2013). Moreover, topographic heterogeneity is a significant effective variable used as a proxy of habitat heterogeneity by controlling temperature, distribution of soil water, and nutrient availability particularly at high elevations, and thereby controlling species’ occurrence and plant diversity in landscapes (Hofer et al. 2011; Bisbing et al.2016; Tashi et al. 2016). This supports our finding that temperature had a negative correlation with conifer richness, but the elevational range and soil nutrients had a strong positive correlation (see Table 1).
The present study showed that, the negative correlation between the human-influence factor and conifer richness was comparatively weak; it is probably to have a significant influence in the areas with high human population density (Panda et al. 2017). The contribution of human-influence explained 4%, and this was greater than that of water availability and climate seasonality indicating the significant effect of human activity on the conifer richness of endemic-threatened species due to logging activities (Schmiedinger et al. 2012; Venteret al. 2016), particularly Abies fargesii var.faxoniana, Abies fabri, Abies recurvate, Picea likiangensis, Picea aurantiaca, Pinus yunnanensis, Pinus henryi, Cupressus chengiana(López-Pujol and Zhao 2004; Miao & West 2004; Threatened Conifers of the World 2019; Farjon et al. 2019). Anthropogenic activity in south-western China is lower than that of Europe, or northern America maybe because most of our study conifers distributed at higher elevations compared to Europe and America, so the human influence is less (Schmiedinger et al. 2012; Threatened Conifers of the World 2019). In comparison, the effect of human influence factor showed less contribution to the richness pattern than energy, topography, and soil factors, and this finding supported by the results of Wang et al.(2010), who studied the tree species richness with tropical and temperate affinities in China. Therefore, China provides future insight and promising model for long-term studies of forest management.