*3.3. Species Richness Geospatial Patterns of Meconopsis*

We found an uneven spatial distribution of *Meconopsis*, with species richness ranging from 0 to 14 species per grid; most of the species were concentrated in the southeastern and southern regions of the study area (Figure 6a), i.e., the Hengduan Mountains, the central and eastern Himalaya, and the southeastern part of the plateau platform adjacent to the Himalaya and Hengduan Mountains, with relatively higher the species richness than other parts. According to the number of species per grid, we divided species richness into three categories: low (0–3), medium (3–8), and high (8–14). From Figure 6b, we could find that the areas with high species richness were mainly forests, while the areas with moderate species richness were forests and grasslands. In the Hengduan Mountains, central and eastern Himalaya, the areas with high and medium species richness were mainly forests, and in the plateau were mainly grasslands. The area with medium–high species richness in all subregions accounted for less than 10%, except for 18.3% in the Hengduan Mountains. Particularly, there was no area with medium–high species richness.

### *3.4. Species Richness Pattern along Environmental Gradients*

The species richness gradient in the study area exhibited a hump-shaped pattern with increasing richness with elevation, peaking at around 3900–4100 m and declining toward the ends of the elevational gradient (Figure 7a). The pattern of *Meconopsis* plant richness along the latitudinal gradient increased remarkably to the maximum and gradually decreased with Gaussian shape fitted (R2 = 0.96, Figure 7b). The richness of *Meconopsis* species showed a more or less monotonous increasing trend along the longitudinal gradient and was found absent beyond 104◦ (Figure 7c). In our study area, their richness varied strongly with the slope gradient with peaks occurring between 6◦ and 11◦, and decreasing in both directions (Figure 7d). The *Meconopsis* plant richness presented a clear hump-shaped pattern along the aspect gradient, reaching its peak at the range of 150◦ to 200◦ (south slope or sunny slope), and it was similar at both the highest (north and northwest slopes) and lowest (north and northeast slopes) aspect belts (Figure 7e). In a word, the polynomial curve fitted precisely for richness patterns of *Meconopsis* plant along the elevation, longitude, slope, and aspect, with reasonable R2 values (R2 > 0.5).

**Figure 6.** Spatial distribution of species richness of *Meconopsis* per 50 km × 50 km grid cell. (**a**) species richness distribution of each grid in the study area; (**b**) the distribution of species richness of *Meconopsis* in different vegetation.

**Figure 7.** *Cont*.

**Figure 7.** Species richness pattern of *Meconopsis* along the studied slope (**a**), latitudinal (**b**), longitudinal (**c**), elevational (**d**), and aspect (**e**) gradient.

### *3.5. Ecological Landscape Fragmentation and Species Richness*

High PD in all subregions indicated a relatively large degree of landscape fragmentation across the pan-Himalaya that was quite similar in all subregions (Figure 8). Specifically, the plateau platform occupied the largest proportion of the study area with the highest PD (152.4069), while the central Himalayan region had the lowest PD (103.3529). In terms of the CONTAG, the value was the highest in eastern Himalaya (66.2219), indicating better connectivity between dominant patches in the landscape, while the CONTAG of the western Himalaya was the lowest (60.1523). The results of PR illustrated that each subregion covered all land-use types. For SHDI, all subregions were greater than 0.8, indicating high landscape heterogeneity in the study area, and the highest SHDI value was in the western Himalayan region (1.3280). According to the four landscape indices, the degree of landscape fragmentation in each subregion from high to low was the western Himalaya, plateau platform, the Hengduan Mountains, the Central Himalaya, and eastern Himalaya. Areas with the highest species richness were the Hengduan Mountains (35) and plateau platform (34); the central Himalaya (20) and eastern Himalaya (19) had similar richness, while the lowest richness was in the western Himalaya.

**Figure 8.** Ecological landscape index and species richness in each sub-region.

### **4. Discussion**

### *4.1. Divergent Environmental Factors Affect the Spatial Distribution of Meconopsis Species*

The long-term joint influence of several factors resulted in the geographical distribution of plants. Climate is the most important factor determining the geographical distribution of plants on a regional scale [60,61]. The findings for MaxEnt indicated that the precipitation of the warmest quarter (Bio 18) had the greatest effect on the distribution of all *Meconopsis* species amongst the 11 environmental variables involved in modeling, and this is consistent with our previous study in Minjiang headwater region [42]. Since precipitation can affect plant growth, morphology, phenology, and biomass accumulation [62–66], in particular for seedling emergence and establishment [67], an appropriate degree of precipitation can supply sufficient water and promote plant growth; however, excessive rainfall can deteriorate the soil permeability [68], creating an anaerobic environment that inhibits the regular respiration of roots [69], and limits plant growth and development by influencing their metabolism [70]. Moreover, waterlogging can also cause stomata to close and photosynthesis to decrease, and increases the energy consumption for respiration, affecting the accumulation of organic matter [71]. In addition, high humidity caused by excess water favors the rapid multiplication of pathogens and the formation of serious diseases that not only threaten the survival of plants but also their distribution patterns on a geographical scale [72]. The species response curves demonstrated that the ecological amplitude of precipitation is small, indicating the narrow tolerance range of *Meconopsis* to precipitation and limiting the geographical distribution range.

Temperature variation affects plant distribution by influencing their germination, water absorption, photosynthesis, transpiration, respiration, reproduction, and growth [73]. Our results showed that temperature seasonality (Bio4) and mean annual temperature (Bio1) were the two dominant temperature-related factors affecting the habitat suitability

of *Meconopsis* species. Elevation was another key variable that had a strong indirect impact on the distribution of *Meconopsis* species. It integrates the effect of temperature, humidity, light, and other indicators to make secondary allocation of resources for influencing species composition and distribution [74,75]. Under the current climate, a potentially suitable area for *Meconopsis* was mainly distributed in the southeast Qinghai-Tibet Plateau and Hengduan Mountains where have similar climate conditions, particularly in terms of hydrothermal coupling. Monsoons, with sufficient rainfall with suitable temperature conditions, bolster the growth and development of alpine vegetation [76], accumulating assimilation products for overwintering and reproduction. Furthermore, high mountains and valleys are distributed in tandem with tremendous elevation variation, which creates a unique climate with cold temperatures at the mountain top and dry-hot conditions at the foot of the mountain. The suitable habitats of *Meconopsis* are mostly distributed in the alpine regions above 3500 m, consistent with their preference for cold and cool ecological habits [77,78]. This comprehensive analysis found that elevation and variables related to precipitation and temperature were the key factors restricting the distribution of *Meconopsis*.

### *4.2. The Richness of Meconopsis Species Varies along Geospatial Gradients*

The decline of plant species richness from the equator to the poles is one of the most striking ecological patterns on Earth [79]. However, along the longitudinal gradient, from west to east, plant species richness increases with increasing wetness in different regions of the study area [80–83]. The highest species richness of *Meconopsis* was founded in Hengduan Mountain, which stands at the intersection of latitudinal and longitudinal curves (Figure 5) providing appropriate hydrothermal conditions, high habitat heterogeneity, and complex terrain [84,85] for the growth of *Meconopsis* species. Interestingly, many areas with medium and high species richness were forests. This contradicted our common knowledge that their favorable habitats are shrublands or grasslands, which might be due to scale variations or spatial resolution since shrub and grassland patches in forests are often classified as forests at a 300 m spatial resolution. Moreover, it is generally accepted that forests, shrubs, deserts, and other plant communities with woody plant coverage of less than 40% should be classified as grassland [86]. However, the grassland area calculated based on the area of herbaceous vegetation here is underestimated.

Topography (elevation, slope, and aspect) has a significant impact on the spatial distribution of plants by influencing habitat temperature and humidity [87,88]. *Meconopsis* species richness revealed a unimodal distribution pattern along the elevational gradients in mountainous regions (Figure 5), similar to prior studies [89–91]. This may be attributed to the appropriate mild habitats for plant growth in mid-elevation [92], and this mid-domain effect was highly pronounced in the Himalaya [91], which brings in more rainfall than higher elevations in monsoon and decreases from the southernmost to the northernmost slopes [93]. The optimum temperature decreases along with the elevation increase, and only a few species can adapt to harsh environments [91,94]. Lower *Meconopsis* species richness beyond mid-elevations can be attributed to the decrease of soil cover with more rocks and slow biogeochemical cycle in contrast with favorable mid-elevation providing higher atmospheric moisture and cooler summer temperature [95]. Some species have specific requirements for slope and aspect [96], which directly or indirectly influence the solar radiation, temperature, and soil conditions [97,98] In concordance with other studies, we found that species richness of *Meconopsis* was higher on the sunlit southern slope as compared to the northern slope [99]. Meanwhile, the steeper slope triggers soil erosion [97] which stunts seed settling and plant growth [100]. Therefore, gentle slopes have higher species richness and plant growth as compared to steeper slopes. [97]. All the aforementioned topographical variables play essential and unique roles in the distribution of *Meconopsis*.

### *4.3. Linkage and Relations between Landscape Heterogeneity and Species Richness*

The complex spatial heterogeneity of mountain areas provides advantageous conditions for biodiversity with a variety of patch types [101]. However, our study did not find a relatively clear relationship between landscape fragmentation and species richness, which might be relevant to inconsistent scale and metastability [29,30,102]. At the landscape scale, the degree of landscape fragmentation seems to have little impact on the species richness of *Meconopsis*. Given that spatial heterogeneity such as topography can significantly affect the richness [103], the effects of landscape fragmentation are probably more pronounced at comparatively smaller scales.

A larger habitat can harbor more species and support richer plant communities with more available resources [104]. Therefore, it is easy to understand why the species richness is higher on the plateau platform than in other parts, whose higher species richness is mainly concentrated in the southeastern part. Blocked by the Himalaya and Gangdise mountains, warm moist air from the Indian Ocean flows into China along the Hengduan Mountains, thus bringing abundant rain to the southeastern of the Qinghai-Tibetan Plateau. While the western Himalaya has the lowest species richness with a quite fragmented landscape, the region is inherently less suitable for *Meconopsis* because of poor hydrothermal conditions [105], and this is consistent with the current potential distribution modeling results. The lower latitudes of the central and eastern Himalaya on the southern slope of the Himalaya with abundant summer rainfall have a higher species richness. [105,106]. The Hengduan Mountains are climatically influenced by the westerly circulation and the Indian and Pacific monsoon circulation, with dry winters and rainy summers, which are ideal for the more abundant *Meconopsis*; despite their smaller area than the plateau platform, the species richness of *Meconopsis* is more abundant.

### **5. Conclusions**

Our study discovered that the potential distribution regions with medium- and highsuitable habitats for *Meconopsis*, under the current climate scenario, were mainly located in the central and eastern Himalaya, Hengduan Mountains, and the southeast edge of the plateau platform. Precipitation of the warmest quarter had the greatest impact on the distribution of *Meconopsis* species. We also found species richness within the genus *Meconopsis* was distributed in a unimodal pattern along geospatial gradients except for the longitudinal gradient in pan-Himalaya. However, no obvious consistent relationships exist between landscape fragmentation and species richness for *Meconopsis*. Our findings not only promote an understanding of the distribution and diversity of *Meconopsis* species but also provide an indispensable foundation for future studies on *Meconopsis* plant functions and the sustainability of alpine ecosystems. This study also provides data and theoretical support for species diversity protection policies in pan-Himalaya and adjacent regions.

**Supplementary Materials:** The following supporting information can be downloaded at https:// www.mdpi.com/article/10.3390/10.3390/d14080661/s1: Table S1: The number of presence locations of each species.

**Author Contributions:** Conceptualization, N.S., C.W. and J.W.; methodology, N.S. and C.W.; software, N.S. and C.W.; validation, N.S., C.W. and J.W.; formal analysis, N.S. and C.W.; investigation, N.S. and C.W.; resources, N.S. and C.W.; data curation, N.S. and C.W.; writing—original draft preparation, N.S.; writing—review and editing, N.S., C.W., J.W., N.W., N.N., L.Z., L.W., J.S., W.D., Y.W. (Yanqiang Wei), W.C. and Y.W. (Yan Wu); visualization, N.S. and C.W.; supervision, J.W.; project administration, J.W.; funding acquisition, J.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Natural Science Foundation of China (31971436), CAS "Light of West China" Program (2021XBZG\_XBQNXZ\_A\_007), State Key Laboratory of Cryospheric Science, Northwest Institute of Eco-Environment and Resources, Chinese Academy Sciences (SKLCS-OP-2021-06), Talented Young Scientist Program (Indian-18-008) by China Science and Technology Exchange Centre, Ministry of Science and Technology, and China Biodiversity Observation Networks (Sino BON).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

**Acknowledgments:** Special thanks to Li Yike from the Chengdu Institute of Biology, Chinese Academy of Sciences for his valuable discussion about Fragstats and other issues.

**Conflicts of Interest:** The authors declare no conflict of interest.

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