1. Introduction
Species diversity can positively affect community stability and ecosystem functions (Sharrock et al. 2014; D’amen et al.2017; Pennington et al. 2017), such as nutrient cycling (Maestreet al. 2012), resilience (Oliver et al. 2015), productivity and carbon storage (Liang et al. 2016; Chen et al. 2018; Liu et al. 2018a). Multi-species forests are better than monoculture to achieve higher carbon storage and productivity. Globally, a 10 % decrease in tree species richness would cause a decline of up to 3.1 % in forests’ productivity, which would equate to 13-23 billion US$/year (constant 2015 US$) (Liang et al. 2016). Moreover, species richness plays a central role in the identification of important areas of plant diversity (IPAs) to achieve Target 5 (protecting IPAs) of the Global Strategy for Plant Conservation (GSPC). IPAs can be identified according to criteria including species richness, endemic or internationally threatened species taking into account the provision of ecosystem services, because Target 5 has links to Target 4 (ecoregion conservation) of the GSPC (CBD 2012; Sharrock et al.2014 ).
Understanding the relationship between species richness and the environment is a key issue in ecology. Although the climate is considered to be one of the major factors driving species distribution and composition, the importance of habitat characteristics at a regional scale, is strongly associated with forest structure and composition (Zellweger et al. 2015). In forestry, species diversity has become the main goal, and this requires an understanding of the impacts of disturbance management on species diversity relative to natural drivers such as edaphic or climatic conditions (Schmiedinger et al. 2012). Multiple drivers such as climate, soil, and human influence interact to affect species diversity pattern and composition (Ulrichet al. 2014; Pennington et al. 2017), and this can be explained based on several hypotheses e.g. climatic seasonality, energy, water, and habitat heterogeneity (Currie et al. 2004; Shresthaet al. 2018). The water-energy hypothesis suggested that an area that has high energy and water availability will promote high species diversity (O’Brien 1998; Francis & Currie 2003). Human influence hypothesis was poorly investigated, meanwhile human disturbance causes habitat fragmentation and may have a stronger impact on species loss than global warming (Schmiedinger et al. 2012; Venter et al.2016). The hypothesis of habitat heterogeneity affects species richness by providing a space niche and more chances for diversifications (Moeslund et al. 2013; Stein et al. 2014). Moreover, geodiversity is a methodology that identifies land for conservation based on geological characteristics that correlate with the maintenance of biodiversity, so geodiversity hypothesis can improve our understanding and ability to model the relationship between species richness and abiotic heterogeneity (Bailey et al. 2017; Knudsonet al. 2018). In addition, hundreds publications have neglected some meaningful eco-physiological variables which have direct relationship to the potential species distribution such as soil available water, nutrients, carbon, potential evapotranspiration (PET), and solar radiation which regulates the composition and productivity of the ecosystem through photosynthesis (Piedallu & Gégout 2008; Modet al. 2016). Furthermore, several studies recommended the incorporation of geodiversity quantitative data such as topographic heterogeneity and soil nutrients into spatial modelling and conservation studies (Mod et al. 2016; Tukiainen et al. 2017; Baileyet al. 2017).
However, some studies have reported either a negative or positive relationship between soil nutrients and species richness (Huston 1980; Xu et al. 2016; van der Sande et al. 2018; Ali et al. 2019), soil nutrients and fertility in driving species richness is prone to controversy. In addition, conservationists have rarely incorporated the quantitative geodiversity data, particularly soil nutrients and carbon stock into conservation prioritization efforts (Comer et al. 2015; Tukiainen et al. 2017). Moreover, recent studies recommended the integration of biodiversity, ecosystem services (e.g. soil fertility), topographic and anthropogenic factors for conservation planning and establishing protected areas (Xu et al. 2017; Tripathi et al. 2019). Since species richness enhance carbon storage (Poorter et al. 2015; Chen et al. 2018; Liuet al. 2018a), so we hypothesized that carbon stock would be the main indicator of richness pattern and link with conifer species richness along with other correlated variables that might be soil nutrients.
Conifer forests are a terrestrial biome occurring in tropical, subtropical, boreal, and temperate climates of the world (Olson et al. 2001). Globally, more than fifty percent of the temperate conifer forests located in Asia primarily in China which includes seven ecoregions, most of them (5 of 7) are threatened and located in south-western China (Farjon & Filer 2013; Wan et al. 2017; Dakhil et al. 2019). These forests are ecologically and economically important for carbon sequestration and stabilizing river banks, providing timber, medicinal products, food, and shelter for a wide range of threatened and endemic animals such as red pandas, musk deer, and giant pandas (Miao & West 2004; WWF 2018). Nearly 50% of conifer species are threatened due to anthropogenic activities, e.g. logging, overgrazing, and tourism (Miao & West 2004; López-Pujol & Zhao 2004; Farjon et al. 2019). Wan et al. (2017) and Dakhilet al. (2019) defined Sichuan, Yunnan and south-eastern part of the Tibetan Plateau as conservation priority areas of conifer forests; so attention should be given to this region because the geographical range of most of the tree species in south-western China will be considerably reduced due to human activities and climate change (Zhanget al. 2014; Dakhil et al. 2019). In addition, the nature reserve system of China primarily focused on mammals, meanwhile habitats of plant diversity were not well covered (Xu et al. 2017), thus we hypothesized that IPAs (areas with high richness of endemic and threatened conifers) might locate outside the nature reserve system, so these IPAs should be proposed as new nature reserves for protection to achieve GSPC targets (Ren et al. 2019). Therefore, the objectives of current study were: 1) to evaluate the patterns of conifer richness at vulnerability and endemism levels in south-western China, 2) to assess the relative importance of the predictors of environmental (energy, climate, water, and soil) and human-influence hypotheses on the conifer richness pattern, 3) to determine which set of predictors best explains the richness pattern across the different richness categories of endemic and threatened species, and 4) to identify the hotspot ecoregions, nature reserves or important conifer areas (IPAs) for conservation priority and proposed nature reserves. To our knowledge, this is the first integrative study incorporated soil quantity data as well as human-influence data into species richness modelling at the vulnerability and endemism levels. Thus, such a combination of these predictor variables would help to understand the collaborative interaction between species richness and environmental as well as the socio-economic processes, and hence, this will provide greater insights into effective conservation planning of the ecosystem.