Boundary layer conductance, leaf temperature and transpiration

conductance to heat transfer (gbH) and water vapor transfer (gbV) in foliated branches of Abies amabilis Dougl. ex J. Forbes, a subalpine forest tree that produces clumped shoot morphology on sun-formed branches. Boundary layer conductances estimated in the field from energy balance measurements increased linearly from approximately 10 mm s−1 at low wind speeds (< 0.1 m s−1) to over 150 mm s−1 at wind speeds of 2.0 m s−1. Boundary layer conductances measured on shoot models in a wind tunnel were consistently higher than field measurements. The difference between wind tunnel values and field measurements was attributable to variation in path length between the two experimental environments. Boundary layer conductance estimated by subtracting stomatal resistance (rsV) measured with a porometer from the total branch vapor phase resistance were unusually small. Sensitivity analysis demonstrated that this method is not suitable for coniferous foliage or when stomatal conductance (gsV) is small compared with gbV. Analysis of the relative magnitudes of gsV and gbV revealed that, under most conditions, A. amabilis branches are well coupled (i.e., gsV is the dominant controller of transpiration). The boundary layer conductance to heat transfer is small enough that leaf temperature can become substantially higher than air temperature when radiation is high and wind speed is low. Over a two-month period, the maximum difference between leaf and air temperatures exceeded 6 °C. Leaf temperature exceeded air temperature by more than 2 °C on 10% of the daylight hours during this period. Consideration of both the photosynthetic temperature response of A. amabilis foliage as well as the summer air temperature conditions in its habitat suggests that these elevated leaf temperatures do not have a significant impact on carbon gain during the growing season.