Patterns and Variability in Tree Seedling Photosynthesis: Implications for Recruitment Under Climate Change

Abstract

Predicting future forests’ structure and functioning is a critical goal for ecologists, and seedling performance under climate change will in large part determine future forest structure and composition. Seedling photosynthetic response will be key in determining if a particular species recruits enough individuals to maintain its populations. During the 2012 growing season we studied photosynthetic responses of seedlings of four dominant tree species to a wide range of environmental conditions based on temporally extensive in situ gas exchange measurements. Despite the large intraspecies variability in observed assimilation rates we found significant species differences in light saturated maximum assimilation rate (1.95 ± 0.1415 μmol m-2 s-1 for Acer saccharum, 2.95 ± 0.17 μmol m-2 s-1 for Carya glabra, 3.86 ± 0.23 μmol m-2 s-1 for Quercus rubra, and 4.28 ± 0.1972 μmol m-2 s-1 for Quercus velutina) and photosynthesis under field light levels (1.66 ± 0.29 μmol m-2 s-1 for A. saccharum to 3.21 ± 0.40 μmol m-2 s-1 Q. velutina). Under increases in temperature, assimilation rate will likely decrease by approximately 0.20 μmol m-2 s-1 for C. glabra, 0.21 μmol m-2 s-1 for Q. rubra, and 0.35 μmol m-2 s-1 for Q. velutina in spring, but not as much or at all in summer and fall. This is likely due in part to concurrent increases in spring dark respiration rates with temperature of 0.15 μmol m-2 s-1 for C. glabra, 0.16 μmol m-2 s-1 for Q. rubra, and 0.16 μmol m-2 s-1 for Q. velutina. However, decreased stomatal conductance in response to drought was likely responsible for the observed lack of response to higher temperatures in summer and fall, as well as the lack of response of A. saccharum to temperature in all three seasons. We also found that, while seasonal variability exists in photosynthetic response, assimilation rate was equal in summer and fall, and that this seasonal variability was greater in the drought tolerant oak species we tested (Q. rubra and Q. velutina) than the other species (C. glabra and A. saccharum). This finding points to the importance of field measurements in evaluating the strength of trends seen in the greenhouse. Lastly, under projected increases in temperature and aridity, drought tolerant species may be at a competitive advantage, due to superior photosynthetic capacity under these conditions. Our findings indicate that seasonal trends in photosynthesis may be altered, and oak species may become more dominant in Northeastern forests under projected increases in temperature and aridity due to climate change.