Vegetation dynamics along drydowns across global climate zones

Abstract. Vegetation greenness responds dynamically to accumulating water deficits, with consequences for terrestrial carbon and water cycling. Yet how greenness changes as a deficit builds within a drydown, how this varies across climates, and how it is changing over time remain poorly resolved. Here, we analyse satellite greenness (MODIS Enhanced Vegetation Index, EVI) against cumulative water deficit (CWD, from MSWEP precipitation and GLEAM evapotranspiration) during annual maximum drydowns across the global vegetated surface from 2000 to 2023. We characterise the EVI–CWD relationship through a high-greenness peak state and a low-greenness state, separating the direction of the greenness response from its steepness. The direction of the EVI–CWD response is strongly organised by climate. Greenness tended to rise with accumulating deficit under energy limitation, covering more than half the vegetated surface, and tended to decline under water limitation. Its steepness reflected both the relative greenness change and the width of the cumulative-deficit window between the two states, so ecosystems differed both in the deficit range separating their two states and in how much greenness changed across them. Long-term trends further showed that the two states did not always change together. Peak-state EVI trends were predominantly positive across all climate-zone and regime combinations, often even where the associated CWD trend was positive. Under water limitation, however, low-state greening was consistently weaker than peak-state greening across climate zones and shifted to browning in Cold Humid regions. This contrast shows that water limitation can become more apparent at the low-greenness end of drydowns, even where peak greenness continues to rise. Resolving greenness along drydown trajectories therefore provides a more complete view of how ecosystems withstand accumulating water deficit than aggregated anomalies or single sensitivity metrics.