Impacts of warming and elevated CO2 on a semi-arid grassland are non-additive, shift with precipitation, and reverse over time

It is unclear how elevated CO2 (eCO(2)) and the corresponding shifts in temperature and precipitation will interact to impact ecosystems over time. During a 7-year experiment in a semi-arid grassland, the response of plant biomass to eCO(2) and warming was largely regulated by interannual precipitation, while the response of plant community composition was more sensitive to experiment duration. The combined effects of eCO(2) and warming on aboveground plant biomass were less positive in wet' growing seasons, but total plant biomass was consistently stimulated by similar to 25% due to unique, supra-additive responses of roots. Independent of precipitation, the combined effects of eCO(2) and warming on C-3 graminoids became increasingly positive and supra-additive over time, reversing an initial shift toward C-4 grasses. Soil resources also responded dynamically and non-additively to eCO(2) and warming, shaping the plant responses. Our results suggest grasslands are poised for drastic changes in function and highlight the need for long-term, factorial experiments.