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

Abstract

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 (Psi(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 > 0.5 kPa. Daytime Psi(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 Psi(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 (> 4 kPa) that occur during periods of regional drought, even if soil water is not locally limiting.

Energy and water vapour exchanges over a mixedwood boreal forest in Ontario, Canada

“…There was an exponential decrease in G S with the increase in VPD, indicating stomatal closure during high atmospheric demand and dry periods. This phenomenon has also been observed in other boreal forests during late afternoon hours (Hogg et al, 2000) and during drought events (Baldocchi et al, 2000). demand was low (VPD < 0Ð8 kPa), indicating the influence of environmental factors on G S .…”

The canopy conductance of a boreal aspen forest, Prince Albert National Park, Canada

“…Second, there was the suggestion that there was some water loss at night from the leafed aspen canopy (also found using sap-flow methods by Hogg et al, 2000). In contrast, the nocturnal hazelnut g c was often negative, indicating condensation on the hazelnut leaves.…”

Phenology of vegetation in Southern England from Envisat MERIS terrestrial chlorophyll index (MTCI) data

“…The timing of the onset and eventual loss (offset) of leaves regularly alters land-surface boundary conditions by changing surface albedo, roughness, and surface water and energy fluxes (Hogg et al 2000, Wilson and Baldocchi 2000, Molod et al 2003. Moreover, change to the associated length of the growing season and increase in primary productivity directly influences the amount of radiatively active gases (particularly CO 2 and H 2 O) in the atmosphere (White et al 1997, Schwartz 1999, Cleland et al 2007, Richardson et al 2010).…”