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

Abstract

We used three methods to measure boundary layer conductance to heat transfer (g(bH)) and water vapor transfer (g(bV)) 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 (r(sV)) 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 (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 branches are well coupled (i.e., g(sV) 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 degrees C. Leaf temperature exceeded air temperature by more than 2 degrees 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.

Oxygen isotope content of CO2 in nocturnal ecosystem respiration: 1. Observations in forests along a precipitation transect in Oregon, USA

“…We present data collected at other sites in Oregon ( [17] The Craig-Gordon model of evaporative enrichment [Craig and Gordon, 1965;Flanagan et al, 1991] Mean summertime values for relative humidity and air temperature were used (days 150-250). We assumed that leaf temperature was identical to air temperature [Jarvis and McNaughton, 1986;Martin et al, 1999], and we neglected leaf boundary layer conductance as described by Flanagan et al [1997]. We used the mean of all observations for d 18 O of xylem water, and estimated d…”

Is leaf pubescence of Cape Proteaceae a xeromorphic or radiation-protective trait?

“…Relatively large resistance through the overall diffusive pathway ensures that an increase in boundary layer resistance due to pubescence may have little effect on gaseous exchange. Conversely, when the ratio of boundary layer resistance to total resistance is relatively high pubescence may significantly decrease the rate of gas exchange (Martin et al 1999). In this case leaf pubescence may limit transpiration and be functional in arid environments.…”

Oxygen isotope content of CO2 in nocturnal ecosystem respiration: 2. Short‐term dynamics of foliar and soil component fluxes in an old‐growth ponderosa pine forest

“…Cernusak et al's [2002] model was based on several parameters obtained from leaf level gas exchange measurements such as leaf transpiration rate, leaf conductance, and leaf water concentration, which unfortunately were not available in the present study. Further, our assumption that leaf temperature equals air temperature, a common assumption for conifers, is likely to fail at low wind speeds [Martin et al, 1999]. Regardless, given the large difference in d foliage and d soil , meaningful determination of the soil respiration fraction ( f ) of total ecosystem respiration can still be achieved with errors of a few % in d…”