COS 91-6 - Effects of stress simulations on hydraulic conductivity in bald cypress tree roots

Abstract: The loss of bald cypress (Taxodium distichum (L.) Rich.) trees within the forested wetlands of coastal Louisiana is a concern as altered hydrology, reduced sedimentation, subsidence, and sea level rise interact, resulting in tree mortality and the transition of bald cypress swamps to other habitat types, such as oligohaline marshes. These wetlands are important to Louisiana for several reasons, including wildlife habitat and storm-surge protection. With these developments, there is a new importance to investigating these effects. Most previous research on stress responses in bald cypress trees had been conducted on seedling or sapling trees in greenhouses and other controlled environments, leaving a research gap on the physiological and developmental stress responses of mature trees in their natural habitat. The goal of this method development was to design a framework for the hydraulic conductivity of bald cypress roots collected in-situ and assess how stress affects root hydraulic function. Specific hydraulic root conductivity is the measure of the rate of water flow per unit sapwood area over a given length for a given water potential gradient. Using the air-seeding method, we assessed the decline of specific hydraulic conductivity with decreasing water potential. This is done using a pressure head of gravity fed water and a volumetric pipette to feed water through a root sample that has been pressurized to an increasing series of pressures, simulating increasing stress. Root vulnerability curves are produced from the collected data. While this method has been applied previously to roots of many upland species of trees, bald cypress root adaptations to the wetland environment made it necessary to reassess multiple details in experimental design and expectations of conductivity rates and responses to stress. It can be concluded that bald cypress trees have a lower tolerance for stress simulations than other conifers such as the loblolly pine (Johnson et al. 2016), as their root vulnerability curves display a higher loss of conductivity at lower pressure values. The P50 values for the bald cypress, the pressure value at which the roots lost 50% of their conductivity, were lower than 1 MPa, in contrast to loblolly pines which have P50 values over 1 MPa. This coincides with the fact that bald cypress is a wetland species, making it less drought stress tolerant. Bald cypress roots also had lower maximum specific conductivity values than upland conifers previously studied, which may be expected because of the presence of aerenchyma in roots.