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.