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

In situ gas-exchange data, for branchlets of white spruce [Picea glauca (Mo ench) Voss.] in a mature mixed-wood boreal forest in central Canada (53 deg rees 44'N 105 degrees 14'W), were subjected to a multiple regression analys is. Vapor pressure deficit (VPD) and branchlet temperature (t(leaf)) were b oth significant predictors (P<0.0001) of stomatal conductance to water vapo r (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 tleaf were aut ocorrelated in these field data, but also to further explore the nature of independent effects of temperature and humidity on water and CO2 exchange i n white spruce, steady-state gas-exchange was performed on well-watered gre enhouse-grown seedlings of white spruce. Results from laboratory experiment s supported the following conclusions: (1) Transpiration (E) increases with VPD to an inflection point that increases linearly with t(leaf). This t(le af) effect on E could not be explained by trends in VPD, RH, A(n) or PFD. R ather, 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, respe ctively. 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 i ncrease in n(-1) with increasing t(leaf). We propose that increases in k ex plain 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(s w) 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 a t low (10 degrees C) and high (40 degrees C) temperatures, and only indirec tly 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 diur nally or seasonally.