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The branch bag method was used to monitor photosynthesis and transpiration of trembling aspen ( Populus tremuloides  Michx.) and hazelnut ( Corylus cornuta  Marsh.) over a 42-day midsummer period in 1996, as part of the Boreal Ecosystem–Atmosphere Study (BOREAS). During the same period, daytime measurements of stomatal conductance ( g  s ) and leaf water potential (Ψ leaf ) were made on these species, and sap flow was monitored in aspen stems by the heat pulse method. Weather conditions during the study period were similar to the long-term average. Despite moist soils, both species showed an inverse relationship between daytime  g  s  and vapor pressure deficit ( D ) when  D  was &gt; 0.5 kPa. Daytime Ψ leaf  was below –2 MPa in aspen and near –1.5 MPa in hazelnut, except on rainy days. These results are consistent with the hypothesis that stomatal responses are constrained by hydraulic resistance from root to leaf, and by the need to maintain Ψ leaf  above a minimum threshold value. Reductions in  g  s  on sunny afternoons with elevated ambient  D  (maximum 2.3 kPa) were associated with a significant decrease in photosynthetic rates. However, day-to-day variation in mean carbon assimilation rate was small in both species, and appeared to be governed more by solar radiation than  D . These results may be generally applicable to healthy aspen stands under normal midsummer conditions in the southern boreal forest. However, strong reductions in carbon uptake may be expected at the more extreme values of  D  (&gt; 4 kPa) that occur during periods of regional drought, even if soil water is not locally limiting.

Responses of trembling aspen and hazelnut to vapor pressure deficit in a boreal deciduous forest