The regulation of stomatal conductance in response to environmental factors, such as drought or flooding, is one of much interest and of increasing concern due to climate change. In this study, we measured xylem sap flux in Fraxinus pennsylvanica (green ash) in a floodplain forest in Oklahoma. This area experiences a large variation in soil moisture including long periods of complete saturation. While species differ in flood tolerance, typically there is a reduction in stomatal conductance until adaptations occur. However, this response varies and can lead to a greater sensitivity to dry conditions after adaptations occur. Using Granier-type sapflux sensors and micrometeorological sensors, we assessed the response of transpiration to different environmental variables. The specific goals of this research were to 1) identify the role soil moisture plays on transpiration at the whole tree level; 2) determine if there is a tradeoff between flood tolerance and response to drought; and 3) evaluate whole canopy stomatal response to drought and flooding events. Temporal trends in VPD, soil moisture content, ET, sap flux, predawn and midday water potentials, as well as transpiration rates were analyzed to address these questions at two sites within the floodplain forest, with one site flooded more frequently than the other.
Results/Conclusions
The results indicate that the trees in the wetter site had higher rates of transpiration, as well as higher sensitivity to VPD than the trees at the drier site. This implies that the individuals located in the areas with higher soil moisture have greater stomatal control, but are also more vulnerable to cavitation. More negative water potential was observed at the wetter site corresponding to higher rates of transpiration. During times of complete soil saturation (flood), stomatal conductance was mostly unaffected in both sites, with transpiration rates reflecting little to no response to the flooding events. This lack of stomatal resistance to contrasting environmental events was unexpected, as there is typically a decrease in conductance in most species during the onset of flooding. The similarity of transpiration rates during times of both flooding and drought suggest that there is little tradeoff to having flood adapted root architecture for this species.