*2.2. Geographic Data Analysis*

Based on the geographic information (longitude and latitude) of study sites in the existing literature described above (66 sites, the detailed geographic information is not available in all the papers), as well as that of Nepalese alder specimens from the Global Biodiversity Information Facility (GBIF, https://www.gbif.org, 184 sites, accessed on 30 September 2022) and the iPlant (http://www.iplant.cn, 71 sites, accessed on 30 October 2022), we analyzed the geographical distribution pattern about this species by using ArcGIS 10.7 for Desktop. The Shuttle Radar Topographic Mission Digital Elevation Model (SRTM DEM), the most comprehensive high-resolution digital topographic database, was applied to obtain the elevation for the above 321 sites. The SRTM DEM data with a spatial resolution of 90 m were obtained from the USGS (https://earthexplorer.usgs.gov/, accessed on 10 October 2022). Slope aspect of the study site was obtained by using visual interpretation of highresolution remote sensing imagery (World Imagery, https://www.arcgis.com/, accessed on 2 November 2022).

### *2.3. Bibliometrics Analysis*

With the help of CiteSpace visualization software (version number: 6.1.R3), keyword clustering analysis was carried out for the English and Chinese literature, respectively. The keyword clustering map of Nepalese alder researches were obtained, and the relevant research hotspots were revealed according to the analysis. Since the literature related to the Nepalese alder was mainly in Chinese (accounting for about 72%), we summarized the development processes about *A. nepalensis* researches based on the published papers in Chinese.

### **3. Geographic Distribution of Nepalese Alder and Corresponding Limiting Factors**

As shown in Figure 2, the Nepalese alder is widely distributed, with a latitude ranging between 38◦47 S and 34◦26 N, and a longitude from 156◦55 W to 177◦43 E. It is mainly concentrated in the southern foothills of the Himalayas and the southwestern part of China (especially Yunnan province) with a middle and low latitude of 20◦01 –30◦32 N, and a longitude of 79◦06 –104◦20 E. Additionally, a few of the records were found in North America, Oceania, and Ecuador in South America. Perhaps due to the synthetic influence of climatic, geological, and topographical factors, such as the southwest monsoon, the east Asia monsoon, and the uplift of the Himalayas, coupled with the shade-intolerant and adaptive characteristics of Nepalese alder, the Himalayas, Hengduan Mountain, and their southern mountains have become the main distribution area of Nepalese alder. Compared with other alder species of the congener (*A. formosana*, *A. trabeculosa*), Nepalese alder has a wider range at similar latitudes and a larger altitude span (more than 3000 m); for example, *A. formosana* is only distributed at altitudes of no more than 2500 m [17], whereas *A. trabeculosa* is only distributed between 200 and 1000 m [18]. Therefore, Nepalese alder is more adaptable to complex and varied habitats.

**Figure 2.** Geographic distribution of Nepalese alder based on information from specimen and literature.

From the respect of its northern boundary, the westernmost is in the district of Korlu in Himachal Pradesh in northeastern India, with a latitude of 32◦60 N. The northern boundary of the central region is mainly located in the Dongjiu area of Bayi District, Nyingchi City, with a latitude of 30◦00 N, whereas those of the eastern region were in Shimian County of Ya'an City, Sichuan Province and Wuchuan Gelao and Miao Autonomous County of Zunyi City, Guizhou Province, with latitudes of 28◦56 N and 28◦55 N, respectively. The meteorological data of the above four sites of northern limits were obtained from the WorldClim website (https://worldclim.org/, accessed on 11 October 2022). After justification according to the differences between gridded and exact altitudes, the annual mean temperatures were calculated as 15.4 ◦C, 9.6 ◦C, 10.8 ◦C, and 14.7 ◦C, respectively. Therefore, it seemed that the annual mean temperature of about 10 ◦C might be the thermal factor shaping its geographic distribution, which is consistent with Yang et al. who reported a low temperature limit of about 10 ◦C for Nepalese alder [4].

In terms of altitudinal distribution, nearly three-quarters of the research sites and specimen collection sites of Nepalese alder were concentrated between 1000 and 2500 m (Figure 3a), and only 12.79% of the sites sat below 1000 m, and a very small number (less than 1.3%) were distributed in alpine regions above 3500 m (mainly on Hengduan Mountain). The highest altitude record, 3860 m, happened in the northwest of Huili County in southern Sichuan Province, at a latitude of 26◦39 N. In addition, several sites with an altitude of more than 3600 m are distributed in Weixi Lisu Autonomous County, Yunnan, with a latitude of about 27◦12 N. Accordingly, it can be seen that the Nepalese alder is mainly distributed in typical mountainous terrain environments, and it is rarely distributed in low mountain plains and alpine areas above 3500 m.

**Figure 3.** (**a**) Altitude and (**b**) slope aspect distribution patterns of Nepalese alder.

Slope aspect may also exert impacts on the distribution of Nepalese alder. Nearly 60% of the sites were distributed in south-facing slopes (Figure 3b), and only 37% of the samples were distributed in north-facing slopes, indicating that the Nepalese alder prefers sunny slope habitats, consistent with its sunward and light-loving characteristics [4]. This is also associated with its distribution in the southern foothills of the Himalayas and the mountainous areas of southwest China, both of which showed the same topographic characteristic of high in the north and low in the south.
