Inter- and intraspecific variation in the response of photosynthesis to elevated [CO2] in a Florida scrub oak community

Abstract

Understanding inter- and intraspecific responses to rising [CO2] is critical for assessing possible future shifts in competition and species dominance, and for accurately predicting future carbon exchange in forest ecosystems. Two clonal, co-dominant evergreen oaks within a Florida scrub-oak ecosystem have regrown entirely under elevated [CO2], providing a rare opportunity to assess inter- and intraspecific variation during canopy closure. Photosynthesis was measured with an open-gas exchange system to assess acclimation of photosynthetic capacity in two co-dominant oaks. There was no acclimatory loss of photosynthetic capacity in Quercus myrtifolia; therefore, with decreased photorespiration in elevated [CO2], a 72% stimulation of net photosynthetic rate was observed. Quercus geminata showed a large acclimatory loss of photosynthetic capacity sufficient to offset any stimulation of light-saturated net photosynthesis due to decreased photorespiration. The photosynthetic rate of Q. geminata leaves grown and measured at elevated [CO2] was no higher than leaves grown at ambient [CO2]. Q. myrtifolia also exhibited an increase in photosynthetic nitrogen use efficiency, which was still present at canopy closure. Intraspecific variation in the response of photosynthesis to elevated [CO2] exists in both species. The role that individual genotypes play in that variation is currently unknown. DNA microsatellite analysis of Q. myrtifolia and Q. geminata is being used to estimate genetic distances between individuals. The results show that at canopy closure in a woody community, elevation of [CO2] causes a substantial difference in the capacity of co-dominants to acquire carbon and energy, with important implications for fitness and future composition of this community.