Spatiotemporal variation of climate of different flanks and elevations of the Qinling–Daba mountains in China during 1969–2018

Climate change exhibits great variation on different flanks and at different elevations in the same mountain range. To investigate the complexity of the geographic patterns of climate and phenology in the Qinling–Daba mountains (QDM), in the North–South transition zone of China, this study analyzed the spatiotemporal distribution characteristics of daily air temperature and precipitation data measured at 118 national weather stations (1969–2018). The principal findings were as follows. (1) Overall, a significant trend of warming was detected in all seasons over the past 50 years, with rates of increase of 0.347, 0.125, 0.200 and 0.302 °C/10a, in spring, summer, autumn and winter, respectively. Precipitation did not show significant variation at most stations in different seasons. (2) The rising rate of air temperature varied considerably between different flanks. Generally, air temperature change on northern flanks was greater than on southern flanks in all seasons. The tendency of air temperature change was greater in spring and winter than in summer and autumn on different flanks in the QDM. (3) The rate of increase in high-elevation regions was greater than in low-elevation regions in summer, autumn and winter, e.g., 0.440, 0.390 and 0.456 °C/10a at 3000–4000 m and 0.205, 0.218 and 0.303 °C/10a at 0–1000 m, respectively. However, in spring, the rate of increase in low-elevation regions were higher than in high-elevation regions, e.g., 0.369 °C/10a at 0–1000 m and 0.317 °C/10a at 3000–4000 m.


Study area
The QDM, located in central China (30°-36°N, 101°-115°E, span six provinces: Gansu, Sichuan, Shaanxi, Chongqing, Henan, and Hubei (Fig. 1). The QDM, consisting primarily of the Qinling Mountains and the Daba Mountains separated by the Han River valley, form a natural boundary between the warm temperate zone and the subtropical zone. Generally, the Qinling Mountains are broadly coincident with the 0 °C isotherm in January, the 800-mm isohyet, and the 2000-h sunshine duration contour in China 12 . Variation in these climatic factors causes the regional vegetation to change gradually from a deciduous broadleaved forest zone to an evergreen broadleaved forest zone across the QDM from north to south 13,14 . However, western parts of the QDM are dominated by cold temperate grassland because of the higher elevation and the drier climate in certain valleys 15 .
A network of 118 national weather stations is distributed over different flanks in the QDM: 31 and 20 stations are located on the southern and northern flanks of the Qinling Mountains, respectively, 13 and 20 stations are distributed on the southern and northern flanks of the Daba Mountains, respectively, and 34 stations are distributed on the western flank of the QDM (Fig. 1).

Materials and methods
Weather stations. A dataset comprising daily temperature and precipitation data ) measured at 118 national weather stations in the QDM was obtained from the National Meteorological Information Center (https:// data. cma. cn/), and the temperature and precipitation data were averaged by year, season and month. As shown in Fig. 1, the weather stations are mainly distributed over six provinces: Gansu, Shaanxi, Henan, Sichuan, Hubei and Chongqing.
DEM data. For this study, digital elevation model (DEM) data for different blocks of the QDM were downloaded from the website of the U.S. Geological Survey (https:// earth explo rer. usgs. gov/) and then spliced into a complete regional DEM covering the entire area of the QDM with 1000-m spatial resolution. To analyze the association between climate change and elevation, the DEM data were classified into four levels: 0-1000, 1000-2000, 2000-3000, and 3000-4000 m. The variation characteristics of the weather stations  were determined for the different classification segments, and the relationship between climate change and elevation specific to the QDM was explored.  (20), and blue dots represent weather stations on the western flank of the Qinling-Daba mountains (34). Notes: the map is created via the software ArcGIS 10.2 and the related research marks and words are added. The data of the map is from the site: https:// earth explo rer. usgs. gov/, https:// data. cma. cn/.  Method. The widely used Manne-Kendall method is effective in detecting change in long-term time series 16 .
The details of the method of treatment are described in Xu et al. 17 In this study, the Manne-Kendall method was applied to detect changes in the long-term trends of temperature and precipitation on different flanks and at different elevations of the QDM using the meteorological data from the 118 national weather stations.  Table 1). However, the rate of increase in air temperature in   www.nature.com/scientificreports/ 61 (69.32%), and 72 (81.82%) of the stations were found to have a significant trend of increase in air temperature at elevations of 0-1000 m in spring, summer, autumn, and winter, respectively, whereas 98.68%, 92.50%, and 100% of stations were found to have a significant trend of increase at elevations of 1000-2000, 2000-3000, and 3000-4000 m, respectively. It was discovered that the area of warming in high-elevation areas was greater than that in low-elevation areas in different seasons. The air temperature trend analysis for different seasons revealed significant trends of increase at different elevations in the QDM during 1969-2018. (Fig. 4, Table 2). However, the rate of increase in mean air temperature varied greatly depending on season and elevation. At higher elevations, we detected higher rates of increase in mean air temperature in summer, autumn, and winter (Fig. 4, Table 2). For example, at elevations of 3000-4000 m, the rate of increase in air temperature in summer, autumn, and winter could reach 0.440, 0.390, and 0.456 °C/10a, respectively, while at elevations of 0-1000 m, the rate of increase was 0.205, 0.218, and 0.303 °C/10a, respectively. Conversely, the rate of increase in air temperature in spring was higher in low-elevation areas (0-1000 and 1000-2000 m) than in high-elevation areas (2000-3000 and 3000-4000 m).

Results
Precipitation changes on different flanks and at different elevations in the QDM during 1969-2018. The Mann-Kendall tests did not indicate significant trends in precipitation (Fig. 5). However, some stations did exhibit notable trends in precipitation in spring, summer, and winter (Fig. 6). As shown in Fig. 7 and listed in Table 3, Zigui and Shiquan stations showed significant trends of increase in summer precipitation, with change rates as high as 47.36, and 22.91 mm/10a, respectively, while one station (Diebu) showed a significant trend of decrease (− 19.43 mm/10a). In contrast, four weather stations (Zigui, Beichuan, Taibai, and Hezuo) indicated increasing changes in winter precipitation. It is important to note that Mount Hua station exhibited a significant trend of decrease in precipitation in both spring and autumn with a

Discussion
Our analysis confirmed that air temperature in the QDM has exhibited a trend of increase over the previous 50 years, but that the rate of temperature increase presents marked variation between different flanks and at different elevations. Overall, the rate of change of air temperature was greater on the northern flank and at higher elevations than on the southern flank and at lower elevations of the QDM in all seasons. For example, the rate of change of air temperature over the previous 50 years was as high as 0.374 and 0.456 °C/10a on the northern flank of the Qinling Mountains and at 3000-4000 m in the QDM, respectively, but as low as 0.267 and 0.303 °C/10a on the southern flank of the Qinling Mountains and at 0-1000 m in the QDM, respectively. A similar result was also identified in relation to the Qinghai-Tibet Plateau by Liu and Yao 18,19 . Additionally, research in the Tianshan Mountains, Swiss Alps, and Colorado Rockies by Xu et al. 20 , Beniston et al. 21 and Rangwala et al. 22 , respectively, also found that the rate of climate warming in high-elevation areas has been higher than that in low-elevation areas. A possible reason is that air temperature change on northern flanks and at relatively high elevations is more sensitive to driving factors such as snow/ice coverage 22 , cloud 18,23 , water vapor 24,25 , carbon dioxide 26,27 , and soil moisture 28 . The findings of this study highlighted the variation in air temperature increase in different seasons. In comparison with low-elevation areas, the rate of increase in air temperature was higher at high elevations in summer, autumn, and winter. Conversely, in spring, it was higher at lower elevations than at higher elevations. Dong Danhong et al. 29 also found that the change in the rate of increase in air temperature at lower elevations was greater than that at higher elevations in spring. Possible causes of this phenomenon include human activities 30 and solar radiation 31,32 . However, clarification of how these factors might affect the change in air temperature at different elevations in spring needs further research. The air temperature tendency on different flanks of the QDM was greater in spring and winter than in summer and autumn. The findings of this study indicated that the increase in air temperature was more pronounced in spring and winter, consistent with the results of Gao and Zhu et al. regarding air temperature change in the QDM and on the Qinghai-Tibet Plateau, respectively 8,33 . Plausible explanations for these results include the normalized difference vegetation index 34 , latent heat generated by local precipitation 35 , and air temperature inversion due to topography 36 . Anthropogenic factors such as surface albedo variation associated with land use change could explain the variation of air temperature in different seasons in  The seasonal rates of air temperature increase that differ between the various subregions and at different elevations in the QDM, will affect changes in vegetation cover and biodiversity. The rate of air temperature change was found to be greater on the northern flank than on the southern flank, and greater at higher elevations than at lower elevations. Therefore, it will result in faster increase in vegetation coverage at higher elevations on the northern flank than that at lower elevations on the southern flank, as verified by Kullman 43 and Walther 44 . Accordingly, carbon sequestration might be greater at higher elevations than at lower elevations, which has also been verified by recent research 45 . In the future, high-elevation vegetation might migrate upward, resulting in increased species diversity.

Conclusions
We used climate statistical analysis methods to analyze the spatiotemporal trends of air temperature and precipitation during 1969-2018 on different flanks and elevations in the QDM. The following conclusions were obtained.
(1) Overall, an obvious trend of warming in different seasons has occurred over the previous 50 years in the QDM, but the rate of air temperature increase has varied markedly on different flanks and at different elevations. Precipitation has not changed significantly in most areas of the QDM during 1969 -2018, except at eight stations that showed a significant change in precipitation to a certain degree in different seasons. (2) The seasonal rate of increase in air temperature presented notable variations on the different flanks of the QDM. Generally, the rate of air temperature change was greater on the northern flank than on the southern flank in all seasons in the QDM. Specifically, the air temperature tendency was greater in spring and winter than in summer and autumn on the different flanks in the QDM. (3) There was an increasing trend of seasonal air temperature at different elevations in different seasons in the QDM. With increasing elevation, the rate of increase in mean air temperature showed a tendency of increase in summer, autumn and winter, whereas the rate of increase in mean air temperature in spring was higher at lower elevations than at the higher elevations.
Received: 9 November 2021; Accepted: 13 April 2022 Table 3. Seasonal rate of change in mean precipitation (mm/10a) at weather stations with a significant trend in different seasons in the Qinling-Daba mountains during 1969-2018. * and ** indicate that the climate trends are significant at the 0.05 and 0.01 levels, respectively, based on the Mann-Kendall test for long-term trends.