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

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.