Temporal repackaging of rainfall magnifies negative impacts of vapor pressure deficit on semiarid ecosystem productivity

Authors: ZHANG, F - University Of Arizona; Biederman, Joel; Pierce, Nathan; GUO, J.S. - Claremont Colleges; POTTS, D.L. - Buffalo State College; Scott, Russell; FU, Y.H. - Beijing Normal University; SMITH, W.K. - University Of Arizona

Submitted to: New Phytologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/16/2025
Publication Date: 7/30/2025
Citation: Zhang, F., Biederman, J.A., Pierce, N.A., Guo, J., Potts, D., Scott, R.L., Fu, Y., Smith, W. 2025. Temporal repackaging of rainfall magnifies negative impacts of vapor pressure deficit on semiarid ecosystem productivity. New Phytologist. 248(2):615-627. https://doi.org/10.1111/nph.70431.
DOI: https://doi.org/10.1111/nph.70431

Interpretive Summary: Many portions of the semiarid US Southwest are receiving rainfall that is repackaged into fewer but larger rainstorms, regardless of any trends in the total seasonal or annual rainfall amounts. Meanwhile, continuing warming trends are driving increases in atmospheric dryness, which tends to reduce rangeland productivity. 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 the sensitivity of plant productivity to atmospheric dryness. We found that plant productivity in two dry years was lower than in two humid years, despite application of the same experimentally controlled rainfall in all years. Reduced productivity was exacerbated under fewer, larger rainfalls. Soil moisture and plant community data suggest this was because fewer, larger rainfall treatments tend to cause deeper soil moisture infiltration, favoring deeper-rooted perennial bunchgrass plants, which keeps the ecosystem productivity going even when surface conditions are very dry. These results imply that if rainfall arrives in fewer, larger storms, this will exacerbate the inhibition of plant productivity by ongoing trends towards increasingly dry air.

Technical Abstract: Many drylands are experiencing less frequent but larger rainfall events alongside rising temperatures and vapor pressure deficit (VPD). How these shifts influence dryland productivity remains unclear.Using a 4-yr field experiment in a semiarid grassland, we examined how infrequent but large rainfalls shape VPD constraints on photosynthesis. Ambient conditions over the course of the study spanned 2 yr of extremely high VPD and 2 yr of relatively low VPD, providing a unique opportunity to test the effects of VPD on ecosystem gross primary productivity (GPP) under controlled soil moisture conditions.GPP declined 39'±'4% under high VPD even though irrigation treatments and soil moisture levels were unchanged. Daily GPP sensitivity to VPD was strongest under infrequent but large rainfall events, with a 58%'±'19% increase in sensitivity compared to normal rainfall. Another facet of this four-year study was that deep-rooted perennials became increasingly dominant through time under infrequent but large rainfalls independent of VPD. Meanwhile, shallow-rooted annuals became more dominant in frequent but small rainfall treatments and varied with VPD.These findings underscore how temporally repackaging rainfall into fewer, larger events exacerbates VPD constraints on photosynthesis by driving shifts in vegetation structure, and how an intensified hydrological cycle may impact dryland productivity.