Life history tradeoffs are widespread in plant ecology, yet we rarely understand the physiological traits that drive these trade-offs, and how these mechanisms can inform predictions for how communities will respond to climate change. I investigated whether competing physiological demands for carbon and water help explain a growth-survival tradeoff in five species of summer annual plants in the Chihuahuan Desert. These species have been shown to have a tradeoff between growth at high soil moisture and survival at low soil moisture. I measured photosynthetic and transpiration rates in the field across a range of soil moisture conditions. Specifically, I hypothesize that maintaining higher photosynthetic rates at low soil moisture allow high-survival species to persist. However, this comes at the cost of lower photosynthetic rates and slower growth when soil moisture increases compared to fast-growing species.
Results/Conclusions
As hypothesized, species with low-survival maintained photosynthesis rates 2-3 times lower than their high-survival counterparts in dry conditions. As soil moisture increased, fast-growing species rapidly increased photosynthetic rates, but this increase only allowed them to surpass the photosynthesis of two of the three slow-growing species in this system. In addition, water use efficiency (photosynthesis/transpiration) changed across soil moisture conditions corresponding with life history strategy. Fast-growing, low-survival species’ water use efficiency increased in wet conditions, while slower-growing, high-survival species maximized their water use efficiency in dry conditions. Climate change in the Chihuahuan Desert is predicted to increase rainfall event size, but also the number of dry days without rainfall between events. Increasing rainfall event size is likely to favor fast-growing, low-survival species, which are better able to capitalize on periods of high soil moisture. However, increasing numbers of dry days between rain events may counteract this favoring slow-growing, high-survival species. These dual effects may help to maintain plant diversity in this community in the face of drastic climate change.