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Grass-dominated, dry rangelands account for over 30% of Earth's terrestrial surface (9,10) and provide most of the forage for the world's domestic livestock. Among the most important of these include [...], Global warming is predicted to induce desiccation in many world regions through increases in evaporative demand (1-3). Rising C[O.sub.2] may counter that trend by improving plant water-use efficiency (4,5). However, it is not clear how important this C[O.sub.2]-enhanced water use efficiency might be in offsetting warming-induced desiccation because higher C[O.sub.2] also leads to higher plant biomass, and therefore greater transpirational surface (2,6,7). Furthermore, although warming is predicted to favour warm-season, [C.sub.4] grasses, rising C[O.sub.2] should favour [C.sub.3], or cool-season plants (8). Here we show in a semi-arid grassland that elevated C[O.sub.2] can completely reverse the desiccating effects of moderate warming. Although enrichment of air to 600 p.p.m.v. C[O.sub.2] increased soil water content (SWC), 1.5/3.0 °C day/night warming resulted in desiccation, such that combined C[O.sub.2] enrichment and warming had no effect on SWC relative to control plots. As predicted, elevated C[O.sub.2] favoured [C.sub.3] grasses and enhanced stand productivity, whereas warming favoured [C.sub.4] grasses. Combined warming and C[O.sub.2] enrichment stimulated above-ground growth of [C.sub.4] grasses in 2 of 3 years when soil moisture most limited plant productivity. The results indicate that in a warmer, C[O.sub.2]-enriched world, both SWC and productivity in semi-arid grasslands may be higher than previously expected.

Premise: The impact of elevated CO2 concentration ([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 C3 plant species in the Prairie Heating and CO2 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: Anet in the C3 subshrub Artemisia frigida and the C3 forb Sphaeralcea coccinea was stimulated by elevated [CO2] under ambient and warmed temperature treatments. Warming by itself reduced Anet in all species during the dry year, but stimulated photosynthesis in S. coccinea in the wet year. In contrast, Anet in the C3 grass Pascopyrum smithii was 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 of Anet in this semi‐arid temperate grassland, emphasizing the important role of water relations in driving grassland responses to global change. [ABSTRACT FROM AUTHOR]