Effects of shoot water potential (Ψ) and leaf-to-atmosphere vapor pressure difference (VPD) on gas exchange of jack pine (Pinus banksiana Lamb.), black spruce (Picea mariana (Mill.) B.S.P.), and aspen (Populus tremuloides Michx.) were investigated at the northern edge of the boreal forest in Manitoba, Canada. Laboratory measurements on cut branches showed that net photosynthesis (An) and mesophyll conductance (gm) of jack pine and gm of black spruce did not respond to Ψ until a threshold Ψ was reached below which they decreased linearly. Photosynthesis of black spruce decreased slowly with decreasing Ψ above the threshold and declined more rapidly thereafter. The threshold Ψ was lower in black spruce than in jack pine. However, stomatal conductance (gs) of black spruce decreased continuously with decreasing Ψ, whereas gs of jack pine showed a threshold response. Mesophyll limitations were primarily responsible for the decline in An at low Ψ for jack pine and black spruce in the middle of the growing season, but stomatal limitations became more important later in the season. Field measurements on in situ branches on warm sunny days showed that both conifer species maintained Ψ above the corresponding threshold and there was no evidence of Ψ limitation on An of jack pine, black spruce or aspen.
Vapor pressure difference was important in regulating gas exchange in all three species. An empirical model was used to quantify the gs response to VPD. When parameterized with laboratory data for the conifers, the model also fit the corresponding field data. When parameterized with field data, the model showed that stomata of aspen were the most sensitive of the three species to VPD, and stomata of black spruce were the least sensitive. For jack pine and aspen, stomata of foliage in the upper canopy were significantly more sensitive than stomata of foliage in the lower canopy. Vapor pressure difference had a greater impact on An of aspen than on An of the conifers as a result of aspen’s greater stomatal sensitivity to VPD and greater slope of the relationship between An and intercellular CO2 concentration (Ci). During the 1994 growing season, VPD averaged 1.0 kPa, corresponding to ratios of Ci to ambient CO2 of 0.77, 0.71 and 0.81 for jack pine, black spruce and aspen, respectively. We conclude that increases in VPD at the leaf surface in response to climate change should affect the absolute CO2 and H2O fluxes per unit leaf area of the aspen component of a boreal forest landscape more than those of the conifer component.