Glacial refugia as indicators of climate change: The effects of temperature on carbon balance of Canaan and Fraser fir

Background/Question/Methods Refugial tree species from the last glacial maximum exist in high altitude stands at their southernmost distributions in the southern Appalachians. We were interested in how future climate change might affect these species; thus, we conducted a greenhouse experiment to determine the effect of increasing temperature on photosynthetic efficiency, needle respiration, and growth of two refugial tree species, Fraser fir (Abies fraseri) and Canaan fir (Abies balsamea var. phanerolepis). We hypothesized that the northern species, Canaan fir, would show a greater sensitivity to warmer temperatures than the southern species, Fraser fir. Each species was grown in greenhouses set to low and high temperature regimes (27/20°C and 32/25°C day/night, respectively). After 6 weeks of growth in the two temperature treatments, light-saturated photosynthesis (A_sat) and dark respiration (R_d) were measured on fully-expanded needles of seedlings. The short-term effect of elevated temperature on A_sat and R_d was determined at the same time on seedlings grown in the low temperature treatment exposed to elevated temperatures for 24 hr immediately prior to gas exchange measurements. After 16 weeks of temperature treatment, seedlings were harvested; tissues were separated, and leaf area and dry biomass were determined.

Results/Conclusions Elevated temperature had a strong negative effect on biomass production of both refugial tree species with significant reductions in root (27-30%) and leaf mass (40-44%). Rates of dark respiration (R_d) under the high temperature treatment were 52% and 78% higher for Canaan fir and Fraser fir, respectively, compared to the low temperature treatment. Dark respiration of needles of both species did not show a strong acclimation response to temperature. Light-saturated photosynthesis (A_sat) declined by 29% for Canaan fir and 50% for Fraser fir following short-term exposure to the high temperature treatment. However, photosynthesis of seedlings grown in the low and high temperature treatments was not significantly different, indicating a strong acclimation response for both species. Our hypothesis that there would be a differential response to temperatures by the two species was not supported by our data. We conclude that seedling recruitment of both of these two high elevation refugial tree species may be negatively affected by increasing temperatures due to the negative impacts of temperature on biomass production. Although photosynthesis appears to be able to acclimate to the higher temperatures, we did not observe an acclimation response for dark respiration in either species.