Spatiotemporal distribution of net primary productivity and its driving factors in the Nanliu River basin in the Beibu Gulf

By analyzing the spatiotemporal dynamic characteristics of net primary productivity and its driving mechanism in the Nanliu River basin of the Beibu Gulf, the goal of this study was to provide a scientific basis for the protection of the ecological environment and the response to climate change in the region. Based on the light energy utilization model（CASA）, the net primary productivity（NPP） of the Nanliu River basin in the Beibu Gulf from 2000 to 2015 was estimated using remote sensing data, meteorological data, and vegetation type data. The spatiotemporal variation characteristics, the future trend of NPP and its driving factors in the study area were analyzed with the aid of mathematical statistics, such as the Theil-Sen trend, Mann Kendall test, and Hurst index. The results indicated:（1） Regarding temporal variation, the annual average NPP increased with a fluctuation in the study area from 2000 to 2015, with a growth rate of 44.03 g C m-2（10 a）-1, which is faster than that of the Guangxi autonomous region. The annual average NPP in the upstream and downstream of the river basin was faster than that of the entire region, whereas that in the middle reaches was slower than that of the entire region.（2） Regarding spatial distribution, the distribution law of NPP showed obvious regional differentiation, with the net primary productivity of vegetation in the middle reaches being the highest（1098.99 g C/m2）, followed by that downstream（1041.71 g C/m2）, and that upstream smallest（1013.22 g C/m2）. The Sen trend of NPP was from-77.10 to 74.80 g C/m2, and the net primary productivity of the basin showed an increasing trend.（3） Regarding the volatility of NPP, the CV of NPP was larger, being from 0.01 to 0.39. The expansion area of urban and rural construction land around the Hongchao River reservoir, Xiaojiang reservoir, and Yulin City was in a state of high fluctuation, whereas the Liuwandashan mountain in the middle of the basin and in the the Wuhuang mountain belt were in a state of low fluctuation.（4） Regarding the future trend of NPP, the NPP value of the Hurst index was from 0 to 0.99, with a mean of 0.70, and it presented a right unimodal distribution, which indicated that the overall NPP would take on a continuously increasing trend in the future.（5） Regarding the driving mechanism, the watershed NPP had a significant positive correlation with the annual average precipitation, and had a non-significant negative correlation with the annual average temper