Ongoing changes in climate place new stresses on forest ecosystems, thereby complicating forest management plans. The Adaptive Silviculture for Climate Change (ASCC) project was established to develop and evaluate forest management plans designed to encourage ecosystem-level tolerance to such changes. At a longleaf pine (P. palustris Mill.) dominant site in southwestern Georgia, the ASCC project is focused on tolerance to drought, an increasingly important climatological feature of the southeastern United States. One management plan under evaluation at this site uses thinnings and seedling plantings to shift the hardwood component of the forest towards more xeric species, including turkey oak (Quercus laevis Walt.). This study addresses the feasibility of regenerating turkey oak seedlings in a longleaf pine ecosystem and examines the influence of planting location by investigating the impacts of overstory metrics on seedling leaf gas exchange. It was hypothesized that changes in seedling physiology would indicate both facilitative and competitive interactions with the overstory, and that the balance between these interactions would depend on the stressfulness of environmental conditions. Seedling leaf gas exchange and microenvironment were measured over one growing season and examined in relation to overstory metrics for indications of competition and facilitation.
Results/Conclusions
In situ leaf net photosynthetic rate (Pnet) of turkey oak seedlings responded positively to increases in canopy openness. In addition, average Pnet and stomatal conductance (gs) were lower on dates exhibiting more stressful environmental conditions (low soil volumetric water content [VWC] and high vapor pressure deficit [VPD]). However, differences in environmental conditions between measurement dates did not affect the observed relationship between canopy openness and Pnet. VPD was not significantly affected by any overstory metrics; however, VWC was positively correlated with increasing canopy openness. Furthermore, the relationship between canopy openness and VWC was weakened on two measurement dates exhibiting more stressful environmental conditions. Results indicate that VWC and seedling Pnet and gs were negatively impacted by the overstory. Changes in environmental stressors were not found to have an effect on the relationship between the overstory and seedling leaf gas exchange, providing little evidence of facilitation and suggesting that competitive interactions dominate. Efforts to transition the longleaf pine ecosystem towards greater drought tolerance may therefore be most effective when the planting of turkey oak seedlings is concentrated in canopy openings. Such improvements in management efforts may aid in conserving this highly threatened and biodiverse ecosystem.