Patterns of plant rehydration and growth following pulses of soil moisture availability

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.

Patterns of plant rehydration and growth following pulses of soil moisture availability

Plant hydraulic and photosynthetic responses to individual rain pulses are not well understood because pulse experiments are sparse. Understanding individual pulse responses would inform how rainfall intermittency impacts terrestrial biogeochemical cycles, especially in drylands which play a large role in global atmospheric carbon uptake interannual 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, which supports isolated field experiments. The response dependence on moisture pulse characteristics also implies ecosystem sensitivity to intra-annual rainfall intensity and frequency, which are expected to shift in a future climate.

Phenology of Croton blachetianus Baill in a dry tropical forest associated with the dynamics of temporal variability of rainfall

In the semi-arid environment, the synchronism and the magnitude of the precipitation pulses are indispensable for the ecological processes, mainly due to the availability of water in the soil for the plants and to the microbiological activity of the soil. The present study aimed to determine the vegetative and reproductive phenological behavior of Croton blachetianus Baill in areas of Caatinga. The hypothesis was that the occurrence of the phenophases is synchronized with the spatial and temporal distribution of the precipitation pulses, under semi-arid conditions. For the phenological study, 50 tree individuals were selected, marked and monitored weekly, recording the presence and absence of the phenophases of sprouting, appearance of flower buds, flowering (anthesis), fruiting and senescence, whose data were related to the water pulses and inter-pulses. The method proposed by Fournier was used to estimate the percentage of the intensity of the phenophases in each individual. The phenological data of intensity of each phenophase (sprouting, flower-bud, flowering, fruiting and senescence) were related to the distribution of rain pulses and through Spearman’s correlation coefficient, using the number of individuals under each phenophase and the daily climate data. The vegetative and reproductive phenological behavior of C. blachetianus reflected the seasonal pattern of precipitation. The intensity and duration of the phenophases depended on the extent and frequency of the rain pulses during the rainy season. Total leaf senescence happened when the precipitation inter-pulses intensified as the dry season progressed, characterizing deciduousness.

Influence of rain pulse characteristics over intrastorm throughfall hot moments

Forest canopy alters the amount of rainfall reaching the surface by redistributing it as throughfall. Throughfall is critical to watershed ecological variables (soil moisture, stream water discharge/chemistry, and stormflow pathways) and controlled by canopy structural interactions with meteorological conditions across temporal scales (from seasonal to within-event). This work uses complete linkage cluster analysis to identify intrastorm rain pulses of distinct meteorological conditions (beginning-of-storm and internal-to-storm pulses that are atmospherically dry, moderate, or wet), relates each cluster to intrastorm throughfall responses, then applies multiple correspondence analyses (MCAs) to a range of meteorological thresholds (median intensity, coefficient of variation (CV) of intensity, mean wind-driven droplet inclination angle, and CV of wind speed) for identification of interacting storm conditions corresponding to hot moments in throughfall generation (≥ 80% of rainfall). Equalling/exceeding rain intensity thresholds (median and CV) corresponded with throughfall hot moments across all rain pulse types. Under these intensity conditions, two wind mechanisms produced significant correspondences: (1) high wind-driven droplet inclination angles under steady wind increased surface wetting; and (2) sporadic winds shook entrained droplets from surfaces. Correspondences with these threshold conditions were greatest for pulses of moderate vapour pressure deficit (VPD), but weakest under high VPD. Weaker correspondences between throughfall hot moments and meteorological thresholds for high VPD pulses may be because canopy structures were not included in the MCA. In that vein, strongest meteorological threshold correspondences to throughfall hot moments at our site may be a function of heavy T. usneoides coverage. Future applications of MCA within other forests are, therefore, recommended to characterize how throughfall hot moments may be affected along drainage paths dependent on different structures (leaves, twigs, branches, etc.).

Hydrologic control of the oxygen isotope ratio of ecosystem respiration in a semi-arid woodland

We conducted high frequency measurements of the δ18O value of atmospheric CO2 from a juniper (Juniperus monosperma) woodland in New Mexico, USA, over a four-year period to investigate climatic and physiological regulation of the δ18O value of ecosystem respiration (δR). Rain pulses reset δR with the dominant water source isotope composition, followed by progressive enrichment of δR. Transpiration (ET) was significantly related to post-pulse δR enrichment because the leaf water δ18O value showed strong enrichment with increasing vapor pressure deficit that occurs following rain. Post-pulse δR enrichment was correlated with both ET and the ratio of ET to soil evaporation (ET/ES). In contrast, the soil water δ18O value was relatively stable and δR enrichment was not correlated with ES. Model simulations captured the large post-pulse δR enrichments only when the offset between xylem and leaf water δ18O value was modeled explicitly and when a gross flux model for CO2 retro-diffusion was included. Drought impacts δR through the balance between evaporative demand, which enriches δR, and low soil moisture availability, which attenuates δR enrichment through reduced ET. The net result, observed throughout all four years of our study, was a negative correlation of post-precipitation δR enrichment with increasing drought.

Hydrologic control of the oxygen isotope ratio of ecosystem respiration in a semi-arid woodland

We conducted high frequency measurements of the δ18O value of atmospheric CO2 from a juniper (Juniperus monosperma) woodland in New Mexico, USA, over a four-year period to investigate climatic and physiological regulation of the δ18O value of ecosystem respiration (δR). Rain pulses reset δR with the dominant water source isotope composition, followed by progressive enrichment of δR. Transpiration (ET) was significantly related to post-pulse δR enrichment because leaf water δ18O value showed strong enrichment with increasing vapor pressure deficit that occurs following rain. Post-pulse δR enrichment was correlated with both ET and the ratio of ET to soil evaporation (ET / ES). In contrast, soil water δ18O value was relatively stable and δR enrichment was not correlated with ES. Model simulations captured the large post-pulse δR enrichments only when the offset between xylem and leaf water δ18O value was modeled explicitly and when a gross flux model for CO2 retro-diffusion was included. Drought impacts δR through the balance between evaporative demand, which enriches δR, and low soil moisture availability, which attenuates δR enrichment through reduced ET. The net result, observed throughout all four years of our study, was a negative correlation of post-precipitation δR enrichment with increasing drought.