Boundary layer conductance, leaf temperature and transpiration of Abies amabilis branches

We used three methods to measure boundary layer conductance to heat transfe r (g(bH)) and water vapor transfer (g(bV)) in foliated branches of Abies am abilis Dougl, ex J. Forbes, a subalpine forest tree that produces clumped s hoot morphology on sun-formed branches. Boundary layer conductances estimat ed in the field from energy balance measurements increased linearly from ap proximately 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 measurem ents. The difference between wind tunnel values and field measurements was attributable to variation in path length between the two experimental envir onments. Boundary layer conductance estimated by subtracting stomatal resis tance (r(sV)) measured with a porometer from the total branch vapor phase r esistance were unusually small. Sensitivity analysis demonstrated that this method is not suitable for coniferous foliage or when stomatal conductance (g(sV)) is small compared with g(bV). Analysis of the relative magnitudes of g(sV) and g(bV) revealed that, under most conditions, A. amabilis branch es are well coupled (i.e., g(sV) is the dominant controller of transpiratio n). The boundary layer conductance to heat transfer is small enough that le af temperature can become substantially higher than air temperature when ra diation is high and wind speed is low. Over a two-month period, the maximum difference between leaf and air temperatures exceeded 6 degrees C. Leaf te mperature exceeded air temperature by more than 2 degrees C on 10% of the d aylight hours during this period. Consideration of both the photosynthetic temperature response of A. amabilis foliage as well as the summer air tempe rature conditions in its habitat suggests that these elevated leaf temperat ures do not have a significant impact on carbon gain during the growing sea son.