5 CONCLUSIONS
We found that the response in tree sap flux density to decreasing soil water content across zones was dependent on the species, year, and vapour pressure deficit, tree-level transpiration was strongly linked to sapwood area, and mid-slope sap flux density will likely provide the best estimate for ET. In the dry year, species sap flux density in loblolly pine, sweetgum, Virginia pine, white oak decreased from the buffer zone to the upland-slope. In contrast, sap flux density in red maple and tulip poplar increased from the buffer zone to the mid-slope, primarily due to increased atmospheric evaporative demand.
Water use by trees could intensify soil hydrological drought (i.e., lack of soil moisture) during short-term dry periods of low precipitation. Under normal climatic conditions, red maples and tulip poplars will likely create more soil water storage and benefit storm water abatement when compared to other common trees in the region. However, the advantages of these species might be less critical during increasing or prolonged droughts.
This study improves our broader understanding of the relationship between species-specific transpiration and soil moisture. Linking soil moisture, tree water use, and climatic variability at the watershed level is rarely done, but is critical to refining transpiration estimates, managing the effects of drought, and understanding hydrological processes in unmanaged and managed watersheds across various regions. New plantations are being planned across the southern United States to meet the rising demand for wood production. Findings from this project could help public and private landowners decide which trees might be better to maximize the benefits and costs related to water in tree planting across the Piedmont.