Coupling whole-tree transpiration and canopy photosynthesis in coniferous and broad-leaved tree species

We used sap flow as a measure of whole-tree function to examine how coniferous and broad-leaved species in mixed temperate forests differ in canopy-level transpiration and photosynthetic rates. We used heat dissipation probes to measure whole-tree sap flow in three species throughout one full year and then combined these measurements with micrometeorological monitoring and leaf-level gas exchange to determine whole-tree carbon gain. Both broad-leaved species (red oak, Quercus rubra L.; red maple, Acer rubrum L.) had two- to four-fold greater annual fluxes of water and carbon on a ground area basis than did the conifer (eastern hemlock, Tsuga canadensis (L.) Carriere), with red oak trees additionally showing 60-80% higher fluxes than red maple. Despite fixing one-third of its carbon when broad-leaved species were leafless, hemlock was not able to compensate for its low photosynthetic rates during the growing season. Productivity measures derived from annual growth rings and eddy covariance confirmed that whole-tree sap flow provided a valuable estimate of both the magnitude of current forest fluxes and differences in individual species' fluxes. Our results indicate that the predicted loss of hemlock from mixed temperate forests could potentially increase whole-forest water loss and carbon gain by two- to four-fold, provided sufficient nitrogen and water remain available to support such a change.