Space-time variability of vegetation and their multi-faceted drought impacts in the tropical and subtropical regions

Drought is a climate-related slow-onset hazard event and has a significant impact on agricultural production and ecosystem health. Mainland Southeast Asia is a tropical and subtropical region of major cropland and vegetation ecosystems, and this region is increasingly vulnerable to drought-related hazards. This study assessed space-time variability of vegetation dynamics and their drought impacts using satellite-based vegetation condition time series and multi-temporal drought indices from 2000 to 2022 over the MSEA region. Specifically, we examined the vegetation dynamics and their responses to multi-temporal (short-term and long-term) drought indices in consideration of different land cover types, land-use transitions, and elevation characteristics. We also used an explanable machine learning method to quantify the impacts of multi-faceted droughts on natural and undisturbed vegetation ecosystems. Our results revealed that vegetation in the MSEA region suffered from multi-year drought-induced stress, but overally nearly 70% of the region experienced a greening trend over the study period. Most declining vegetation areas are observed in forest and rainfed croplands in Cambodia and southern Laos whereas Vietnam witnessed a greening trend. Vegetation-drought analysis indicated that recent land-use transitions and lower altitude areas had higher responses of vegetation to droughts. In natural and undisturbed ecosystems, short-term drought disturbances had the largest impact on vegetation, accounting for nearly 93% of observed variations. The largest influential factors among the examined drought indices was identified as the SPEI-3 and TCI, accounting for around 35% and 20% of the observed changes in vegetation, respectively. Notably, the SPEI-3 highlights that favorable wet conditions can result in an enhancement of vegetation condition by up to 15%, while severe drought occurrences can lead to a significant reduction of up to 20% in vegetation condition.