Atmospheric Drying Dominates Recent Climatic Constraints on Vegetation Productivity in the Humid Subtropical Nanling Mountains

Atmospheric drought, defined here as days on which vapor pressure deficit (VPD) exceeds its 90th percentile, is usually studied in water-limited regions yet threatens vegetation where water is abundant. Using solar-induced chlorophyll fluorescence (SIF) from the contiguous SIF (CSIF) dataset, MODIS gross primary productivity (GPP), and meteorology in the Nanling Mountains, China, we quantify the dominant climatic signal and vegetation response. Extreme hot days showed no trend, whereas extreme drought days rose nearly tenfold, from 10.6 to 101.8 d yr−1, partly reflecting a 2002–2003 humidity step; both VPD tails widened. Hydrological drought (via the standardized precipitation evapotranspiration index, SPEI-3) showed no regional trend, and the two trend fields were uncorrelated. A counterfactual reconstruction validated the record; humidity alone contributed +1.06 d yr−1 (46% of the rise), temperature alone +0.12 (not significant), and the remainder reflects their joint covariation. Path analysis showed associations consistent with drought suppressing GPP mainly via SIF (indirect effect −0.174; direct effect not significant). Canopy greenness (near-infrared reflectance of vegetation, NIRv) was unresponsive in dry months, topsoil moisture showed no trend, and MODIS GPP moved in the opposite direction to CSIF in dry months (+2.42% versus −2.33%); both declined in the 2022 flash drought, CSIF five times more (−9.6% versus −1.9%).