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.