Response of the Shortgrass Steppe to Changes in Rainfall Seasonality

Studies in temperate grassland ecosystems have shown that differences in composition of C₃ and C₄ plant functional types can have important influences on ecosystem pools and processes. We used a plant community dynamics model (STEPPE) linked to a biogeochemical cycling model (CENTURY) to determine how ecosystem properties in shortgrass steppe are influenced by plant functional type composition. Because of phenological differences between C₃ and C₄ plants, we additionally simulated the effects of precipitation seasonality on plant communities and examined how C₃ and C₄ composition interacts with precipitation to affect ecosystems. The model output suggests that differences in C₃ and C₄ composition can lead to differences in soil organic carbon (C) and nitrogen (N) within 1000 simulation years. Soil organic C and N (g C and N m^{− 2} to 0.2-m depth) were least in a 100% C₄ community compared with a 100% C₃ community and a mixed C₃–C₄ community. A change in the time of maximum precipitation from summer to spring in a simulated shortgrass steppe slightly favored C₃ plants over C₄ plants. The proportion of total net primary production accounted for by C₃ plants increased from 21% to 25% after 200 years, when 90 mm of precipitation was switched from summer to spring. Soil organic matter (SOM) was relatively stable in the C₄-dominated communities with respect to changes in precipitation seasonality, whereasSOM in the C₃ community was sensitive to precipitation seasonality changes. These results suggest an important interaction between plant community composition and precipitation seasonality on SOM, with phenology playing a key role. Received 9 June 1998; accepted 6 January 1999