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).