grasses prosper as carbon dioxide eliminates desiccation in warmed semi-arid grassland

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.