Nonlinear trends of vegetation changes in different geomorphologic zones and land use types of the Yangtze River basin, China

How land use changes and geomorphologic zones impact on vegetation nonlinear trends remains unclear in economically developed areas with complicated terrain. This paper explores the nonlinear trends of vegetation changes with normalized vegetation index (NDVI) in the Yangtze River basin, China, and further analyzes the effect of geomorphologic zones and land use changes on the nonlinear trend. The results show that: (1) Although monotonic increasing is the main trend type of vegetation NDVI (32.46%), reversal trends from decreasing to increasing and from increasing to decreasing account for 11.87% and 24.95%, respectively. (2) The vegetation change is mainly monotonically increasing in low altitude and relief zones, while that is mainly a reversal trend in high altitude and relief zones, indicating an increased risk of vegetation degradation with altitude and relief increasing. (3) The trends in most land use types are mainly monotonically increasing, but those in urban, forest, and grassland are mainly from increasing to decreasing, with area percentages as high as 32.29%, 27.25%, and 35.97%, indicating degradation risk. (4) The conversion of cropland and wetland to forestland has greatly promoted the vegetation restoration. However, a risk of vegetation degradation exists in land conversions between grassland and other land use types, especially the afforestation of grassland. Over all, considering the effects of both geomorphic zones and land use changes can help us better explore the driving of the nonlinear trends of vegetation changes and understand the process of vegetation dynamics.