Genetic variation in responses to increasing temperatures and elevated atmospheric CO2: Results from red maple (Acer rubrum) collected from across the eastern US

Background/Question/Methods Genetic variation in plant responses to increasing atmospheric CO₂ and rising global temperatures may determine how future global change influences plant evolution and ecosystem productivity.  Previous work has observed genetic variation separately in response to high CO₂ and also temperature, but no study has yet examined the concurrent impacts of elevated CO₂ and increased temperature on tree fitness.  We present some of the first results determining how levels of relatedness – the maternal family, the population, and the provenance – affect variation in the CO₂ and temperature responses of a species.  We collected Acer rubrum (red maple) seeds from over thirty populations along a north-south transect in the eastern United States, ranging from New Hampshire to Georgia.  We grew them in six glasshouses at the Harvard University campus. This experiment focused on the germination, growth, biomass allocation, and survivorship responses of red maple to ambient (390 µl/l) and elevated (750 µl/l) concentrations of CO₂ as well as to three temperature regimes:  23^oC day/18^oC night, 26^oC/22^oC, and 29^oC/26^oC – approximating mean August temperatures in Massachusetts, Virginia, and Georgia.  We ask whether the impacts of CO₂ on fitness (namely germination, growth and survivorship) will amplify or negate the effects of rising temperatures. 

Results/Conclusions Thus far we are finding that the largest impact of elevated CO₂ and increased temperatures is on tree seedling survivorship, with trees growing at elevated CO₂ and either 26^oC or 29^oC having the highest survivorship: 90±0.04% (p=0.003) and 91±0.02% (p=0.01), respectively. However, the impacts of CO₂ and temperature on growth and survivorship differed significantly among families, populations, and provenances of origin.  Models predicting future responses of tree species to global warming need to consider both effects of rising CO₂ as well as intra-specific genetic variability in responses of tree species to both temperature and atmospheric CO₂ concentrations.