Climate warming alters photosynthetic responses to elevated CO 2 in prairie plants

Premise The impact of elevated CO 2 concentration ([CO 2 ]) and climate warming on plant productivity in dryland ecosystems is influenced strongly by soil moisture availability. We predicted that the influence of warming on the stimulation of photosynthesis by elevated [CO 2 ] in prairie plants would operate primarily through direct and indirect effects on soil water. Methods We measured light‐saturated photosynthesis ( A net ), stomatal conductance ( g s ), maximum Rubisco carboxylation rate ( V cmax ), maximum electron transport capacity ( J max ) and related variables in four C 3 plant species in the Prairie Heating and CO 2 Enrichment (PHACE) experiment in southeastern Wyoming. Measurements were conducted over two growing seasons that differed in the amount of precipitation and soil moisture content. Results A net in the C 3 subshrub Artemisia frigida and the C 3 forb Sphaeralcea coccinea was stimulated by elevated [CO 2 ] under ambient and warmed temperature treatments. Warming by itself reduced A net in all species during the dry year, but stimulated photosynthesis in S. coccinea in the wet year. In contrast, A net in the C 3 grass Pascopyrum smithii was not stimulated by elevated [CO 2 ] or warming under wet or dry conditions. Photosynthetic downregulation under elevated [CO 2 ] in this species countered the potential stimulatory effect under improved water relations. Warming also reduced the magnitude of CO 2 ‐induced down‐regulation in this grass, possibly by sustaining high levels of carbon utilization. Conclusions Direct and indirect effects of elevated [CO 2 ] and warming on soil water was an overriding factor influencing patterns of A net in this semi‐arid temperate grassland, emphasizing the important role of water relations in driving grassland responses to global change.