Response of a semi-arid grassland to altered seasonal patterns of water availability

Background/Question/Methods

Climate change is altering seasonal dynamics in a wide range of ecosystem with consequences that include shifts in phenology, timing of nutrient availability, and changes in plant community composition. Current research has primarily focused on temperature as the key driver for these shifts because of the strong directional trend with climate warming, however, alterations in the availability of water across seasons is an unappreciated aspect of climate change that can significantly influence ecosystem functioning. While changes in the seasonal availability of water are expected to be globally pervasive, arid- and semi-arid ecosystems may be particularly vulnerable because they are already water-limited and often have species with distinct seasons of growth. Here, we designed an experiment to understand how shifts in the seasonal availability of water affected ecosystem function in a northern mixed grass prairie in southeastern Wyoming. We reduced spring rainfall (April – June 2021) using rainout shelters and returned the excluded precipitation during the summer (July-September), effectively shifting spring rainfall to summer rainfall. We then assessed responses in soil moisture, soil respiration, leaf water potentials for key C3 and C4 grasses, above-ground net primary production (ANPP), and shifts in plant phenology and community composition.

Results/Conclusions

In 2021, total growing season precipitation (April – September) was 270 mm with 200mm occurring from April-June, making this year a very wet spring followed by a dry summer. Our treatment excluded 200mm from April-June and added that amount back from July-September in a similar pattern. This shift in precipitation influenced soil water availability over time, leading to increased soil respiration in the summer months, and sustained canopy greenness throughout the growing season. Despite these responses, there were no significant differences in total or C3 ANPP between treatment and control plots. This result is likely driven by extremely high levels of spring soil moisture, which occurred before the rainout shelters were deployed, and sustained growth of the dominant C3 grasses. However, in plots with a high percent cover of C4 grasses, C4 ANPP increased significantly in response to greater summer rainfall. We conclude that shifts in the seasonality of water availability with climate change will alter carbon cycling, shift seasonal patterns of canopy albedo, and differentially impact C3 vs. C4 species in this regionally extensive semi-arid grassland.