Solar-induced chlorophyll fluorescence reveals a functional recovery lag behind canopy structural recovery after flash drought on the North China plain

Increasing flash drought frequency poses a serious threat to ecosystem stability. Traditional greenness-based remote sensing methods are slow to detect early suppression of photosynthetic physiology and its recovery. While the earlier response of Solar-Induced Chlorophyll Fluorescence (SIF) to drought stress is well documented, the asymmetry between functional and structural recovery after drought termination has not been systematically quantified. Here we address this gap by comparing SIF and the Normalized Difference Vegetation Index (NDVI) within a unified resistance-resilience framework during the 2015 flash drought on the North China Plain. SIF showed strong spatiotemporal consistency with gross primary productivity (GPP, R 2 = 0.95). Quantitative assessment revealed, for the first time, that recovery times differed markedly among indicators (SIF 30.7 d, NDVI 20.3 d, GPP 40.0 d). This ordering, with NDVI fastest, SIF intermediate, and GPP slowest, reflects a hierarchical recovery cascade. Structural re-greening occurs first, followed by photochemical efficiency restoration as captured by SIF, and finally full re-establishment of carbon assimilation metabolism as represented by GPP. Both functional recoveries, SIF and GPP, thus lag behind structural recovery. Both also showed high spatial consistency, identifying ecologically fragile areas such as the sandy lands of the Yellow River floodplain. Air temperature was the primary environmental predictor associated with SIF variations. These findings suggest that SIF enables early detection of photosynthetic decline and, more importantly, provides the first quantitative evidence that functional recovery lags behind structural recovery, a critical insight for post-drought ecological assessment.