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Notes: Concurrent, independent measurements of stomatal conductance (gs), transpiration (E) and microenvironmental variables were used to characterize control of crown transpiration in four tree species growing in a moist, lowland tropical forest. Access to the upper forest canopy was provided by a construction crane equipped with a gondola. Estimates of boundary layer conductance (gb) obtained with two independent methods permitted control of E to be partitioned quantitatively between gs and gb using a dimensionless decoupling coefficient (Ω) ranging from zero to 1. A combination of high gs (c. 300–600 mmol m−2 s−1) and low wind speed, and therefore relatively low gb (c. 100–800 mmol m−2 s−1), strongly decoupled E from control by stomata in all four species (Ω= 0.7–0.9). Photosynthetic water-use efficiency was predicted to increase rather than decrease with increasing gs because gb was relatively low and internal conductance to CO2 transfer was relatively high. Responses of gs to humidity were apparent only when the leaf surface, and not the bulk air, was used as the reference point for determination of external vapour pressure. However, independent measurements of crown conductance (gc), a total vapour phase conductance that included stomatal and boundary layer components, revealed a clear decline in gc with increasing leaf-to-bulk air vapour pressure difference (Va because the external reference points for determination of gc and Va were compatible. The relationships between gc and Vc and between gs and Vs appeared to be distinct for each species. However, when gs and gc were normalized by the branch-specific ratio of leaf area to sapwood area (LA/SA), a morphological index of potential transpirational demand relative to water transport capacity, a common relationship between conductance and evaporative demand for all four species emerged. Taken together, these results implied that, at a given combination of LA/SA and evaporative demand scaled to the appropriate reference point, the vapour phase conductance and therefore transpiration rates on a leaf area basis were identical in all four contrasting species studied.

Notes: It has been proposed that the stomatal response to humidity relies on sensing of the transpiration rate itself rather than relative humidity or the saturation deficit per se. We used independent measurements of stomatal conductance (gs), transpiration (E), and leaf-to-air vapour pressure difference (V) in a hybrid poplar canopy to evaluate relationships between gs and E and between gs and V. Relationships between E, V and total vapour phase conductance or crown conductance (gc) were also assessed. Conductance measurements were made on exposed and partially shaded branches over a wide range of incident solar radiation. In exposed branches, gs appeared to decline linearly with increasing E and increasing V at both high and low irradiance. However, in a partially shaded branch, a bimodal relationship between gs and E was observed in which gs continued to decrease after E had reached a maximum value and begun to decrease. The relationship between gs and V for this branch was linear. Plots of gc against E always yielded bimodal or somewhat variable relationships, whereas plots of gc against V were invariably linear. It was not possible to derive a unique relationship between conductance and E or V because prevailing radiation partially determined the operating range for conductance. Normalization of data by radiation served to linearize responses observed within the same day or type of day, but even after normalization, data collected on partly cloudy days could not be used to predict stomatal behaviour on clear days and vice versa. An additional unidentified factor was thus also involved in determining operating ranges of conductance on days with different overall radiation regimes. We suggest that the simplest mechanism to account for the observed humidity responses is stomatal sensing of the epidermal or cuticular transpiration rate rather than the bulk leaf or stomatal transpiration rate.