Geographical distribution patterns of orchids in the Hengduan Mountains
Within global biodiversity hotspots, diversity and threats distribute unevenly (Cañadas et al., 2014; Harris et al., 2005; Murray-Smith et al., 2009), and the harsh reality is that although funding for biodiversity is increasing, there is still a large gap between it and the actual resource needs (Waldron et al., 2013). Hence, understanding the geographic distribution patterns of species and identifying regional conservation priorities, are more conducive to the use of special funding, optimizing the structure of funding, and improving the efficiency of conservation.
The Hengduan Mountains are one of the global biodiversity hotspots within which the orchid family is represented brilliantly (David, 2014; Marchese, 2015; Yu et al., 2020). Like other hotspots, the Hengduan Mountains have geographically diverse and highly heterogeneous environments to create favorable conditions for plant diversity, species formation, and dispersal (Wang et al., 2012). They are commonly considered important drivers of diversity (Crain & White, 2013; Perez-Escobar et al., 2017). This may explain the breadth of orchid distribution in the Hengduan Mountains under either modeling strategy. According to the suitability maps, the terrestrial and overall orchids have a similar spatial distribution pattern. It is consistent with the research about the local orchid flora that the temperate terrestrial orchids occupy a critical component in the Hengduan Mountains (Lang, 1990). The results of the double ranking allow us to obtain some critical geographical regions of orchids, mainly concentrated in the Minshan Mountain System in the northeastern part of the study area, gradually extending southwestward to the Shaluri Mountain System (the widest range of the Hengduan Mountains) in the central part of the study area. These diverse mountainous areas could be consistent with the assertion of the peak level of diversity in the mountains (Acharya et al., 2011; Zizka & Antonelli, 2018). Other regional orchids’ geographic research has demonstrated the rapid growth of mountain ranges and geological activities as the main drivers of orchid evolution and species formation (Crain & Fernandez, 2020; Dodson, 2003; Kirby, 2011).
The geographic distribution pattern of the mycoheterotrophic orchids largely coincided with the spatial patterns of the two above, but the double-ranking results showed inconsistent critical regions. For mycoheterotrophic orchids, the middle and south ends of the Minshan Mountain Range (the easternmost mountain range of the Hengduan Mountains) in the northeastern part of the study area are the critical regions for their geographic distribution (Jiuzhaigou and Pingwu counties). This region belongs to a typical subtropical mountain climate with cold winters and cool summers, abundant rainfall but insufficient heat, and coniferous forests developed in high mountain valleys. Combined with the model variable importance results (see Appendix S3.4 for details), vegetation and bio10 (the hottest quarterly mean temperature) play a more significant role in the distribution model of mycoheterotrophic orchids. From this, we infer that the environmental preference of mycoheterotrophic orchids is for colder and wetter environments compared to terrestrial orchids in terms of temperature and precipitation. And may be more specialized to mycorrhizal environments generated by specific vegetation (Djordjevic et al., 2020; Kelly et al., 2013; Selosse et al., 2004). The geographic distribution pattern of epiphytic orchids is dissimilar. The southwestern mountains of the study area are the critical region for their distribution, which belongs to the Nujiang River valley and the Gaoligong Mountain system. The north-south longitudinal valley provides favorable conditions for the penetration of warm and humid airflow brought by the southwestern monsoon. The high temperature and abundant precipitation provide sufficient survival opportunities for epiphytic orchids. The analysis of environmental variables also showed that bioclimatic variables representing precipitation (bio13 and bio15) play a more significant role in the model of epiphytic orchids. This also coincides with the physiological characteristics of epiphytic orchids, where high temperatures and sufficient precipitation are the main ecological requirements (Zhang et al., 2018; Zotz & Hietz, 2001).
The environmental preferences of different life forms of orchids lead to geographically distinct spatial distribution patterns, suggesting that various geographic attributes could support separate centers of orchid diversity (Crain & Fernandez, 2020). It further illustrates that pre-classification of orchid occurrences to achieve a more accurate match with environmental information enables distinction of these centers of diversity and better identification of conservation priorities.