Temperature and humidity effects on branchlet gas-exchange in white spruce: an explanation for the increase in transpiration with branchlet temperature

Abstract 

In situ gas-exchange data, for branchlets of white spruce [Picea glauca (Moench) Voss.] in a mature mixed-wood boreal forest in central Canada (53°44´N 105°14´W), were subjected to a multiple regression analysis. Vapor pressure deficit (VPD) and branchlet temperature (t_leaf) were both significant predictors (P<0.0001) of stomatal conductance to water vapor (g_sw) and net photosynthesis (A_n), together explaining 67 and 64% of the variation in g_sw and A_n, respectively. Since VPD and t_leaf were autocorrelated in these field data, but also to further explore the nature of independent effects of temperature and humidity on water and CO₂ exchange in white spruce, steady-state gas-exchange was performed on well-watered greenhouse-grown seedlings of white spruce. Results from laboratory experiments supported the following conclusions: (1) Transpiration (E) increases with VPD to an inflection point that increases linearly with t_leaf. This t_leaf effect on E could not be explained by trends in VPD, RH, A_n or PFD. Rather, our data support a model in which E and g_sw are influenced by the balance between ’supply’ and ’loss’ of water to and from leaf tissue, respectively. The supply of water appears to be in accordance with Darcy’s law, where supply of water is proportional to the driving gradient in pressure/ tension, specific permeability (k), and inverse of water viscosity (n ^–1). Approximately half of the increase in E could be explained by the linear increase in n ^–1 with increasing t_leaf. We propose that increases in k explain the remainder of the increase in E with t_leaf. (2) VPD and t_leaf appear to have independent effects on g_sw. In contrast, RH effects on g_sw or E were subtle and could be explained by a combination of effects of t_leaf and VPD. (3) A_n was affected primarily by t_leaf, being reduced at low (10°C) and high (40°C) temperatures, and only indirectly by humidity parameters via stomatal conductance, viz. intercellular CO₂ concentrations. Our results have implications for the prediction of water fluxes from plants and canopies in areas where plant temperatures vary diurnally or seasonally.