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

Mueller, K. E.; Blumenthal, D. M.; Pendall, E.; Carrillo, Y.; Dijkstra, F. A.; Williams, D. G.; Follett, R. F.; Morgan, J. A.

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

2016

It is unclear how elevated CO₂ (eCO₂) 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 eCO₂ 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 eCO₂ 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 eCO₂ and warming on C₃ graminoids became increasingly positive and supra‐additive over time, reversing an initial shift toward C₄ grasses. Soil resources also responded dynamically and non‐additively to eCO₂ 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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