The competitive advantage of a constitutive CAM species over a C₄ grass species under drought and CO₂ enrichment

Abstract

Plants 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.

Lagged and dormant season climate better predict plant vital rates than climate during the growing season

“…Examples include the reported long, lagged window in which precipitation has a negative effect on the growth of C. flava . Yu et al (2019) showed that under wetter conditions, B. eriopoda , a C 4 grass, had a competitive advantage over C. imbricata . Indirect effects can also include interspecific (Aschehoug et al, 2016) or intraspecific density dependence (Dahlgren et al, 2016).…”

Demographic back‐casting reveals that subtle dimensions of climate change have strong effects on population viability

“…During our study, we observed recruitment to be almost exclusively from seed (sexual and asexual recruits are easily distinguishable), though it is possible that regeneration dynamics were different under historical grazing regimes. Grazing may have also promoted cactus populations through the release of competitive interactions with grasses (Yu et al., 2019). Thus, one hypothesis is that C. imbricata achieved current densities under the historical land use regime, and cannot maintain these densities in the absence of cattle grazing.…”

Dissecting succulence: Crassulacean acid metabolism and hydraulic capacitance are independent adaptations in Clusia leaves

“…With the advent of global warming, succulent species are becoming increasingly competitive with sympatric nonsucculent plants. For example, Mesembryanthemum crystallinum (Aizoaceae) and Cylindropuntia imbricate (Cactaceae) begin to outcompete sympatric C 3 and C 4 grasses under drier conditions (Huang et al, 2020;Yu et al, 2019).…”