Divergent vegetation variation and the response to extreme climate events in the National Nature Reserves in Southwest China, 1961–2019

•Annual average NDVI demonstrate significant increasing trend in the three subregions.•NDVI values more than 0.8 mainly concentration on the 1049–2349 m, 947–2547 m, 617–2817 m in the three subregions.•The combination of extreme climate indices can better explain the NDVI change on the month and year scale in the three subregions. Southwest China (SWC) is a critical hotspot of biodiversity and highly sensitive to climate change. To better protect biodiversity, many National Nature Reserves have been established to maintain viable populations of species and minimize their habitat loss. However, the response of the normalized difference vegetation index (NDVI) in National Nature Reserves to extreme climate events has not been described. We analyzed NDVI changes and made several key observations: (1) from 1998 to 2019, annual average NDVI trends of 0.00421 yr−1, 0.00514 yr−1, and 0.00529 yr−1 were significantly increasing and NDVI values more than 0.8 were mainly concentrated at 1049–2349 m, 947–2547 m, 617–2817 m, in the National Nature Reserves of Hengduan Mountain (NNRHDM), Sichuan basin (NNRSCB), and Yunnan-Guizhou Plateau (NNRYGP), respectively. The change trends in different altitude bins were more significant in the NNRYGP; (2) extreme cold (warm) indices significantly decreased (increased) in the NNRHDM, NNRSCB and NNRYGP, extreme precipitation indices increased in the NNRHDM, and decreased in the NNRSCB and NNRYGP; (3) the combination of extreme temperature indices can better explain NDVI change in the NNRSCB and NNRYGP, and the combination of extreme temperature and precipitation indices can better explain the NDVI change in the NNRHDM on the monthly scale. Overall, the NDVI green trend is more obvious in the NNRYGP than in the NNRHDM and NNRSCB, and is more sensitive to extreme climate change, especially extreme temperature. Our findings showed that although the National Nature Reserves have achieved remarkable results in conservation, future work should focus on the development of adaptation and alleviation strategies to prevent extreme climate events (especially extreme temperature events) that could promote vegetation degradation in these sensitive areas.