Variability of tree transpiration across three zones in a southeastern
U.S. Piedmont watershed
Abstract
Quantifying species-specific tree transpiration across watershed zones
is important for estimating watershed evapotranspiration (ET) and
predicting drought effects on vegetation. The objectives of this study
are to 1) assess sap flux density (Js) and tree-level transpiration (Ts)
across three contrasting zones (riparian buffer, mid-hillslope, and
upland-hillslope), 2) determine how species-specific Js responds to
vapor pressure deficit (VPD), and 3) compare watershed-level
transpiration (Tw) derived from each zone. We measured Js and Ts in
eight tree species in the three zones in a 12-ha forested watershed. In
the dry year of 2015, loblolly pine (Pinus taeda), Virginia pine (Pinus
virginiana), and sweetgum (Liquidambar styraciflua) Js rates were
significantly higher in the buffer when compared to the other two zones.
In contrast, Js in tulip poplar (Liriodendron tulipifera) and red maple
(Acer rubrum) were significantly lower in the buffer than in the
mid-hillslope. Daily Ts varied by zone and ranged from 10 to 93 liters
in the dry year and 9 to 122 liters in the wet year. Js responded
nonlinearly to VPD in all trees and zones. Annual Tw based on scaled-Js
data was 447 mm, 377 mm, and 340 mm for the buffer, mid-hillslope, and
upland-hillslope, respectively. We conclude that large spatial
variability in Js and scaled Tw were driven by differences in soil
moisture at each zone and forest composition. Consequently, spatial
heterogeneity of vegetation and soil moisture must be considered when
accurately quantifying watershed level ET.