PS 7-69
Patterns and variability in seedling carbon assimilation: Implications for tree recruitment under climate change
Predicting future forests’ structure and functioning is a critical goal for ecologists. Information on seedling recruitment will be crucial for determining the composition and structure of future forest ecosystems. Seedling photosynthetic response to changing environmental conditions is key in determining recruitment in populations experiencing a changing climate. We studied photosynthetic responses of sugar maple (Acer saccharum), pignut hickory (Carya glabra), northern red oak (Quercus rubra), and eastern black oak (Quercus velutina) seedlings to a range of environmental conditions using temporally extensive in situ gas exchange measurements. We incorporated environmental variables into the Farquhar model of photosynthesis in a Bayesian hierarchical framework and allowed most parameters to vary seasonally to address two questions: (1) How do commonly estimated photosynthetic parameters (gm, Rd, Vcmax, Jmax) vary through the growing season among seedlings in field conditions? (2) How are these parameters affected by light availability, temperature, soil moisture, and vapor pressure deficit (VPD) within and among seasons? In addition, to understand how photosynthetic assimilation rates of our species might be impacted under projected increases in temperature due to climate change (3), we used posterior estimates of model parameters to estimate net assimilation rates under current temperature averages and +3ºC scenarios.
Results/Conclusions
Maximum Rubisco carboxylation (Vcmax) and electron transport (Jmax) rates showed significant seasonal variation, but differed from patterns observed in other studies of adult trees. Vapor pressure deficit and soil moisture limited Jmax and Vcmax across the growing season for all four species. Results from predictions indicated that seedlings would experience large declines in summer assimilation rates under a 3 ºC increase in temperature, while spring and fall assimilation rates may increase. In particular, our models predict decreases in the summer assimilation rate in gap habitats with at least 90% probability, and with 20% to 99.9% probability in understory habitats depending on species. Predictions also show 70% probability of increases in photosynthetic rates in fall, and 52% probability of increases in spring in understory habitats. Our results indicate that under projected increases in temperature and aridity, drought tolerant species will be at a competitive advantage due to superior assimilation rates under these conditions. Our findings indicate that all species will be impacted, but oak species may become more dominant in Northeastern forests under projected increases in temperature, though as growing seasons become longer, the effects of climate change on seedling photosynthesis may be complex.