1. Introduction
It is highly likely that global temperatures will exceed a 1.5 °C increase in the future [
1]. The Intergovernmental Panel on Climate Change (IPCC) has emphasized in its Sixth Assessment Report (AR6) that surpassing the 1.5 °C threshold would cause the extinction of indigenous species and the loss of local habitats, posing a significant threat to future biodiversity [
2]. High latitude regions, due to their cold climate, experience a faster warming rate than the global average. The effects of climate change on the plateau region are far-reaching, encompassing the melting of glaciers, a downward shift in snowlines, and the expansion of desertification, among others. These changes pose a significant threat to the region’s biodiversity [
3]. High-altitude medicinal plants, which are integral to the plateau’s ecological makeup, are particularly vulnerable and face an elevated risk of extinction compared to others [
4]. In order to further safeguard global biodiversity and foster regional economic development, it is imperative to comprehend the interplay between the potential geographic distribution of species and climate change. This knowledge will enable us to identify the potential geographic distribution areas for species in the face of future climate change scenarios and devise adaptive conservation strategies.
Gymnosia orchidis Lindl. (
G. orchidis) is a vulnerable species (VU) belonging to the Gymnadenia genus in the
Orchidaceae family. It holds the highest extinction risk among numerous plant species [
5,
6].
G. orchidis is known as “Wangla” in Tibetan medicine. The rhizomes of this plant contain compounds such as dactylorhin B and dactylorhin A [
7]. It possesses significant medicinal value, including the treatment of lung deficiency, bleeding, and cough. It also plays a crucial role in the combined treatment of type 2 diabetes (T2DM), where its tuberous root and pumpkin seeds are utilized [
8]. Relevant studies have indicated a positive correlation between the compounds found in
G. orchidis and its growth age. Furthermore, moderate light intensity has been shown to enhance the production of chemical substances [
7,
9]. However, research on
G. orchidis has primarily focused on its pharmacological actions, with limited publications on predicting its potential suitable zones [
10]. This knowledge gap is concerning given the species’ longer growth cycle and reliance on wild resources, making it increasingly rare [
8,
11]. The wild resources of
G. orchidis are continuously declining, with low production but increasing demand each year [
12]. Therefore, urgent attention is required for the conservation and cultivation of
G. orchidis. The temporal and spatial heterogeneity of
G. orchidis is pronounced, with suitability continuously expanding from the MH (Mid-Holocene). The major distribution areas are situated in Qinghai, Yunnan, Tibet, and Sichuan, in China. With its growth environments being hillside forests and alpine grasslands at an altitude of 2800–4100 m, there is a wide variation in the quality of regional resources [
11,
13]. Hence, it is crucial to have a comprehensive understanding of the potential geographical distribution and the suitable habitat conditions of endangered medicinal plants as a prerequisite for conducting effective conservation work. However, in practice, there is a limited availability of data on the geographic distribution of endangered medicinal plants on the plateau region.
The Qinghai–Tibet Plateau (QTP), located at 67–105° E and 25–40° N, is a crucial region for
G. orchidis distribution. With an average elevation of over 4000 m above sea level, the QTP exhibits distinctive characteristics, including high altitude, diverse vegetation distribution, and pronounced zonal differences [
13,
14,
15]. The eastern and southeastern regions of the QTP are renowned for their abundant plant biodiversity. Nevertheless, the suitable habitat range for medicinal plants in these areas has been progressively shrinking, particularly in the eastern parts [
16]. This decline can be attributed to the intricate topography and land characteristics of the region [
17], as well as its plentiful water resources and favorable climate. Moreover, the relatively stable environment during glaciation periods has had an impact on the distribution patterns of medicinal plants [
18]. These factors contribute to the creation of unique conditions that provide a suitable habitat for
G. orchidis. Moreover, the QTP functions as a highly sensitive and vulnerable ecological screen to climate variability, not only in China but also across Asia as a whole [
19]. Recent studies have indicated a warming and wetting trend in the QTP over the past 40 years, which is projected to continue. These changes not only impact the physiological and biochemical processes of various plant species, increasing the risk of extinction for endangered species like
G. orchidis, but also have the potential to alter the global vegetation ecological pattern, posing significant threats to organism and ecosystem diversity [
20]. Consequently, it is crucial to explore potential suitable areas for endangered species in the future to facilitate biodiversity protection and maintain ecosystem balance in this region [
21].
The complex and diverse environment of the QTP presents difficulties in surveying numerous medicinal plants, and the absence of accurate species distribution maps adds to the challenges faced in conservation work [
4]. Species distribution modeling (SDM) is employed as the primary method to simulate suitable habitat areas and predict distribution changes based on environmental variables [
22]. Commonly used SDMs are the Genetic Algorithm for Rule-set Production (GARP), Climate and Expertise (Climex), and Maximum Entropy model (MaxEnt) [
23,
24]. Among the SDM approaches, the MaxEnt is widely used due to its simplicity, rapid processing, and high prediction accuracy, even with limited distribution points [
25,
26,
27]. The MaxEnt has good applications in predicting prehistoric geology and future climate scenarios and is particularly suitable for predicting suitable areas for endangered species, medicinal plants, and alpine plants, as well as assessing their response to future climate changes [
4,
28,
29,
30].
This study focuses on G. orchidis and aims to simulate the current distribution pattern of its suitable areas in the QTP using the MaxEnt. Furthermore, it seeks to predict the potential suitable areas in paleo and future time periods to evaluate the spatial distribution of G. orchidis on a temporal scale. The ultimate goal is to develop a numerical model that accurately predicts the suitable areas for G. orchidis, facilitating informed introductions and resource conservation efforts. This study has the potential to catalyze regional biodiversity conservation and contribute to the development of related industries.
4. Discussion
Hydrothermal conditions have been identified as the most important variables influencing the geographical distribution of plants, particularly terrestrial plants [
35]. These conditions also play a key role in explaining the association between species richness and terrain [
48]. In our study, we found that annual precipitation (Bio12) and mean temperature of coldest quarter (Bio11) were the dominant variables influencing the distribution of
G. orchidis. This suggests that temperature and rainfall are the driving mechanisms for
G. orchidis. Secondary variables such as min temperature of coldest month (Bio6), temperature seasonality (Bio4), and precipitation of coldest quarter (Bio19) further indicate that rainfall and temperature changes during the coldest quarter can influence the growth of
G. orchidis. These findings align with the overall habitat characteristics of the
Orchidaceae family [
49] (
Table 1). This study found that
G. orchidis has a limit threshold for cold and drought resistance at −14.0 °C and 11 mm, respectively; this phenomenon is likely a result of the combined effects of the uplift of the plateau during the Neogene period (Miocene and Pliocene) and the climatic fluctuations throughout the Quaternary period. These geological and climatic factors have played a crucial role in shaping the evolution of alpine plants, enabling them to develop unique traits such as cold and drought resistance [
16]. Furthermore, it also found that
G. orchidis shows a preference for southern slopes and an annual precipitation of 900 mm. This phenomenon highlights that the suitable habitat for these plants tends to be in regions with moderate sunlight exposure and ample availability of water and favorable thermal conditions [
50]. This could be attributed to the fact that sunlight exposure can increase the production of chemical compounds in
G. orchidis and promote its growth and development [
9]. Additionally, studies have suggested that the mountains on the southern slope of the study area experience higher temperatures and precipitation compared to the northern slope [
51,
52,
53]. This indicates that the southern slope provides a sheltered environment to resist extreme cold conditions, which may be related to the diverse climate types in this region [
49]. It is important to note that this study focused solely on the influence of climate, topography, and soil type on species distribution and did not consider biological factors such as interspecific relationships and anthropogenic forcing. Other factors, including genetic variation and soil environments, should be considered in future studies [
35,
49,
54,
55,
56]. Therefore, further research is needed to explore the dominant factors affecting
G. orchidis.
The moderate and high mountainous areas in regions with high precipitation, temperature, and humidity provide suitable habitats for rare species, including
G. orchidis [
57]. Our study predicts that the potential suitable areas for
G. orchidis will change under different climate scenarios over time, showing significant fluctuations in suitability. The results from the MaxEnt indicate that the distribution range of
G. orchidis along mountains such as Henduan, Yunlin, and Longmen has consistently expanded, especially during the Mid-Holocene (MH) when increased precipitation driven by solar radiation occurred [
15]. Previous studies have also suggested that terrestrial herbs gradually became dominant on the QTP during the MH. The northwestward movement of the species’ centroids further indicates the expansion of suitable areas for
G. orchidis (
Figure 10) [
13]. During the glacial period, the suitability for
G. orchidis was reduced, likely due to extensive ice coverage on the QTP [
58]. The increase in highly suitable areas of
G. orchidis during the LGM was more likely attributed to the southern slope of the mountains serving as a glacial refuge for
G. orchidis [
51]. Under different future scenarios, the suitable areas for
G. orchidis continue to expand towards the northwest, including the provinces of Qinghai and Tibet, particularly in the SSP370 scenario (projecting a temperature rise of 3.6 °C by the year 2100), where the expansion ratio reaches 46.19%. However, there is a loss of suitable areas in the SSP126 scenario (projecting a temperature rise of 1.8 °C by the year 2100), with contraction areas mainly concentrated in the Himalayas, possibly due to reduced precipitation [
50,
59]. Significant changes are observed in the SSP585 scenario (projecting a temperature rise of 4.4 °C by the year 2100), with the rate of change in suitable areas ranging from 1.4% to 22.3%. The centroid of
G. orchidis in the 2050s approaches the centroid of the SSP126 scenario in the 2070s, indicating that the climatic conditions of the SSP126 scenario in the 2070s may be similar to those in the 2050s under the SSP585 scenario on the QTP. The multidimensional migration of centroids also suggests that changes in suitability may be influenced by carbon dioxide (CO
2) emissions [
33,
36,
60]. Increased CO
2 concentrations can improve the survival of
G. orchidis and align with the future growth trend of the Orchidaceae family as a whole [
49]. However, human activities can disrupt suitable areas and force species migration, which is a complex issue. If a suitable environment is not found during the migration process,
G. orchidis may be at risk of extinction [
57,
61]. As an extremely rare and environmentally sensitive Tibetan medicinal resource, the habitat conditions of
G. orchidis are scarce and difficult to recover once lost. Therefore, proactive measures should be taken to reduce emissions and protect existing habitats.
ENMeval, as a criterion for quantitative evaluations of ecological niche models (ENMs), is widely used in optimizing the MaxEnt to avoid multicollinearity and improve the accuracy of predictions. In our study, we utilized an optimized MaxEnt to simulate and predict the suitability of
G. orchidis under different scenarios. The high values of the AUC (area under the curve) greater than 0.9 indicate an excellent predictive performance. However, it is important to note that MaxEnts are based solely on existing data, which may span a large period of time. They may lack consideration for actual conditions such as biological interactions, genetic variability, and species dispersal capacity, which can result in a wider prediction range for species distribution areas [
55]. Therefore, future studies could further refine the model and incorporate more field validation to obtain more comprehensive details about the species, leading to better conservation assessments.
5. Conclusions
In this study, the optimized MaxEnt was used to analyze the changes in the distribution pattern of G. orchidis in ancient and modern periods and make predictions for suitable areas under multiple future scenarios. The results highlighted that annual precipitation (Bio12) and mean temperature of the coldest quarter (Bio11) were the dominant factors, accounting for 55.1% and 23.4% of the influence, respectively. The range of Bio12 varied from 613 mm to 2466 mm, while Bio11 ranged from −5.8 °C to 8.5 °C. This study revealed a significant expansion of the suitable areas for G. orchidis from the past to the future. In the past, mountains such as Henduan, Yunlin, and Longmen served as glacial shelters for G. orchidis, and the centroids of its distribution shifted northward. Furthermore, the trend of northward migration is projected to persist. Under the future scenarios, there was a substantial expansion in the SSP370 scenario (30.33% to 46.19%), followed by the SSP585 scenario (1.41% to 22.3%). However, there was a contraction in suitable areas under the SSP126 scenario. The centroids of G. orchidis exhibited multidirectional movement, with the greatest distance observed in the SSP585 scenario (100.38/km). In summary, G. orchidis is expected to have better prospects in the future, but it is crucial to preserve its suitable habitats and avoid human-induced destruction. The conservation of G. orchidis not only has profound impacts on economic development but also plays a crucial role in safeguarding biodiversity and maintaining the stability of ecosystem structures worldwide.