Shifts in plant phenology have been one of the most widely reported responses of organisms to current climate change. However, few studies have addressed the implications that such phenological trends might have on plant communities. In the case of forest ecosystems, seedlings of deciduous trees often rely on periods of high light availability at the beginning and end of the growing season to fix most of that year’s carbon. Climate change has led to a global increase in growing season, however if phenological shifts differ among species, and the time between seedling and canopy leaf-out increases for some but decreases for others, species may begin to diverge in their capacity to maintain a positive carbon balance, thereby shifting recruitment dynamics in these forests. Here we measured in situ carbon assimilation rates along the growing season on seedlings of two coexisting species common in North American eastern forests, Acer saccharum and Quercus rubra. We then analyzed each species’ carbon assimilation performance along the growing season and also as a function of the environmental conditions under which each seedling was growing (light and soil moisture environments).
Our analysis confirms that seasonal differences exist between seedlings of the two study species with respect to their carbon assimilation rates. Acer saccharum seedlings had their highest net photosynthetic rates at the beginning of the growing season, were largely inactive during the dry part of the growing season, and were unable to capitalize on late fall light availability as most of the seedlings had already senesced their leaves. Quercus rubra seedlings, in contrast, largely failed to capitalize on early spring light availability, but were able to maintain elevated assimilation rates during the summer and into the late fall. Overall, our results suggest that climate change-induced shifts in foliar phenology could differentially alter carbon assimilation dynamics among temperate hardwood forest tree seedlings, and therefore may alter recruitment.