Evaluating the cumulative and time-lag effects of vegetation response to drought in the Lancang-Mekong River basin

• Vegetation in the LMRB showed increasing drought tolerance from upstream to downstream except for the MRLR. • The basin averaged 4.6 months of cumulative drought, with grassland having the highest tolerance of 5.0 months. • The basin's average drought time-lag was 3.9 months, with cropland being the most sensitive at 3.7 months. • SIF was the most sensitive indicator to drought-induced time-lag effects, responding within 2.8 months. The Lancang-Mekong River Basin (LMRB), a critical transboundary river basin in Southeast Asia, plays an essential role in maintaining ecological services and regional biodiversity. In recent decades, the LMRB has experienced frequent and intensified drought events due to climate change. Vegetation responses to drought typically involve cumulative and time-lag effects, yet these effects have not been quantitatively assessed in large basins with high heterogeneity of vegetation and climate using multiple vegetation remote sensing indices. To address this gap, this study investigated the cumulative and time-lag effects of vegetation responses to meteorological drought across multiple climatic zones in the LMRB from 2001 to 2020, using four vegetation indices (EVI, kNDVI, NDVI, and SIF) and the standardized precipitation evapotranspiration index (SPEI). The results showed that vegetation types with longer cumulative drought response durations exhibited greater drought tolerance, with grasslands showing the highest tolerance (average: 5.0 months) compared to the basin-wide mean of 4.6 months. The average drought tolerance of vegetation in the Lancang-Mekong River Basin generally increased from upstream to downstream except for the Lower Mekong River Basin (MRLR). Moreover, the average number of months of drought time-lag in the basin was 3.9 months, with cropland having the shortest lag time of 3.7 months, indicating that cropland was the most sensitive to drought compared to other vegetation types. Overall, cumulative drought effects on vegetation were more pronounced than lagged effects across the basin. In addition, the response of solar induced fluorescence (SIF) to drought was different from EVI, kNDVI, and NDVI. SIF was the most sensitive to drought induced time-lag effects, responding within 2.8 months. This study shed light on the understanding of vegetation-climate relationships in basins such as the LMRB and provided favorable theoretical support for addressing drought risk in the broader context of global climate change.