Rainfall temporal repackaging in a semi-arid grassland delayed peak photosynthesis timing but maintained its magnitude

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Fangyue Zhang and Joel A. Biederman, Southwest Watershed Research Center, USDA-ARS, Tucson, AZ, Fangyue Zhang, Nathan A. Pierce and William K Smith, School of Natural Resources and the Environment, University of Arizona, Tucson, AZ, Daniel Potts, Biology, SUNY Buffalo State, Buffalo, NY

Background/Question/Methods
Against a backdrop of rising temperature, the US Southwest is experiencing fewer, larger precipitation events and longer dry intervals between rainfall events. How such temporal ‘repackaging’ of rainfall alters the magnitude and timing of maximum gross primary productivity (GPPmax) remains unknown. Addressing this knowledge gap is critical since changes to GPPmax magnitude and timing can impact a range of ecosystem services and management decisions including forage production, carbon sequestration, synchrony of plant-pollinator interactions and the intensity and timing of grazing.
Here we used a field-based rainfall manipulation experiment in a semi-arid grassland in southern Arizona, US. Sixty experimental plots under 100% rainfall exclusion shelters received equal amounts of total irrigation during July-September, but each of four treatments varied the size of individual rain events (5 to 60 mm) and corresponding dry intervals (3.5 to 21 days). We measured soil moisture at three depths and green chromatic coordinate (GCC) continuously. We made weekly measurements of whole-plot CO2 flux using portable chambers and thereby calculated GPP. Plant community composition was surveyed both early and late in the growing season, while plant traits were measured at the season’s end.
Results/Conclusions
We found (1) the magnitude of seasonal GPPmax was related to the abundance and biomass of deeper-rooted perennial bunchgrasses, although we did not detect significant GPPmax differences across rainfall treatments. (2) As compared to the climatic normal rainfall timing, lower-frequency/larger events led to deeper water infiltration, shifted the timing of GPPmax later by 16 days, and altered end-of-season community composition to include greater diversity of shallow-rooted annual plants. (3) whole-plot GPP magnitude and timing were well tracked by GCC using a network of low-cost, automated RGB cameras.
We conclude that seasonal GPPmax timing was delayed under few-large events compared with a climatic normal rainfall pattern. Our results suggest that temporal repackaging of rainfall into few events may postpone the seasonal GPPmax of semiarid grasslands due to the slower growth response of perennial grasses to soil moisture changes and the increases of annual plant diversity. The timing and magnitude of rainfall events and resulting temporal patterns of soil moisture relative to critical times for plant growth, biomass accumulation, and plant life histories, may regulate ecosystem responses to altered rainfall patterns. Our results indicated that semiarid grasslands could respond immediately to already-observed and model-forecasted shifts to more extreme-duration drought, and future management decisions should consider this response for optimal use and timing of grassland resources.