Detecting the interactions between vegetation greenness and drought globally

Drought changes can profoundly influence vegetation growth while vegetation, in turn, may impact drought by modulating water and energy exchanges between the land and atmosphere. Previous studies have primarily focused on the unidirectional effects of drought on vegetation dynamics. However, the impacts of vegetation on drought and their bidirectional dependencies have remained underexplored. In this paper, we employ the Leaf Area Index (LAI) and two drought indices (the Standardized Precipitation Index, SPI, and, the Standardized Precipitation Evapotranspiration Index, SPEI) to investigate the interaction between vegetation greenness and drought globally over the period 2001 to 2020. The results demonstrate a statistically significant and consistent trend between LAI and SPI (SPEI) in 66.9% (54.01%) of global vegetated lands. SPI commonly displays a 2-month delayed response to LAI, whereas SPEI shows no evident lag, implying a more rapid response of SPEI to LAI compared to SPI. A substantial proportion of global vegetated areas, around 66% for LAI-SPI and 52% for LAI-SPEI, exhibit a significant positive relationship. The Granger causality test reveals a bidirectional causal relationship between vegetation greenness and drought across approximately 20% of global vegetated areas. Vegetation greenness is identified as the Granger cause of SPI (SPEI) in >60% (30%) of global vegetated lands, providing statistical observational evidence indicating that vegetation can influence drought in many regions within terrestrial ecosystems. Interestingly, the ratio of areas where LAI serves as the Granger cause of SPI is more extensive than the corresponding ratio where SPI is identified as the Granger cause of LAI, a pattern opposite to that observed with SPEI. Overall, our investigation into the interactions between vegetation greenness and drought holds significance for enhancing our understanding of land-atmosphere interplays in the context of climate change and contributes to advancing sustainable development for ecosystem services.