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Article

Global Warming Drives Expansion of Endangered Spruce Forest on the Tibetan Plateau

by
Huayong Zhang
1,2,*,
Hang Yuan
1,
Hengchao Zou
1,
Xinyu Zhu
3,
Yihe Zhang
4,
Zhongyu Wang
1 and
Zhao Liu
2
1
Research Center for Engineering Ecology and Nonlinear Science, North China Electric Power University, Beijing 102206, China
2
Theoretical Ecology and Engineering Ecology Research Group, School of Life Sciences, Shandong University, Qingdao 250100, China
3
Dalian Eco-Environmental Affairs Service Center, No. 58 Lianshan Street, Shahekou District, Dalian 116026, China
4
School of Engineering, RMIT University, P.O. Box 71, Bundoora, VIC 3083, Australia
*
Author to whom correspondence should be addressed.
Sustainability 2024, 16(5), 2164; https://doi.org/10.3390/su16052164
Submission received: 2 February 2024 / Revised: 1 March 2024 / Accepted: 4 March 2024 / Published: 5 March 2024
Browse Figures
Versions Notes

Abstract

:
Global warming is critical to the distribution pattern of endangered plants; therefore, understanding the future changes in the adaptive areas of endangered spruce and driving factors on the Tibetan Plateau is of great research significance for spruce species conservation and sustainability. In this study, variations in the distribution pattern of four endangered spruce species (Picea. Balfouriana, Picea. Linzhiensis, Picea. Complanata, and Picea. Aurantiaca) on the Tibetan Plateau were predicted by the MaxEnt model, and the important environmental variables affecting its geographic distribution were analyzed. We found that under the current climate conditions, the four endangered spruce species were mainly situated in the southern and southeastern Tibetan Plateau. The mean temperature of the coldest quarter was a key environmental variable affecting the geographic distribution of four endangered spruce species, with suitable growth ranges of −9–8 °C for P. balfouriana and −6–5 °C for P. linzhiensis, P. complanata, and P. aurantiaca. Under different future climate pathways, the highly suitable habitat of four endangered spruce was mainly situated in the east, south, and southeast districts of the Tibetan Plateau. With the suitable growth range of key variables continuing to expand on the Tibetan Plateau, the area of suitable habitat for each of the four endangered spruce species increases to varying degrees. Compared with the current climate, four endangered spruce species will expand to the northwest of the Tibetan Plateau under different future climate scenarios, and the degree of expansion will increase with the increase in temperature. This study not only reveals the response of suitable habitats of four endangered spruce species to global warming, but also provides scientific insights for spruce population conservation and sustainable development.

1. Introduction

Climatic factors are essential for plant growth, development, and geographical distribution [1,2,3]. In recent decades, elevated atmospheric CO2 concentrations have led to events such as global warming, altered precipitation patterns, and increasing extreme weather events [4,5]. Community dynamics and geographic ranges of plant species have thus been affected, especially for the endangered plants profoundly [6,7,8]. Global temperatures will continue to rise in the 21st century and precipitation will increase at middle and high latitudes of the Northern Hemisphere [9,10,11], these changes will further affect vegetation growth and distribution [12]. Habitat loss and fragmentation of species due to future climate change are already a serious threat to endangered plants with small natural ranges, exacerbating their risk of extinction [13,14], which in turn affects the sustainability development of the ecosystem [15,16,17]. Therefore, changes in the spatial distributions of endangered plants in the context of rising global temperatures deserve further study.
Species Distribution Modeling (SDM) has been widely used in predicting species response to changing environments. Based on different algorithmic rules and prediction purposes, SDM derives diversified prediction models such as the Maximum Entropy Model (MaxEnt), genetic algorithmic model (GARP), ecological niche factor analysis model (ENFA), as well as bioclimatic and domain models, and others. [18,19]. Among them, the MaxEnt model is a geographically scaled spatial distribution model, and the sample size did not significantly affect its accuracy in predicting the spatial distribution of species [18,20]. At low sample sizes, its accuracy is 1.5 times that of GARP [21,22]. MaxEnt has been widely applied in predicting the suitable and dispersal pathways of invasive species of the future, suitable growing areas for economically important plants, and prioritized conservation areas for endangered species and the response of species spatial distribution to future climate change [23,24,25,26].
The Tibetan Plateau is recognized as an early warning and sensitive area for global climate change due to its unique geography [27,28,29]. Due to the increasing temperature in the next 100 years [9], woody plants on the Tibetan Plateau will migrate northwestward into inside the plateau, and gradually replace herbaceous plants [30,31]. Picea likiangensis var. Balfouriana (P. balfouriana), Picea likiangensis var. Linzhiensis (P. linzhiensis), Picea brachytyla Pritz. var. Complanata (P. complanata), and Picea aurantiaca (P. aurantiaca), as endangered spruce on the Tibetan Plateau, play an irreplaceable role in the construction of local subalpine coniferous forests and the improvement of the living environment for wildlife habitat [32,33,34]. They also play an indispensable role in maintaining the ecological safety of the Asian water tower and the upper of the Yangtze River [35]. Spruce forests, with their narrower ecological amplitude and sensitivity to climate change [36,37], are among the most threatened in the context of global warming, especially in the highly climate-sensitive region of the Tibetan Plateau [38,39,40]. Therefore, it is of great research importance to recognize the variations in the suitable distribution pattern of endangered spruce on the Tibetan Plateau. Most previous works have focused on exploring the physiological activities and morphological characteristics of spruce species in response to warming and drought by pot experiments [37,41]. However, the suitable distribution pattern of endangered spruce species on the Tibetan Plateau and environmental driving variables remains unclear.
We predict the suitable habitable zones of four endangered spruce species (P. balfouriana, P. linzhiensis, P. complanata, and P. aurantiaca) on the Tibetan Plateau under different periods by the MaxEnt model. The primary aims of our study include the following: (1) to study the current spatial distribution pattern of endangered spruce and the driving variables; (2) to predict the future distribution pattern of endangered spruce under different climate scenarios and its response to key variables; and (3) to analyze the expansion trend of an endangered spruce forest against the backdrop of global warming. This research can provide a scientific reference value for the sustainable development of the ecosystem and conservation of endangered spruce on the Tibetan Plateau under future climate change.

2. Methods and Materials

2.1. Data Processing and Collection

The boundary of the Tibetan Plateau was downloaded from the National Data Centre for the Tibetan Plateau (https://data.tpdc.ac.cn/home (visited on 23 May 2023)). The provincial boundary was acquired from the Resource and Environment Sciences Data Center (http://www.resdc.cn/ (visited on 23 April 2023)). The natural distribution point data of P. balfouriana, P. linzhiensis, P. complanata, and P. aurantiaca were received from the Vegetation Atlas of China (1:1,000,000), published by Geography Press, the Global Biodiversity Information Network (GBIF: https://www.gbif.org (visited on 5 June 2023)) and the Digital Herbarium of China (CVH: https://www.cvh.ac.cn (visited on 5 June 2023). Data without an exact location in the GBIF were first removed, and then latitude and longitude were acquired based on the geographic location of the species. Next, invalid data and duplicate data were removed using the ArcGIS SDM tool. We set up a 5 km × 5 km (2.5′) grid buffer and one geographic point was kept for every grid. The data of 178 effective distribution points of four endangered spruce species were finally obtained, as shown in Figure 1.
This study used 32 environmental variables, including climatic, terrain, and soil factors (Table S1). The current climate factors were acquired from the WorldClim 2.1 version (http://www.worldclim.org/ (visited on 25 May 2023)). Current climate factors are nineteen average bioclimatic variables from 1970 to 2000. The future climate factors were acquired from the WorldClim 2.1 version (http://www.worldclim.org/ (visited on 25 May 2023)), which in every scenario includes 19 average bioclimate factors. We used the future climate prediction data from CMIP6 to predict the future distribution of P. balfouriana, P. linzhiensis, P. complanata, and P. aurantiaca. Future climate data are bioclimatic variables under different carbon emission scenarios and shared socio-economic pathways (SSPs) reflect future climate change, including the five main SSPs (SSP119, SSP126, SSP245, SSP370, and SSP585) [42]. So as to avoid error and uncertainty [43], we selected three bioclimatic variables representing low (SSP126), medium (SSP370), and high (SSP585) carbon emission trends under the future climate scenarios during 2041–2100. Topography factors were acquired from the WorldClim 2.1 version (http://www.worldclim.org/ (visited on 25 May 2023)). Soil factors were acquired from the Soil Database (http://www.iiasa.ac.at/web/home/research/researchPrograms/water/HWSD.html visited on 2 June 2023)). The spatial resolution of each of the 32 environmental variables is 2.5′.

2.2. Selecting of Environmental Variables

In order to avoid strong correlations between excess variables leading to overfitting of the final simulation results, the 32 environmental factors used in this study were first imported into the MaxEnt model for simulation and screened for environmental variables with percentage contributions ≥1% [44,45]. Next, we calculated the correlation between environmental variables by using the spatial analyst tools of ArcGIS 10.8 software. If the relevance coefficient of the two variables was bigger than 0.8, only the environmental variable that was important in influencing spruce distribution was retained [46,47,48]. Finally, we selected 19 mutually independent environmental factors to construct a predictive model of the suitable distribution of the four endangered spruce species (Table 1).

2.3. Classification and Calculate of Suitable Habitats

Environmental data (ASC format) and distribution data (CSV format) were inputted into the MaxEnt model, and the response curve was confirmed and constructed by the Jackknife method. In the MaxEnt model, 75% of the geographic distribution data were randomly selected as the training set for building the model, and the remaining 25% of the geographic distribution data as the test set for verification. Assessing the accuracy of the results based on the area under the curve (AUC) values of the characteristics of the subjects’ work (ROC) [49]. The AUC value ranges from 0 to 1, and the bigger the value of AUC, the better the prediction of the MaxEnt model. A value of AUC greater than 0.9 signifies a perfect prediction [50].
Based on the natural discontinuity classification method (Jenks) of ArcGIS software, we used the ASC data outputted by the MaxEnt model to divide the highly suitable habitat, moderately suitable habitat, lowly suitable habitat, and not suitable habitat for P. balfouriana, P. linzhiensis, P. complanata, and P. aurantiaca under different periods [45,51] (Table S2). We calculated the area of the different divided areas by ArcGIS 10.8 software according to the approach raised by Zayneb Soilhi et al. [52].

2.4. Recognize of Driving Variables

The importance value of the variables obtained by the Jackknife method was applied to quantitatively analyze the influence of environmental factors on the geographical distribution of forest types and to screen out the dominant environmental factors [53]. The Jackknife test in MaxEnt sequentially employs and rules out a specific variable to build a new model [54,55]. Higher values of regularized training gain for “including only this variable” demonstrate higher predictability accuracy and are more important for predicting species distribution. In this study, the dominant environmental variables that influence species distribution were used to evaluate using the Jackknife method and contribution rate and analyze the importance of environmental variables. Generally, when the existence probability value was >0.2 in response curves, the corresponding ecological variable range values were suitable for plant growth.

2.5. Changes in Spatial Patterns of Suitable Habitat

Based on the natural discontinuity classification method (Jenks) of ArcGIS software, we obtained suitable and non-suitable habitats for four endangered spruce species. Changes in spatial patterns in the suitable areas of P. balfouriana, P. linzhiensis, P. complanata, and P. aurantiaca were investigated under different future climate scenarios by using the ArcGIS (10.8) grid processing tool [45,51]. Three categories of alterations in suitable habitats were defined, namely, area of gain, area of loss, and remaining area. The future changes in suitable habitats were calculated based on current conditions. Under different future climate pathways, the spatial pattern of a suitable area changes as follows: changes from a suitable area to an unsuitable area were considered to be a loss of area, changes from an unsuitable area to a suitable area were considered to be a gain area, and the other changes were considered to be remaining area.

3. Results

3.1. Current Distribution Patterns of Endangered Spruce and Driving Variables

Based on the results, we found that the MaxEnt model had a good simulation performance, and the method can accurately predict the suitable habitat of P. balfouriana, P. linzhiensis, P. complanata, and P. aurantiaca (Figure S1). According to the classification of suitable habitats acquired by the results (Figure 2), suitable habitat of four endangered spruce species was mainly situated at the eastern and southeastern Tibetan Plateau, with the area of suitable habitats accounting for 23.35%, 10.50%, 8.12%, and 6.33% of the Tibetan Plateau (Table S3). The highly suitable area of P. balfouriana was mainly situated in the eastern area of the Tibet Plateau, which accounted for about 6.15% of the Tibetan Plateau. The highly suitable habitat of P. linzhiensis was mainly situated at the southern Tibetan Plateau, with the highly suitable habitat area accounting for about 1.79% of the Plateau. The highly suitable areas of P. aurantiaca and P. complanata were mainly situated at the eastern and southern Tibetan Plateau, which for about 1.58% and 0.82% of the Tibetan plateau, respectively (Table S3).
According to the results of the regularized training gain with only one variable (Figure 3) and the contribution rate of environmental variables (Table S4), the primary environmental elements influencing the geographic distribution of P. balfouriana, P. linzhiensis, and P. complanata were the mean temperature of coldest quarter (Bio11), annual precipitation (Bio12) and temperature seasonality (Bio4). The major environmental factors influencing the geographic distribution of P. aurantiaca were the mean temperature of the coldest quarter (Bio11), annual precipitation (Bio12), and topsoil sodicity (T_Esp). The regularized training gain of the mean temperature of the coldest quarter (Bio11) was the biggest, followed by the annual precipitation (Bio12), temperature seasonality (Bio4), and topsoil sodicity(T_Esp), indicating that the mean temperature of the coldest quarter was the key variable influencing the geographic distribution of the four endangered spruce species (Figure 3).
Based on the environmental variables response curves (Figure 4), it can be concluded that the range of mean temperature of the coldest quarter suitable for the growth of P. balfouriana was −9 °C–8 °C, and the optimum value was −2 °C. The remaining three species of spruce can grow within −6–5 °C, and the optimum value was 0 °C. The annual precipitation of 770 mm was the most suitable for the growth of the four endangered spruce species, P. linzhiensis can grow in the range of 400 mm–1000 mm, P. balfouriana and P. aurantiaca can grow in the range of 500 mm–1500 mm, and P. complanata can grow in the range of 600 mm–2000 mm. The highest degree of suitability of P. balfouriana, P. linzhiensis, and P. complanata was achieved with temperature seasonality between 570–590. But the growth of P. linzhiensis and P. complanata was firstly inhibited when temperature seasonality was greater than 650, while P. balfouriana was unsuitable for growth when temperature seasonality was greater than 760. The highest fitness level of P. aurantiaca was found when the topsoil sodicity content was around 1%, and greater than 3% rapidly led to low fitness.

3.2. Future Distribution Pattern and Key Variable Responses

Under the SSP126 pathway, the suitable habitat of the four endangered spruce species is expected to be distributed in the southern and southeastern districts of the Tibet Plateau, the suitable habitat area of the four endangered spruce species will first decrease and then increase from 2041 to 2100 (Table S5). The highly suitable area of four endangered spruce is expected to be distributed in the eastern, southern, and southeastern Tibetan Plateau (Figure 5). The mean temperature of the coldest quarter, as a key environmental variable, influences the distribution of the four endangered spruce species. Because the mean temperature of the coldest quarter response interval of P. balfouriana is 6 °C larger than that of the remaining three species of spruce, the suitable growth range of mean temperature of the coldest quarter for P. balfouriana on the Tibetan Plateau is larger than that of the remaining three species of spruce. With the suitable growth range of the mean temperature of the coldest quarter, temperature first decreases and then increases over time on the Tibetan Plateau, and over 88.36% of the total suitable habitat area is projected to fall within the suitable growth range of the mean temperature of coldest quarter (Table S6).
Under the SSP370 pathway, the suitable habitat area of P. balfouriana and P. linzhiensis will first decrease and then increase, and the suitable habitat area of P. complanata and P. aurantiaca will keep increasing from 2041 to 2100 (Table S5). The highly suitable area for P. balfouriana, P. complanata, and P. aurantiaca is expected to concentrate and be distributed in the eastern and southeastern Tibetan Plateau. The highly suitable area for P. linzhiensis is expected to concentrate and be distributed in the southern Tibetan Plateau (Figure 6). With the suitable growth range of the mean temperature of the coldest quarter, the temperature increases over time on the Tibetan Plateau, over 91.50% of the total suitable habitat area for endangered spruce is predicted to fall within its suitable growth range (Table S6).
Under the SSP585 pathway, the suitable habitat area of four endangered spruce species will maintain an increasing trend from 2041 to 2100. Under the three different future climate pathways, the suitable habitat area of four endangered spruce species will reach its largest values under the 2081–2100 SSP585 climate, accounting for 39.86%, 17.59%, 20.37%, and 26.46% of the Tibetan Plateau, respectively (Table S5). The highly suitable habitat of P. aurantiaca is expected to mainly be located in the eastern region of the Plateau, while the other three endangered spruce species will mainly be located in the southern and southeastern regions of the Tibet Plateau (Figure 7). With the suitable growth range of the mean temperature of the coldest quarter temperature increasing rapidly over time on the Tibetan Plateau, over 92.78% of the total suitable habitat area for endangered spruce is predicted to fall within its suitable growth range (Table S6).

3.3. Future Expansion of Endangered Spruce Forest

According to Figure 8, Figure 9 and Figure 10, the suitable habitats of the four endangered spruce species under the three future climate scenarios are in an expanding trend, and all of them will expand towards the northwest of the Tibetan Plateau. Under the SSP126 climate, the areas of expansion of P. balfouriana, P. complanata, and P. aurantiaca are significantly larger than the areas of contraction, and all of them show strong expansion towards the northwest of the Tibetan Plateau. The area of expansion of P. linzhiensis is slightly larger than the area of contraction, indicating weak expansion towards the northwest of the Tibetan Plateau. Under the 2061–2080 SSP126 climate scenario, the four endangered spruce species expand the smallest, accounting for 3.26%, 3.06%, 2.63%, and 5.25% of the Tibetan Plateau area, respectively. Under the SSP370 and SSP585 climate, the expansion area of the four endangered spruce species is significantly larger than the contraction area, and it shows a strong expansion towards the northwest of the Tibetan Plateau. Under the SSP585 climate scenario from 2081 to 2100, the four endangered spruce species expand the most, accounting for 17.31%, 10.69%, 13.53%, and 21.22% of the area of the Tibetan Plateau, respectively (Table S7).

4. Discussion

The MaxEnt model is reliable in predicting the response of species’ spatial distribution to future climate change [56,57]. The average AUC values outputted by the MaxEnt model for the four endangered spruce species in this study are 0.926, 0.973, 0.977, and 0.988 indicating that the prediction is nearly perfect. Thus, it can be concluded that this study can accurately predict the suitable habitats of P. balfouriana, P. linzhiensis, P. complanata, and P. aurantiaca under different periods. Based on the Jackknife method and contribution rate of variables, the environmental elements affecting the spatial distribution of four endangered spruce species are the temperature factors, the regularized training gain value is the biggest, followed by the precipitation factor. On the Tibetan Plateau, previous research has shown that temperature and precipitation are more important in influencing spruce distribution, which is broadly in line with this study [44,58,59]. This study can clearly reveal the relationship between suitable habitats and environmental factors.
The suitable area of four endangered spruce is mainly situated in the south and southeast regions of the Tibetan Plateau under the current climate pathway. P. balfouriana’s high habitat size is the greatest, making up 6.15% of the Tibetan Plateau, primarily located in the southeastern region of the plateau. This result is in general agreement with a biomass study of P. balfouriana [60]. The highly suitable habitats of P. linzhiensis and P. complanata are mainly situated in the southern Tibetan Plateau, which is consistent with the geographical distribution in the study of vegetation in the Tibetan Plateau [61]. The highly suitable area of P. aurantiaca is the smallest, only accounting for 0.84% of the Tibet Plateau, and is situated at the eastern Plateau. The distribution areas of spruce in the vegetation of China are mainly located in western Sichuan, northeastern Yunnan, southeastern Tibet, and southeastern Qinghai, which is consistent with the result obtained in the current study [62]. Based on this study and previous research, it can be concluded that the southeast area of the Tibetan Plateau is suitable for spruce growth under the current climate [60,61,62].
As global temperatures rise, spruce is significantly more sensitive to temperature than precipitation on the Tibetan Plateau [36,44,58]. The key environmental variable influencing the growth and geographic distribution of endangered spruce on the Tibetan Plateau in this study is the mean temperature of the coldest quarter, followed by the annual precipitation and temperature seasonality. The mean temperature of the coldest quarter is suitable for the growth of P. balfouriana in the range of −9°C–8°C, and the remaining three species of spruce can grow in the range of −6 °C–5 °C. P. balfouriana shows the best growth adaptation when the mean temperature of the coldest quarter is −2 °C. When the mean temperature of the coldest quarter rises to 0 °C, P. linzhiensis, P. complanata, and P. aurantiaca begin to show their best growth adaptations. In the study of picea.purpurea [59], the mean temperature of the coldest quarter was −7 °C, which is less than the optimal growth temperature of the four endangered spruce species in this study. The four endangered spruce species grow most appropriately when the annual precipitation is 800 mm, P. linzhiensis can grow in the range of 400 mm–1000 mm, which is 500–1000 mm lower than the suitable range of the remaining three endangered spruce. The annual precipitation optimum values for the four endangered spruce species are generally similar to the results of previous research about spruce [36,59]. Furthermore, against the backdrop of global warming, the temperature extremes are increasing. Occasional extreme high or low temperatures can severely affect the growth of endangered spruce, resulting in discomfort or even death. This will lead to the migration of endangered spruce to suitable growing areas, resulting in a change in the geographic distribution pattern of endangered spruce [63].
Against the backdrop of global warming, some research about spruce on the Tibetan Plateau has shown that the suitable habitat for spruce is in a trend of increase [58,64]. Under different future climate pathways in 2041–2100, the suitable area for four endangered spruce species is expected to be distributed in the southern and southeastern Tibetan Plateau, and the size of suitable habitats for endangered spruce will increase to different extents. The suitable habitat area of four endangered spruce species was the largest under the 2081–2100 SSP585 climate pathway, accounting for 39.86%, 17.59%, 20.37%, and 26.46% of the Tibetan Plateau, respectively. In some research on predicting suitable habitats for spruce on the Tibetan Plateau, it was found that highly suitable areas for spruce were mainly situated in the eastern, southern, and southeastern Tibetan Plateau [58,64]. In the present study, the highly suitable habitat of P. balfouriana is mainly situated in the southeastern area of the Tibetan Plateau, and P. complanata is scattered, located at the eastern and southern Tibetan Plateau. The highly suitable habitat of P. linzhiensis and P. aurantiaca is mainly situated in the southern and eastern regions of the Tibetan Plateau. With the suitable growth range of the mean temperature of the coldest quarter temperature increases over time on the Tibetan Plateau, over 88.36% of the total suitable area of endangered spruce is projected to fall within the suitable growth range. Further, this indicates that the expansion of the suitable growth range of the mean temperature of the coldest quarter leads to an increase in the total area of suitable habitat [36,65,66].
Previous studies have shown that during the interglacial and postglacial periods, a large-scale expansion of picea.purpurea in alpine areas accompanied by increasing temperatures occurred [67]. Furthermore, expansion has also been found in studies of picea.purpurea and picea.smithiana in the context of future global warming [44,58]. Comparing with the suitable area for the four endangered spruce species under the current climate scenario, the four endangered spruce species show different degrees of expansion towards the northwest of the Tibetan Plateau under the three different future climate pathways, which follows the expansion trend [44,58,68]. It has been shown that the northward migration of plants may be created by global warming [69]. Under the three different climate pathways from 2041 to 2100, the expansion area of P. complanata, P. complanata, and P. aurantiaca will be larger than the contraction area, it shows strong expansion towards the northwest of the Tibetan Plateau. The P. linzhiensis area shows a weak expansion under the SSP126 climate scenario and a strong expansion trend under the SSP370 and SSP585 climate scenarios. The four endangered spruce species expand the most under the 2081–2100 SSP585 climate scenario, accounting for 17.31%, 10.69%, 13.53%, and 21.22% of the Tibetan Plateau, respectively. As global temperatures continue to rise [70], woody plants on the Tibetan Plateau will migrate northwestward into the inside the plateau, and gradually replace herbaceous plants [30,31]. Some studies have suggested that global warming may result in cooler and wetter climates in high-latitude areas, but more arid climates in mid-latitude areas [71,72]. Since the endangered spruce in this study is suitable for growth in a cool and humid environment, the suitable habitat for spruce is generally expanding to the high latitudes northwest of the Tibetan Plateau.
In addition to climatic, topographic, and soil factors, there are other factors that have an impact on the prediction of endangered spruce habitat that were not considered in this study, such as biotic factors such as interspecific competitiveness, biotic interactions, and evolutionary adaptation of species to environmental factors, as well as anthropogenic factors such as the density of roads and settlements [26,73]. Since the MaxEnt model requires only species presence data for modeling and lacks species absence data to constrain the model, future work will need to incorporate more factors and newer geographical distribution records for predicting suitable areas of plant distribution. Furthermore, in future plant community studies, integrating the physiological and biochemical roles of plants themselves, interactions between organisms, ecosystem changes, and anthropogenic factors will further deepen our understanding of the long-term climate change impacts on plant communities in high-altitude districts such as the Tibetan Plateau, thereby providing new ideas for future plant community research [73].

5. Conclusions

The Tibetan Plateau is recognized as an early sensitive and warning area for global climate change due to its unique geography. The endangered spruce on the Tibetan Plateau plays an indispensable role in the construction of local subalpine coniferous forests. In our research, the MaxEnt model was utilized to analyze the alterations in the suitable distribution pattern of four endangered spruce on the Tibetan Plateau and their driving variables in the context of rising global temperatures. Our analysis revealed that the four endangered spruce species are predominantly found in the south and southeast districts of the Tibetan Plateau under current and projected future climatic pathways. The mean temperature of the coldest quarter is a key variable influencing the geographic distribution of four endangered spruce species, followed by annual precipitation and temperature seasonality. Under the three different climate pathways (SSP126, SSP370, and SSP585), the area of suitable habitat for endangered spruce increases with the concentration of carbon emissions increases. The four endangered spruce species will expand to the northwest of the Tibetan Plateau under the future climate, and the degree of expansion of the four endangered spruce species will increase with the increase in temperature. Understanding the suitable migration and dispersal processes of endangered spruce on the Tibetan Plateau will help to understand the possible distributional changes of spruce species against the backdrop of rising global temperatures, which is of important significance in providing a reference for the protection and management of endangered spruce, and that has a great benefit for sustainable development for endangered spruce species on the Tibetan Plateau.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/su16052164/s1, Figure S1. Average AUC training values for four endangered spruce species, A for P. balfouriana, B for P. linzhiensis, C for P. complanata, D for P. aurantiaca. Table S1. Environment variables. Table S2. Criteria for the classification of endangered spruce suitable areas. Table S3. Area (×104 km2) and proportion (%) of current suitable habitats for P. balfouriana, P. linzhiensis, P. brachytyla and P. aurantiaca. Table S4. Contribution of Environmental Variables. Table S5. Area (×104 km2) and proportion (%) of suitable habitats under different future climate scenarios. Table S6. Area (×104 km2) and proportion (%) of endangered spruce suitable habitat within the suitable growth range of the key variable under future climate scenarios. Table S7. Changes of distribution area (×104 km2) and proportion (%) of P. balfouriana, P. linzhiensis, P. complanata and P. aurantiaca under different future climate scenarios.

Author Contributions

Conceptualization, H.Z. (Huayong Zhang); methodology, H.Z. (Huayong Zhang); software, H.Y.; validation, H.Z. (Huayong Zhang), H.Z. (Hengchao Zou), X.Z. and Y.Z.; formal analysis, H.Y.; writing—original draft preparation, H.Z. (Huayong Zhang), H.Y. and H.Z. (Hengchao Zou); writing—review and editing, H.Z. (Huayong Zhang), H.Y., H.Z. (Hengchao Zou), Z.W., X.Z., Y.Z. and Z.L.; visualization, H.Y.; supervision, H.Z. (Huayong Zhang) and H.Z. (Hengchao Zou); funding acquisition, H.Z. (Huayong Zhang). All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by National Water Pollution Control and Treatment Science and Technology Major Project (2017ZX07101) and the Discipline Construction Program of Huayong Zhang, Distinguished Professor of Shandong University, School of Life Sciences (61200082363001).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All links to input data are reported in the manuscript and all output data are available upon request to the authors.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Distribution records of four endangered spruce species on the Tibetan Plateau.
Figure 1. Distribution records of four endangered spruce species on the Tibetan Plateau.
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Figure 2. Current suitable distribution for the four endangered spruce species.
Figure 2. Current suitable distribution for the four endangered spruce species.
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Figure 3. Regularized training gain for endangered spruce. (a) P. balfouriana; (b) P. linzhiensis; (c) P. complanata; (d) P. aurantiaca.
Figure 3. Regularized training gain for endangered spruce. (a) P. balfouriana; (b) P. linzhiensis; (c) P. complanata; (d) P. aurantiaca.
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Figure 4. Response curves of four endangered spruce species. (a) P. balfouriana; (b) P. linzhiensis; (c) P. complanata; (d) P. aurantiaca.
Figure 4. Response curves of four endangered spruce species. (a) P. balfouriana; (b) P. linzhiensis; (c) P. complanata; (d) P. aurantiaca.
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Figure 5. Distribution of suitable habitat and key variable response under the SSP126 climate pathway.
Figure 5. Distribution of suitable habitat and key variable response under the SSP126 climate pathway.
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Figure 6. Distribution of suitable habitat and key variable response under the SSP370 climate pathway.
Figure 6. Distribution of suitable habitat and key variable response under the SSP370 climate pathway.
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Figure 7. Distribution of suitable habitat and key variable response under the SSP585 climate pathway.
Figure 7. Distribution of suitable habitat and key variable response under the SSP585 climate pathway.
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Figure 8. Change of suitable habitat under the SSP126 climate pathway.
Figure 8. Change of suitable habitat under the SSP126 climate pathway.
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Figure 9. Change of suitable habitat under the SSP370 climate pathway.
Figure 9. Change of suitable habitat under the SSP370 climate pathway.
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Figure 10. Change of suitable habitat under the SSP585 climate pathway.
Figure 10. Change of suitable habitat under the SSP585 climate pathway.
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Table 1. Environmental variables for the model.
Table 1. Environmental variables for the model.
CategoryVariableDescription UnitSpecies
P. balfourianaP. linzhiensisP. complanataP. aurantiaca
ClimateBio2Mean diurnal range°C√ √√
Bio3Isothermality%√
Bio4Temperature seasonality\√√√
Bio5Max temperature of warmest month°C √
Bio7Temperature annual range°C √
Bio11Mean temperature of coldest quarter °C√√√√
Bio12Annual precipitationmm√√√√
Bio14Precipitation of driest monthmm √
Bio15Precipitation seasonality% √
Bio17Precipitation of driest quarter mm √
Bio19Precipitation of coldest quarter mm √
TopographyEleElevationm √
AspAspect° √√√
SloSlope°√ √
SoilT_EspTopsoil Sodicity (ESP)%√√ √
S_GravelSubsoil Gravel Content%√
S_ClaySubsoil Clay Fraction% √√
T_GravelTopsoil Gravel Content% √√
T_SiltTopsoil Silt Fraction% √
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Zhang, H.; Yuan, H.; Zou, H.; Zhu, X.; Zhang, Y.; Wang, Z.; Liu, Z. Global Warming Drives Expansion of Endangered Spruce Forest on the Tibetan Plateau. Sustainability 2024, 16, 2164. https://doi.org/10.3390/su16052164

AMA Style

Zhang H, Yuan H, Zou H, Zhu X, Zhang Y, Wang Z, Liu Z. Global Warming Drives Expansion of Endangered Spruce Forest on the Tibetan Plateau. Sustainability. 2024; 16(5):2164. https://doi.org/10.3390/su16052164

Chicago/Turabian Style

Zhang, Huayong, Hang Yuan, Hengchao Zou, Xinyu Zhu, Yihe Zhang, Zhongyu Wang, and Zhao Liu. 2024. "Global Warming Drives Expansion of Endangered Spruce Forest on the Tibetan Plateau" Sustainability 16, no. 5: 2164. https://doi.org/10.3390/su16052164

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