With increases in global temperatures, flooding, and prolonged drought events in the southeastern United States, it is expected that bottomland hardwood forests will experience a drastic shift in composition and productivity. Historic deforestation of this ecosystem has led to a demand for restoration and conservation efforts. The success of forest restoration depends on the individual species’ ability to adapt to a changing climate. As environmental conditions shift, certain tree species may experience an increase in productivity or could be more negatively affected over more resilient species, leading to a shift in species composition, water use, and carbon uptake. The goal of this research is to use sap flow, leaf characteristics, and photosynthetic rates to study the species-specific factors that influence physiological functioning of eight co-occurring bottomland hardwood species. Sap flow was measured on 24 trees using the Granier heat dissipation method and compared with concurrent measurements of vapor pressure deficit and soil moisture.. Measurements were taken continuously over the course of two growing seasons which included a drought and saturated soil conditions.
Results/Conclusions
We found that shagbark hickory used the most water, 29% more than the next highest consumer, willow oak. We found that sap flow of American elm, winged elm, willow oak, cherrybark oak, swamp chestnut oak, and water oak exhibited varying relationships with vapor pressure deficit under low or ample soil moisture conditions. This suggests that these species may be significantly less productive under low soil moisture conditions, suggesting they are more sensitive to long-term droughts. Green ash and shagbark hickory did not vary in response, suggesting these species are more drought-tolerant. Regarding leaf characteristics, the stomatal density of green ash was significantly lower than all other species. The more drought-sensitive species had the largest stomatal densities and lengths. In terms of leaf gas exchange, green ash and shagbark hickory exhibited the highest photosynthetic and transpiration rates, as well as the lowest water-use efficiencies compared to the other species. Taken together, these responses can be used to estimate forest water budgets given stand species composition or to predict individual species resilience or adaptation to a changing climate, which can improve land surface models and identify species in this forest type that will be most successful under future climate conditions.