C_4 grasses prosper as carbon dioxide eliminates desiccation in warmed semi-arid grassland

摘要

Global warming is predicted to induce desiccation in many world regions through increases in evaporative demand1"3. Rising CO_2 may counter that trend by improving plant water-use efficiency~(4,5). However, it is not clear how important this CO2-enhanced water use efficiency might be in offsetting warming-induced desiccation because higher CO_2 also leads to higher plant biomass, and therefore greater transpirational surface2'6'7. Furthermore, although warming is predicted to favour warm-season, C_4 grasses, rising CO_2 should favour C_3, or cool-season plants8. Here we show in a semi-arid grassland that elevated CO_2 can completely reverse the desiccating effects of moderate warming. Although enrichment of air to 600 p.p.m.v. CO_2 increased soil water content (SWC), 1.5/3.0 ° C dayight warming resulted in desiccation, such that combined CO_2 enrichment and warming had no effect on SWC relative to control plots. As predicted, elevated CO_2 favoured C_3 grasses and enhanced stand productivity, whereas warming favoured C_4 grasses. Combined warming and CO_2 enrichment stimulated above-ground growth of C_4 grasses in 2 of 3 years when soil moisture most limited plant productivity. The results indicate that in a warmer, CO_2-enriched world, both SWC and productivity in semi-arid grasslands may be higher than previously expected.