The relative importance of climate change and the physiological effects of CO2 on freezing tolerance for the future distribution of Yucca brevifolia

We modeled potential changes in geographic distribution due to increased atmospheric CO2 via climate change as well as direct physiological effects. Numerous studies have quantitatively predicted how the geographic distribution of plant species will shift in response to climate change, but few have also included the direct effects of atmospheric CO2 concentrations on plant physiology. We modeled the role that increased seedling freezing tolerance caused by exposure to elevated CO2 would play in determining the future range of the Joshua Tree (Yucca brevifolia). Results from greenhouse experiments were used to define how a doubling of present-day atmospheric CO2 concentrations changes the low-temperature tolerance. We used discriminant analysis to predict Y. brevifolia distribution as a function of climate based on correlations between observational climate data and the current range of this species. We generate a scenario of future climate under doubled CO2 conditions with a general circulation model (GCM) and used this as input for the predictive distribution model. The model predicts that under future climate, the distribution of this species will change dramatically, and that the total area it occupies will decrease slightly. When the direct effects of CO2 on seedling freezing tolerance are included, the model predicts a different and slightly larger future distribution, indicating that the direct effects of CO2 on this aspect of plant physiology will likely play a significant but secondary role in determining the future distribution of Y. brevifolia.