Effects of climate change and human activities on net primary productivity in the Northern Slope of Tianshan, Xinjiang, China

Abstract: The northern slope of Tianshan has become an important development base of agriculture and animal husbandry, particularly for the key area of "one belt, one road". The investigation of Net primary productivity (NPP) in this area is absolutely essential to the sustainable development of agriculture and animal husbandry, due to the NPP can serve as a significant indicator of ecological environment evaluation. However, most previous studies of NPP mainly focused on the simulation of NPP under a model in a specific period. Therefore, this study aims to analyze the change of NPP from 2004 to 2015, and further investigate the impact of climate change and human activities on the NPP change. The remote sensing image data in the long time series was used to quantify the global variation of NPP subjected to the climate change and human activities. The specific procedure was as follows: First, the image data collected from a MODIS C6 remote sensing was used to simulate the actual net primary productivity (NPPA) for 12 years, and the simulated NPPA was verified by biomass data measured from the field, indicating that the MODIS C6 remote sensing data was feasible to simulate NPPA. Then, the change trends of temperature, precipitation, and NPPA were analyzed in the past 12 years, to clarify the influence of temperature and precipitation on NPPA. Finally, the net primary productivity (NPPH) was selected to explain the effect of human activities on the Northern Slope of Tianshan. The results show that: the actual NPPA at the Northern Slope of Tianshan ranged from 128.67 to 170.49 g/(m2·a) from the period of 2004 to 2015, with an average annual value of 144.71 g/(m2·a). There were obvious differences in the spatial characteristics of NPPA in different regions, showing a trend of high in the West and low in the East. The annual average of NPPA in the western region was the highest, indicating that the central and western regions were the main supply areas of NPPA on the Northern Slope of Tianshan. Specifically, the change rates of temperature and precipitation in the East were -0.13 ℃/a and 8.89 mm/a, whereas, those in the central region were the smallest, with the change rates of -0.09 ℃/a and 8.70 mm/a, and those in the western region were the largest, with the change rates of-0.15 ℃/a and 11.21 mm/a, respectively. It infers that the effect of precipitation on NPPA was greater than that of temperature. The average of annual NPPH on the northern slope of Tianshan ranged from -163.44-543.74 g/(m2·a), with an annual average of 13.72 g/(m2·a), while an average annual change rate of-1.24 g/(m2·a). Specifically, 44.15% of them were affected by the positive human activities, mainly distributed in Yiwu, Balikun, Wenquan county, and the south of North Slope of Tianshan along the Tianshan Mountain; 55.85% of negative impact areas were mainly distributed in the north and central part of the north slope of Tianshan. The influence of human activities on NPP change was gradually developing towards the direction of promoting effect in the northern slope of Tianshan. The typical areas triggered by increasing human activities were mainly distributed in the middle and east of the northern slope of Tianshan, while, the areas of increasing negative effects were scattered in Kuitun and Wusu County. The finding can provide a new way to explore the spatial pattern, process, and function of NPP under natural and human disturbance.