Abstract: Global warming has increased the frequency and spatial extent of droughts, disrupting vegetation growth through atmospheric and soil pathways. Compound droughts have more complex and severe impacts on vegetation than individual droughts and have recently received widespread attention. However, the response of vegetation to individual droughts is controversial, and the contribution of compound droughts to vegetation productivity remains unclear. This study defined atmospheric, soil, and compound droughts based on vapor pressure deficit and soil moisture, and investigated their trends in China from 1982 to 2018 using piecewise linear regression. Using gross primary productivity products derived from the Global Land Surface Satellite (GLASS_GPP) and the Near-Infrared Reflectance of Vegetation (NIRv_GPP), drought impacts on GPP were quantified to identify the dominant factor in GPP reduction. All three drought types showed significant increasing trends with turning points in 1998 and 2009 and were negatively correlated with GPP. From 1998 to 2009, drought frequency increased most, with GLASS_GPP (–2.02 g C m^-2 year^-1) and NIRv_GPP (–0.90 g C m^-2 year^-1) declining most sharply, mainly in North China, Southwest China, and Central China. Although the negative impacts of all drought types intensified over time, compound droughts were identified as a major contributor to GPP reduction in China. While the influence of other factors may vary regionally, our results provide new evidence highlighting the prominent role of compound droughts in driving declines in vegetation productivity and offer valuable insights for assessing their impact on future terrestrial carbon uptake.

Key words: vapor pressure deficit, soil moisture, atmospheric droughts, plant growth, carbon cycle, climate change