Abstract
Abstract. Plant hydraulic and photosynthetic responses to individual rain pulses are not well understood because field experiments of pulse behavior are sparse. Understanding individual pulse responses would inform how rainfall intermittency impacts terrestrial biogeochemical cycles, especially in drylands, which play a large role in interannual global atmospheric carbon uptake variability. Using satellite-based estimates of predawn plant and soil water content from the Soil Moisture Active Passive (SMAP) satellite, we quantify the timescales of plant water content increases following rainfall pulses, which we expect bear the signature of whole-plant mechanisms. In wetter regions, we find that plant water content increases rapidly and dries along with soil moisture, which we attribute to predawn soil–plant water potential equilibrium. Global drylands, by contrast, show multi-day plant water content increases after rain pulses. Shorter increases are more common following dry initial soil conditions. These are attributed to slow plant rehydration due to high plant resistances using a plant hydraulic model. Longer multi-day dryland plant water content increases are attributed to pulse-driven growth, following larger rain pulses and wetter initial soil conditions. These dryland responses reflect widespread drought recovery rehydration responses and individual pulse-driven growth responses, as supported by previous isolated field experiments. The response dependence on moisture pulse characteristics, especially in drylands, also shows ecosystem sensitivity to intra-annual rainfall intensity and frequency, which are shifting with climate change.
Highlights
A changing climate is likely to shift mean annual rainfall, and the frequency and intensity of rainfall events (Donat et al, 2016; Giorgi et al, 2019; Trenberth, 2011)
In regions with tp ≥ 1 d, vegetation optical depth (VOD) typically begins increasing during the rain pulse period instead of with a lag after soil moisture drying begins
In assessing what differentiates rapid responses and short VOD increases that appear driven by only rehydration, we find short VOD increases have slightly larger pulse magnitudes (Fig. 6b) and drier antecedent soil moisture than rapid responses (Fig. 6a)
Summary
A changing climate is likely to shift mean annual rainfall, and the frequency and intensity of rainfall events (Donat et al, 2016; Giorgi et al, 2019; Trenberth, 2011). Changing rainfall frequency with the same annual rainfall can impact terrestrial carbon uptake (Fay et al, 2003; Knapp et al, 2002), suggesting ecosystem sensitivity to characteristics of individual rain events. This motivates characterizing plant responses to individual moisture availability pulses across climates and biomes. This is especially the case for semi-arid herbaceous plants which respond primarily to individual pulses, likely occurring under a pulse reserve paradigm of individual rainfall events triggering photosynthetic responses and storages (Collins et al, 2014; Feldman et al, 2018; Noy-Meir, 1973). Vegetation responses on these shorter timescales are less well understood than the more commonly studied responses to monthly or annual water anomalies
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