*3.4. The Changes of the Intersection Distribution of Three Species*

Due to the large distribution area in the northeastern of QTP for three species, we analyzed the changes of intersection distributions for three species under future climate scenarios. Figure 6 showed the modeled vegetation fractional cover and spatial distribution of suitable habitat under future climate scenarios in the 2050s and 2070s in the northeastern of QTP. There was a decreasing trend for the intersection distribution area of three species (34,745 km<sup>2</sup> for SSP2.6, 15,441 km<sup>2</sup> for SSP4.5 and 7822 km<sup>2</sup> for SSP8.5 in the 2050s; 18,584 km2 for SSP2.6, 17,060 km<sup>2</sup> for SSP4.5 and 15,440 km2 for SSP8.5 in 2070s), which expanded their distribution area to the northeast. Under SSP8.5, the distribution for *Potentilla parvifolia* enlarged but the distribution for *Picea crassifolia* and *Sabina przewalskii* contracted. The total suitable habitat area for *Potentilla parvifolia* in the QTP would increase from 97,353.72 km<sup>2</sup> under SSP2.6 to 119,069.06 km2 under SSP8.5 in the 2070s. However, the total suitable habitat area for *Sabina przewalskii* in the QTP would shrink from 31,205.97 km<sup>2</sup> under SSP2.6 to 20,496.67 km2 under SSP8.5 in the 2070s.

**Figure 6.** Intersection for *Picea crassifolia*, *Sabina przewalskii* and *Potentilla parvifolia* under SSP2.6, SSP4.5 and SSP8.5 scenarios.

### **4. Discussion**

### *4.1. Influence of Environmental Variables on the Potential Distribution of Three Species*

It is widely known that the species distributions are not only determined by climatic factors but also impacted by local topography, human activities and species interactions [37]. Terrain characteristics, i.e., slope, altitude, and aspect are key environmental variables for shaping the vegetation distribution by changing moisture and heat especially for alpine trees [38,39]. In this study, analyses of environmental variables showed that aspect and elevation are critical factors restricting the distribution of the three species. The three species live in different aspects and their suitable habitats so they have their own ecological characteristics. Species distribution is primarily affected by elevation and aspect in alpine forest ecosystems [40]. According to the results of this study, *Picea crassifolia* is distributed between 2691 and 3375 m. The mean elevation of the highest habitat Suitability of *Picea crassifolia* under SSP8.5 in the 2050s is about 2849 m, which is similar to the previous study [41].

Temperature and precipitation are two major climate factors affecting the species distribution, especially growth-season temperatures, cold tolerance and the available water supply for alpine trees [42]. The results showed that precipitation of driest month (Bio14), annual precipitation (Bio12), temperature seasonality (Bio4) are major climatic factors that influence the distribution of *Picea crassifolia*, *Sabina przewalskii* and *Potentilla parvifolia*. Higher precipitation of the driest month and annual precipitation have a positive impact on species distribution. Temperature and precipitation are the key factors influencing species distribution in the drier upper sites. However, species distribution is more restricted by precipitation than the temperature in the wetter upper sites [37]). Temperature seasonality is positively related to elevation and strong seasonal variation in temperature may inhibit the growth of trees [43].

Soil provides the necessary space and nutrients for species to survive and limits their distributions [44]. The soil thickness at different sites is the reason for the spatial difference of species distribution [45]. Soil thickness ≥40 cm can store enough available water to allow trees to survive during drought periods [40]. In this study, we used physical and chemical characteristics of subsoil (30–100 cm) variables to further evaluate the suitable distribution of three species. We found subsoil CEC (clay), subsoil bulk density and subsoil CEC (soil) have a little influence on species distributions the QTP.

### *4.2. Average Elevation Changes of Potential Suitable Habitat for Three Model Species*

The average elevation in potentially suitable habitat for three species showed a slight upward shift under many climate scenarios in the 2050s and 2070s as compared to the average elevation of their potential distribution at current. Especially the average elevation in potentially suitable habitat for *Potentilla parvifolia* all increased except under SSP8.5 (3552 m) in the 2050s. These results are similar to other studies, which show plant species shift to higher elevation and cooler habitats responding to climate warming [46,47]. In order to adapt to climate change at local, regional, and global scales, alpine species shape the mechanism of shifting suitable climatic niches to relatively cooler habitats [48]. For three species in this study, the modeled predictions indicate species would shift to a higher elevation to occupy the current climate niche by the 2070s.

The changes of the average elevation in potentially suitable habitat for trees (*Picea crassifolia* and *Sabina przewalskii*) under different climate scenarios were not obvious, while the mean elevation in potentially suitable habitat for shrub (*Potentilla parvifolia*) had a basically rising from current to 2070s. The modeling results suggested there would be a competitive relationship between shrubs and trees. The existence of shrubs restricts the growth of trees to higher altitudes, so the average altitude of *Picea crassifolia* and *Sabina przewalskii* did not increase further with climate warming. The previous study found the changes in the mean elevation could be influenced by other factors rather than climate alone [49].

### *4.3. Influence of Other Factors on the Potential Distribution of Three Species*

The vegetation is currently growing to a higher altitude [50,51], and this expansion will probably go on in the future. It is mainly responsible for climate change due to temperature or water availability [52,53]. In this study, we found that the positive interaction between shrubs and trees can promote the upward movement of vegetation. These interactions occur at slightly higher altitudes [54,55]. Shrubs are expected to expand to a higher elevation than trees with the same critical survival temperature. The snow cover is the protection of shrubs because it alleviates the influence of the temperature on shrubs [56]. The interaction between shrubs and trees may become more and more important to explain changes in the species composition and structure on the QTP. The expansion of shrubs could be discontinuous spatially and temporally which actually covers up tree expansion.

In addition, the two Ips species (*Ips nitidus* Eggers and *Ips shangrila* Cognato and Sun) are the most destructive secondary bark beetles on *Picea crassifolia* and always cause mortality of trees by their cooperation [57]. Increasing human interventions, such as harvesting, grazing and mining, may also result in distribution changes of the three species. The human population on the QTP has expanded dramatically in the past decades. Some suitable habitats for alpine species were converted to other land uses, such as pastures or settlements [40].

### **5. Conclusions**

In this study, we explored the influence of climate change on two dominant alpine trees (*Picea crassifolia* Kom and *Sabina przewalskii* Kom) and one dominant alpine shrub (*Potentilla parvifolia* Fisch) under different climate scenarios on the Qinghai–Tibetan Plateau. The predicted current potential distribution of three species was basically located in the northeastern of Qinghai–Tibetan Plateau, and the distribution of three species was most impacted by aspect, elevation, temperature seasonality, annual precipitation, precipitation of driest month, Subsoil CEC (clay), Subsoil bulk density and Subsoil CEC (soil). There were significant differences in the potential distribution of the three species under four climate scenarios in the 2050s and 2070s including expanding, shifting, and shrinking. The mean elevation in potentially suitable habitat for *Potentilla parvifolia* basically rose from the current period to the 2070s. Our study provides an important reference for the conservation of *Picea crassifolia*, *Sabina przewalskii*, *Potentilla parvifolia* and other dominant plant species under climate change. However, our research only used the friendly MaxEnt model without considering other models. In future studies, we will select the ensemble

model which can improve the reliability and accuracy of forecast results to further predict species distribution.

**Author Contributions:** H.H. carried out the main data search, processing, and paper writing work; Y.W. proposed the paper ideas and carried out the paper revision work; W.W. gave valuable comments in the paper writing and helped to collect and check the data; C.W. carried out the paper revision work. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by [the Strategic Priority Research Program of Chinese Academy of Sciences] grant number [XDA19040500]; [the Second Tibetan Plateau Scientific Expedition and Research (STEP) Program] grant number [2019QZKK0502]; [the 2020 Joint Research Project of Three-River National Park of the Chinese Academy of Sciences and the People's Government of Qinghai Province] grant number [LHZX-2020-08]; [the Qinghai Province Research Project] grant number [2019-ZJ-913].

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

**Data Availability Statement:** All environmental variables used in the manuscript are already publicly accessible, and we provided the download address in the manuscript; relevant sampling site information can be found in the online version.

**Acknowledgments:** Our deepest gratitude goes to the anonymous reviewers for their careful work and thoughtful suggestions that have helped improve this paper substantially. This research was jointly supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (grant no. XDA19040500), the Second Tibetan Plateau Scientific Expedition and Research (STEP) Program (2019QZKK0502); the 2020 Joint Research Project of Three-River National Park of the Chinese Academy of Sciences and the People's Government of Qinghai Province (LHZX-2020-08) and the Qinghai Province Research Project (2019-ZJ-913). We are very grateful for their generous funding.

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