Beyond precipitation pulses: hydrological memory and atmospheric demand shape vegetation activity and phenology in a Patagonian steppe

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Abstract
Vegetation activity in arid ecosystems is traditionally interpreted as a direct response to precipitation pulses. However, the extent to which vegetation dynamics are regulated by antecedent soil water storage and its interaction with atmospheric demand remains poorly understood. Here, we combined six years of near-surface GCC from PhenoCam, field NDVI, and satellite-derived vegetation indices to examine the ecohydrological controls of vegetation activity and phenological dynamics in a Patagonian steppe in southern Argentina. Vegetation activity and phenological metrics describing both timing and magnitude were more strongly associated with antecedent soil water storage (SWS) integrated across the 2-m soil profile than with precipitation alone. This suggests that vegetation responses are better explained by hydrological memory than by recent rainfall pulses. However, this control changed with atmospheric demand: under low vapor pressure deficit (VPD) vegetation activity was mainly associated with shallow SWS, whereas under high VPD, deeper SWS became the primary driver. Across phenological metrics, SWS showed a pervasive influence, while atmospheric variables exerted more selective but biologically critical effects. In addition, integrated canopy greenness was positively correlated with grass aboveground net primary productivity, supporting its use as a proxy for seasonal productivity. Our results demonstrate that vegetation activity and phenology in the Patagonian steppe are governed by the interaction between antecedent SWS and atmospheric demand. By identifying profile-scale water availability as a key ecohydrological control, this study highlights the need to incorporate both hydrological memory and atmospheric demand into monitoring and modeling approaches under increasing aridity.