TRANSPIRATION OF A BOREAL PINE FOREST MEASURED BY BRANCH BAG, SAP FLOW AND MICROMETEOROLOGICAL METHODS

Three independent methods were used to evaluate transpiration of a bor eal forest: the branch bag, sap flow and eddy covariance methods. The branch bag method encloses several thousand needles and gives a contin uous record of branch transpiration, The sap flow method provides a co ntinuous record of sap velocity and an estimate of tree transpiration. The eddy covariance method typically measures evaporation rates betwe en a forest and the atmosphere. We deployed an extra eddy covariance s ystem below the forest to estimate canopy transpiration by difference, The three systems detected small water vapor fluxes despite a plentif ul supply of energy to drive evaporation. We also observed that transp iration rates were low even when the soil was well supplied with water . Low rates of transpiration were attributed to the canopy's low leaf area index and the marked reduction in stomatal conductance as vapor p ressure deficits increased. Water vapor fluxes, derived from the sap f low method, lagged behind those derived by the branch bag method by 1 to 2 h. The sap flow method also suffered from sampling errors caused by the non-uniformity of flow across the sapwood and the spatial varia bility of sapwood cross section throughout the forest. Despite technic al difficulties associated with hourly measurements, daily totals of t ranspiration agreed well with values derived from micrometeorological systems.