C4 grasses prosper as carbon dioxide eliminates desiccation in warmed semi-arid grassland

Abstract

Global warming is predicted to induce desiccation in many world regions through increases in evaporative demand. Rising CO(2) may counter that trend by improving plant water-use efficiency. However, it is not clear how important this CO(2)-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 surface. Furthermore, although warming is predicted to favour warm-season, C(4) grasses, rising CO(2) should favour C(3), or cool-season plants. 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 day/night 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.

Elevated CO2 further lengthens growing season under warming conditions

“…5 and Extended Data Table 3), in 2010 the water savings may not have been sufficient to increase water availability above a threshold (,10%) that corresponds to the permanent wilting point (11%) (ref. 19). In the warmed ecosystem, elevated CO 2 (CT) led to the greatest increase in autumn soil water content in 2008 and 2009 (Fig.…”

“…Dry conditions during warm years have led to early senescence and even the death of long-lived plants 21,22 . Elevated CO 2 counteracts the negative effect of warming on water availability 19,23 (Fig. 5), often delaying the timing of plant life cycle events 12,13 (Fig.…”

It is getting hotter in here: determining and projecting the impacts of global environmental change on drylands

“…This is because of the complex interactions and contrasting effects of predicted changes in different global change drivers. For instance, while the expected reductions in water availability will likely exacerbate water stress and reduce productivity of dryland vegetation [43][44][45], increases in [CO 2 ] may improve the water use efficiency (WUE) of plants, and thus ameliorate and potentially counterbalance negative effects of reduced soil moisture [46][47][48]. However, improvements in WUE may not suffice to compensate negative effects on soil moisture of increased evapotranspiration and reduced rainfall scenarios, particularly when feedbacks between elevated CO 2 , water availability and vegetation are taken into account [49].…”

Dryland ecohydrology and climate change: critical issues and technical advances

“…Many dryland savannas and mixed cropping systems have a combination of different plant physiognomies, including both C 3 and C 4 plants (Wang et al, 2009b(Wang et al, , 2010a. Since C 3 and C 4 plants respond to CO 2 enrichment and temperature increase differently (Morgan et al, 2011), the combination of plant physiognomy increases the complexity of managing and predicting dryland responses to future environmental changes.…”