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

The branch bag method was used to monitor photosynthesis and transpiration of trembling aspen (Populus tremuloides Michx.) and hazelnut (Corylus cornu ta Marsh.) over a 42-day midsummer period in 1996, as part of the Boreal Ec osystem-Atmosphere Study (BOREAS). During the same period, daytime measurem ents of stomatal conductance (g(s)) and leaf water potential (Psi(leaf)) we re 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 inver se relationship between daytime g(s) and vapor pressure deficit (D) when D was > 0.5 kPa. Daytime Psi(leaf) was below -2 MPa in aspen and near -1.5 MP a 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 Psi(leaf) above a minimum th reshold value. Reductions in g(s) on sunny afternoons with elevated ambient D (maximum 2.3 kPa) were associated with a significant decrease in photosy nthetic rates. However, day-to-day variation in mean carbon assimilation ra te was small in both species, and appeared to be governed more by solar rad iation than D. These results may be generally applicable to healthy aspen s tands under normal midsummer conditions in the southern boreal forest. Howe ver, strong reductions in carbon uptake may be expected at the more extreme values of D (> 4 kPa) that occur during periods of regional drought, even if soil water is not locally limiting.