Abstract:

Situating in an arid area with a fragile eco-environment, Xinjiang is among the regions mostly sensitive to climate change in China. The vertical zonality of vegetation and the stereoscopic change in climate in Xinjiang are obvious because of the large differences in topography. Although studying the vegetation dynamics and the changes in their climatic drivers on different elevation gradients in Xinjiang can positively help us understand the spatial differentiation of arid and semiarid ecosystems in China, previous studies still paid less attention to this. Therefore, on the basis of the MODIS NPP (net primary production) product and meteorological data from 2000 to 2017, the spatiotemporal and vertical distribution differences in vegetation NPP in Xinjiang were revealed using trend and correlation analyses. More importantly, the altitudinal gradient differences in the correlation between vegetation productivity and climate change were explored for the first time in this paper. The results showed the following: (1) in 2000—2017, there was an insignificant growth in vegetation NPP in the study area, with an average annual NPP of 145.96 g C·m^-2, and the growth rate was 4.422 g C·m^-2·(10a)^-1. Constrained by regional differences in topography and hydrothermal conditions, vegetation productivity was greater in the north than in the south and was larger in the mountain area than in the plain area. Vegetation cover varied significantly across altitudes: the main distribution area of vegetation was in the range of 300-1200 m above sea level, and the accumulated area of vegetation was 26.40×10⁶ hm². Vegetation productivity and vegetation cover were inconsistent with their vertical distribution characteristics; vegetation productivity was highest at altitudes between 1600 m and 1700 m. (2) Vegetation NPP was dominated by increasing trend in the areas of elevation <1400 m, and the increasing trend was mainly concentrated in the northern slope of the Tianshan Mountains, the southern margin of Junggar Basin, and the northwestern margin of southern Xinjiang. Vegetation NPP had a slightly decreasing trend at an altitude of 1400 to 3800 m, and the decline in productivity was more prominent at some high-altitude areas such as the Tianshan and Altai Mountains. (3) Rainfall was the major factor that affected the primary production of vegetation in Xinjiang. NPP was positively correlated with precipitation in 66.9% of the vegetated areas (6.6% was significantly correlated), which demonstrated that increased precipitation is effective in promoting vegetation productivity. At different elevation gradients, vegetation NPP was mainly negatively correlated with temperature and positively correlated with precipitation, and both correlations had more prominent gradient differences at altitudes <1400 m. This suggested that vegetation growth was more sensitive to changes in hydrothermal conditions at middle and low altitudes below 1400 m. However, in high-altitude (>3900 m) areas, temperature replaced precipitation as the dominant factor restricting vegetation activities. Under the general trend of global warming, the study of vertical patterns of vegetation systems and their climate-driven mechanisms in ecologically fragile areas like Xinjiang is of great significance for improving environmental management and ecological construction capacity.

Key words: vegetation, net primary production (NPP), climate change, altitudinal differentiation, Xinjiang