Spring phenology and productivity alter vegetation vulnerability under summer droughts over Northern Hemisphere

Abstract

Climate change has intensified droughts, severely reducing vegetation productivity and even shifting the ecosystem from a carbon sink to a carbon source. Thus, understanding the spatial and temporal variations in vegetation responses to droughts is increasingly important. This study conducts coincidence analysis to examine the vulnerability and response time of vegetation to summer droughts from 1982 to 2022 across the Northern Hemisphere (NH) and employs random forest and partial correlation methods to identify their underlying drivers. The results reveal that arid regions and grasslands exhibit higher coincidence rates and shorter response time. Grasslands have the highest coincidence rate (0.38) and shortest response time (23 days), followed by shrublands, savannas, deciduous forests, and evergreen forests. Trends indicate that vegetation coincidence rates increased significantly (0.1/decade from 1993 to 2013), while lagged days decreased (-7.8 days/decade from 1990 to 2005), showing greater vulnerability to droughts. Spring phenology and productivity influence coincidence rate variations in about 27 % of the study area. Higher latitudes and cold regions exhibit stronger correlations between the start of growing season dates (SOS) and coincidence rates, suggesting that earlier growing seasons may enhance resistance to summer droughts in boreal forests. Conversely, in grasslands, earlier SOS negatively correlates with coincidence rates, indicating that rapid vegetation growth increases drought-related losses. These findings highlight the need to consider vegetation phenology interactions in drought assessments to improve ecosystem resilience and predictability.