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.