Dryland ecosystems are more fragile and susceptible to changes in the climatic environment due to the poor soils and sparse vegetation cover (Reynolds et al., 2007). Previous studies have indicated that global drylands will continue to expand as a result of global warming (Huang et al., 2015) and changing rainfall patterns (Yang et al., 2017), leading to the intensification of the degree of land desertification (Huang et al., 2015). However, some studies have recently demonstrated that the expansion of the global dryland areas in the future would not be caused by climate change (Berg and McColl, 2021; Lian et al., 2021), which contradicts the previous mainstream view. This study analyzed the change in the area of drylands in China from 1981 to 2020 using the AI and determined that the dryland areas in China have gradually expanded at an average rate of 0.019 106 km2 (10 a)−1 over the last 40 years. The driving factors for this expansion were primarily MAT and Srad, which is consistent with the previous hypothesis. MAP, on the other hand, was not a primary driving factor of change. The results indicate that the drylands in China have been influenced by the East Asian monsoon in recent years. The increasing MAP thereby increased the area of the semi-arid and dry subhumid areas while decreasing the area of the hyper-arid and arid areas. When considering the total dryland area, both an increase and decrease were associated with MAP. MAP was thus determined to not be a primary factor driving dryland area change. The arid and semi-arid areas expanded at the fastest rate, while the hyper-arid area gradually decreased. Li et al. (2015) indicated a gradual decrease in the area of the hyper-arid region, which is consistent with the findings of this study. The drylands of China are primarily located in the northwestern region, where precipitation is scarce, and evapotranspiration is high due to the perennial continental climate (Guo et al., 2020). Chen et al. (2021) recently discovered that the recent increase in MAP in China’s northwestern drylands was primarily attributed to anomalies in the water vapor transport associated with the East Asian monsoon’s weakening trend (Chen et al., 2021). It is possible that the increasing MAP in the northwest drylands resulted in a decrease in the hyper-arid area. This was also observed in the SEM model. Meanwhile, increasing MAP in the northwest drylands increased the area of the dry subhumid extent. A significant relationship can therefore be observed between MAP and the dry subhumid area.
In both the arid and semi-arid regions, the fastest rate of area expansion was 0.032 × 106 km2 (10 year)−1, where MAT was the primary driver of the change in the area. The rising MAT has been identified as a significant contributor to increased drought (Komor, 1995) while also increasing evapotranspiration and resulting in increased drought conditions, particularly during the warm season (Meehl et al., 2007). According to the PCA analysis, MAT was positively correlated with PET, whereas in the SEM model, MAT had a highly significant positive effect on PET. Therefore, under the future global warming scenario, the increase in PET promoted by MAT may be one of the important reasons for the increase in dryland area. Srad impacted the changes in dryland areas in two distinct ways: (1) there is a correlation between Srad and MAT (He et al., 2013), and Srad affects the area change in the dry zone indirectly by influencing and thus (2) Zhou et al. (2022) recently discovered that the Srad and the Srad derived precipitation in the East Asian monsoon region (Zhou X et al., 2022) affected the MAP in China’s drylands, which was not evident in the study.
The total NPP in China’s drylands primarily increased at an average rate of 10.78 g C·m− 2·yr− 1 over the last two decades, confirming the previous hypothesis. Numerous studies have demonstrated a significant increase in vegetation activity in the mid-and high-latitudes of the Northern Hemisphere, including the majority of China (Hao et al., 2021). These findings coincided with the results of this study. The vegetation NPP growth rate was found to be the fastest in the dry subhumid area, at 5.15 g C·m− 2·yr− 1. This may be attributed to adequate MAP considering it was determined to be the primary driver of area change in the dry subhumid area in the SEM analysis. Although the dry subhumid area only has an annual average area proportion of around 11%, the annual average NPP can reach approximately 42%, demonstrating the high storage capacity of NPP in the dry subhumid area. Potential explanations for the increase in total NPP in the drylands of China may be: (1) the increase may be attributed to the overall increasing trend of the MAP in the drylands, which increased available water and created favorable conditions for plant growth, thereby increasing photosynthetic efficiency and organic dry matter accumulation of the vegetation (Tong et al., 2019); thus, effectively promoting the accumulation of NPP (Liu et al., 2021). (2) the area of the artificial oasis grew, and the NPP of the cultivated vegetation in the study area increased at a rapid rate. The improvement in agricultural productivity and economic level has led to significant changes in vegetation cover and productivity (Li et al., 2013; Li et al., 2014). (3) in recent years, the vegetation cover has increased, and the ecological environment has improved significantly in Northwest China. China has implemented a series of ecological environmental protection measures, such as restoring cropland to forest and grassland (Peng et al., 2021; Peng and Wang, 2004).
Among the vegetation types, the increasing trend in NPP was more pronounced in forest and cropland, while the NPP in the grassland and desert only increased slightly. These results are not consistent with the previous hypothesis. Firstly, converting farmland to forest has increased in recent years, in addition to planting trees and establishing desert highway protection forests (Jin et al., 2018). Western development and the “One Belt, One Road” initiative have accelerated the growth of the western economy and increased the area of construction land and farmland (Lanan et al., 2018). As a result, forests and farmland have contributed significantly to the total NPP (Lan et al., 2021). Secondly, NPP varied significantly between vegetation types. This may be due to a significant variation in the area under each vegetation type. Additionally, the growing seasons of each vegetation type and their distinct biological and ecological characteristics may have affected the carbon sequestering capacity and efficiency of the vegetation, causing the NPP to vary significantly between vegetation types (Iersel et al., 2010). Thirdly, forests are dominated by tall trees, which have relatively greater photosynthetic capacity than other vegetation types. Their NPP is thus greater and contributed more.
Previous research has established a positive correlation between the NPP of all vegetation types in Inner Mongolia and the precipitation. In other words, precipitation was the primary factor that affected the NPP of the vegetation (Zhang et al., 2011; Hao et al., 2021). Although this study did not correlate MAP with NPP of each vegetation type, a positive correlation between NPP and MAP was determined for all vegetation in the drylands. Under the current trend of global warming, the increase in temperature favored an increase in forest NPP to a certain extent, partially compensating for the decline in forest NPP caused by drought (Hao et al., 2021). Meanwhile, the effect of the temperature increase on the vegetation NPP varied in different regions. For example, when temperatures rise appropriately, the snow melts in high-altitude mountains, replenishing water sources and promoting lush vegetation growth. On the other hand, when temperatures fall, soil water evaporation and plant transpiration are weakened, thereby reducing the amount of available water dissipated by vegetation (Zhu et al., 2019). However, in regions such as the majority of the Xinjiang Province, northern Gansu Province, and the northwestern Qinghai Province, where temperatures are high and rainfall is low (Tao et al., 2003), increased temperatures may lead to increased potential evapotranspiration and decreased soil water content, which further reduces available water (Lu et al., 2005). This thereby inhibits vegetation growth and decreases vegetation NPP.
The spatial correlation between NPP and meteorological elements in the drylands was found to be positive, covering a large area and accounting for 51.1% of dryland area. Only 9.6% of the area was found to be significant. Considering that deserts occupy the majority of the dryland areas and desert vegetation is sparse, the NPP was low, even zero in some areas, which resulted in the lack of obvious spatial correlation. The correlation distributions of Srad and PET with NPP exhibited some spatial similarity, primarily indicating a negative correlation. According to the statistical results of the decreasing daily range of global temperature, Moonen concluded that Srad is the key factor affecting evaporation which decreased with the decrease in Srad (Gilgen et al., 1998; Moonen et al., 2002). The spatial correlation between MAT and NPP was low, while the PCA analysis revealed no significant correlation. According to Liu (2021), the temperature was not a limiting factor for grassland growth in the relationship between NPP and MAT of arid grassland vegetation. On a global scale, Gang et al. (2015) discovered that NPP was significantly correlated with annual precipitation but not with temperature in the grassland/desert ecosystems in China, North America, Europe, and Australia. This is entirely consistent with the findings of this study.