4 DISCUSSION
The heat dissipation method was
used to determine several different indicators of water loss from the
study watershed: species-specific sap flux density, tree-level
transpiration, and watershed-level transpiration. Results from each
indicator were compared across species, zones, and years. Even though in
both years the different tree species accounted for the same proportion
of watershed-level transpiration (around 45% for loblolly pine, 14%
for white oak, 12% for tulip poplar and the other species representing
less than 30%), we found a large variability of sap flux over time, and
among trees and spatial zones within the watershed. Such variabilities
are essential for understanding environmental controls on tree
transpiration, plant resilience to drought, and scaling up transpiration
measurements from the tree level to stand and landscape scales.