Differential phenological responses of plant functional types to the temporal repackaging of precipitation in a semiarid grassland

Authors: ZHANG, F. - University Of Arizona; Biederman, Joel; DEVINE, C.J. - University Of Arizona; Pierce, Nathan; YAB, D. - China Renewable Energy Engineering Institute; POTTS, D.L. - Buffalo State College; SMITH, W.K. - University Of Arizona

Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/19/2025
Publication Date: 2/28/2025
Citation: Zhang, F., Biederman, J.A., Devine, C., Pierce, N.A., Yab, D., Potts, D., Smith, W. 2025. Differential phenological responses of plant functional types to the temporal repackaging of precipitation in a semiarid grassland. Plant and Soil. https://doi.org/10.1007/s11104-025-07323-8.
DOI: https://doi.org/10.1007/s11104-025-07323-8

Interpretive Summary: Many portions of the semiarid US Southwest are receiving rainfall that is repackaged into fewer but larger rain storms, regardless of any trends in the total seasonal or annual rainfall amounts. This rainfall repackaging has unknown consequences for the phenology of rangeland ecosystems including green-up timing, growing season length, and brown-down timing at the end of the growing season. Furthermore, rangeland ecosystems are often composed of different plant types that may each respond differently. In this work, we used rainout shelters at the USDA-ARS rangeland experimental facility RainMan to measure how repackaging a fixed growing season rainfall amount of 205 into fewer/larger rainfall events impacted phenology, which was measured daily by automated cameras. We found that fewer/larger rainfalls delayed green-up after first rainfall by an average of 22 days. This was mainly because shallow-rooted annual grasses failed to thrive under few/large rainfalls, whereas deep-rooted perennial grasses thrived on deeply-infiltrated soil moisture but were slower to respond after the onset of first rainfall. Fewer/larger rainfalls increased the growing season length for perennial grasses and shortened it for annual grasses. These results imply that regardless of rainfall totals, climate change through rainfall repackaging is likely to increase the relative importance of deeper-rooted perennial plants at the expense of shallow-rooted annual plants.

Technical Abstract: Large portions of the western United States have witnessed extended dry intervals between rainfall events, resulting from an intensified hydrological cycle associated with global warming. Semi-arid ecosystems in these regions are anticipated to be especially responsive to the temporal repackaging of rainfall due to their high sensitivity to soil moisture. How such rainfall temporal repackaging alters canopy and plant phenology remains unknown. Addressing this knowledge gap is critical, as phenological shifts may have significant implications for a wide range of ecosystem services and management decisions. We conducted a field manipulation experiment to evaluate the canopy and plant phenological response to summer rainfall repackaging (from many/small events to few/large events while maintaining a fixed total seasonal amount) in a semi-arid mixed annual/perennial bunchgrass ecosystem. We monitored canopy and plant greenness by combining automated, high frequency repeat digital images at both plot and plant functional type levels, and derived phenological metrics including start of season, end of season, and growing season length. We found that canopy onset was delayed ~22 days under few/large events compared to normal historical rainfall pattern. As we expected, the phenology metrics of three plant functional types (perennial grasses, perennial forbs, and annual forbs) showed distinct responses to rainfall repackaging. Specifically, we found a longer growing season length for perennial grasses and a shortened growing season for annuals under few/large events compared to many/small events. Furthermore, phenological metrics of perennial grasses showed greater responsiveness to deep soil water conditions compared to upper soil layers, resulting in a longer growing season length under increased wetness at deep soil depths during few/large events. Our analysis demonstrates the potential for high frequency monitoring of plant functional type phenology to advance our understanding of semi-arid ecosystem response to ongoing rainfall repackaging, and consequently its impact on global biogeochemical cycling.