Water is input into ecosystems as rainfall pulses followed by an interstorm drying period. It is essential to understand ecosystem vegetation responses on this fundamental unit timescale to quantify global water, carbon, and energy cycle sensitivity to shifting rainfall frequency in a changing climate. Previous plant hydraulic and photosynthetic observations following individual rain pulses are confined primarily to individual field studies in drylands (sparse locations and limited duration). As such, little is known about pulse responses on ecosystem scales and beyond drylands, especially compared to seasonal timescales. New satellite observations from NASA’s Soil Moisture Active Passive Mission allow global estimation of pre-dawn vegetation water content and surface soil moisture. Using this dataset augmented with FLUXNET carbon flux observations, we ask: what are the spatial and temporal patterns of ecosystem scale plant response following soil moisture pulses? What hydrologic and physiologic mechanisms may drive these patterns? We use surface soil moisture to identify week-long drydown periods following rainfall. We assess plant water content and carbon flux behavior during these time periods and their relationship to moisture pulse magnitude and antecedent moisture conditions.
Results/Conclusions
Following soil moisture pulses, plant water content tends to increase over multiple days in global drylands. These delays become progressively less frequent in humid regions where rapid plant and soil water coupling occurs (expected under pre-dawn plant and soil water potential equilibrium). In drylands, longer multiday plant water increases (> 3 days) are attributed to growth. This suggests individual storms frequently trigger intermittent growth responses, evidence for the pulse-reserve hypothesis. In contrast, shorter (1-3 day) plant water increases under dry conditions are due to slow plant rehydration, ascribed here to high soil-plant resistances using a plant hydraulic model. A similar dry to wet gradient in behavior is observed in carbon flux observations with drylands exhibiting more frequent, longer carbon uptake following pulses. Unlike wetter environments, pulse characteristics significantly influence dryland plant water and carbon flux behavior. For example, larger pulses on initially wetter soils tend to create the longest, most vigorous dryland plant water and carbon uptake responses. These results together suggest that intermittent pulses of water availability have the strongest influence on dryland vegetation. However, responses extend across climate gradients implicating widespread sensitivity to rainfall timing.