Abstract
AbstractPlants with crassulacean acid metabolism (CAM) are increasing in distribution and abundance in drylands worldwide, but the underlying drivers remain unknown. We investigate the impacts of extreme drought and CO2 enrichment on the competitive relationships between seedlings of Cylindropuntia imbricata (CAM species) and Bouteloua eriopoda (C4 grass), which coexist in semiarid ecosystems across the Southwestern United States. Our experiments under altered water and CO2 water conditions show that C. imbricata positively responded to CO2 enrichment under extreme drought conditions, while B. eriopoda declined from drought stress and did not recover after the drought ended. Conversely, in well‐watered conditions B. eriopoda had a strong competitive advantage on C. imbricata such that the photosynthetic rate and biomass (per individual) of C. imbricata grown with B. eriopoda were lower relative to when growing alone. A meta‐analysis examining multiple plant families across global drylands shows a positive response of CAM photosynthesis and productivity to CO2 enrichment. Collectively, our results suggest that under drought and elevated CO2 concentrations, projected with climate change, the competitive advantage of plant functional groups may shift and the dominance of CAM plants may increase in semiarid ecosystems.
Highlights
Over the last century, human activities associated with fossil fuel burning and land use change have dramatically increased the concentrations of CO2 and other trace gases in the atmosphere, a trend that is expected to continue in the decades to come (Vitousek et al 1997, IPCC 2014)
Meta-analysis The meta-analysis on CO2 enrichment including 19 constitutive crassulacean acid metabolism (CAM) species across global drylands showed an average increase of 51% in daily CO2 uptake and 33% in biomass (Fig. 1; Appendix S1), which is comparable to the biomass response of C3 plants (+45%, 300 species) and greater than the biomass response of C4 plants (+12%, 40 species) to CO2 enrichment (Poorter and Navas 2003)
This study showed the competitive advantage of CAM plants (C. imbricata) over grasses (B. eriopoda) under extreme drought and CO2 enrichment conditions (Fig. 2)
Summary
Human activities associated with fossil fuel burning and land use change have dramatically increased the concentrations of CO2 and other trace gases in the atmosphere, a trend that is expected to continue in the decades to come (Vitousek et al 1997, IPCC 2014). Global climate models predict increased precipitation variability along with more frequent extreme rainfall events, and increased risk of prolonged drought, especially in dryland regions (Smith 2011, IPCC 2014) These changes in environmental conditions combined with human activities have led to significant changes in vegetation cover and plant community composition (Poorter and Navas 2003, D’Odorico et al 2012, Anderegg et al 2013), with important impacts on ecohydrological and geochemical processes, regional climate, and the provision of ecosystem services such as livestock grazing, sheltering of the soil surface, and carbon sequestration (Anderegg et al 2013, Reichstein et al 2013). An empirical evaluation of the response of constitutive CAM plants and their competitive relationships with other functional groups (i.e., C4 plants) to CO2 enrichment and changing climate, is still missing (Poorter and Navas 2003)
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