Fast response of satellite fluorescence-derived plant physiology to drought stress.

The differences in response times between vegetation physiology and vegetation structure to water stress at the global scale remain unclear. Here, we integrate solar-induced chlorophyll fluorescence satellite observations, optical remote sensing indices, and hydrometeorological data to globally disentangle the sequence and related factors of vegetation physiological and structural responses to drought. We isolated fluorescence efficiency by normalizing solar-induced chlorophyll fluorescence by absorbed photosynthetically active radiation; We show that fluorescence efficiency is a robust proxy for ecosystem-level vegetation physiology and responds to drought within ~3 days, while structural changes emerge after ~12 days. The contrast in timing is clearest in humid regions, owing to the sustained soil moisture availability during the initial stage of drought. Physiological responses are more temporally aligned with changes in vapor pressure deficit, whereas structural changes coincide more with soil moisture dynamics, reflecting differing patterns of association under increasing drought stress. These findings advance mechanistic understanding of vegetation drought responses across the continuum from physiological to structural processes.