Impacts of warming and elevated CO 2 on a semi‐arid grassland are non‐additive, shift with precipitation, and reverse over time

Abstract

It is unclear how elevated CO 2 ( 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 ~ 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.

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