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  1. Premise

    The impact of elevated CO2concentration ([CO2]) 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 [CO2] in prairie plants would operate primarily through direct and indirect effects on soil water.

    Methods

    We measured light‐saturated photosynthesis (Anet), stomatal conductance (gs), maximum Rubisco carboxylation rate (Vcmax), maximum electron transport capacity (Jmax) and related variables in four C3plant species in the Prairie Heating and CO2Enrichment (PHACE) experiment in southeastern Wyoming. Measurements were conducted over two growing seasons that differed in the amount of precipitation and soil moisture content.

    Results

    Anetin the C3subshrubArtemisia frigidaand the C3forbSphaeralcea coccineawas stimulated by elevated [CO2] under ambient and warmed temperature treatments. Warming by itself reducedAnetin all species during the dry year, but stimulated photosynthesis inS. coccineain the wet year. In contrast,Anetin the C3grassPascopyrum smithiiwas not stimulated by elevated [CO2] or warming under wet or dry conditions. Photosynthetic downregulation under elevated [CO2] in this species countered the potential stimulatory effect under improved water relations. Warming also reduced the magnitude of CO2‐induced down‐regulation in this grass, possibly by sustaining high levels of carbon utilization.

    Conclusions

    Direct and indirect effects of elevated [CO2] and warming on soil water was an overriding factor influencing patterns ofAnetin this semi‐arid temperate grassland, emphasizing the important role of water relations in driving grassland responses to global change.

     
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