Transpiration and stomatal conductance across a steep climate gradient in the southern Rocky Mountains

Transpiration (E) is regulated over short time periods by stomatal conductance (G(s)) and over multi-year periods by tree- and stand-structural factors such as leaf area, height and density, with upper limits ultimately set by climate. We tested the hypothesis that tree structure, stand structure and G(s) together regulate E per ground area (E-g) within climatic limits using three sites located across a steep climatic gradient: a low-elevation Juniperus woodland, a mid-elevation Pinus forest and a high-elevation Picea forest. We measured leaf area : sapwood area ratio (A(1) : A(s)), height and ecosystem sapwood area : ground area ratio (A(s) : A(g)) to assess long-term structural adjustments, tree-ring carbon isotope ratios (delta C-13) to assess seasonal gas exchange, and whole-tree E and G(s) to assess short-term regulation. We used a hydraulic model based on Darcy's law to interpret the interactive regulation of G(s) and E-g. Common allometric dependencies were found only in the relationship of sapwood area to diameter for pine and spruce; there were strong site differences for allometric relationships of sapwood area to basal area, A(1) : A(s) and A(s) : A(g). On a sapwood area basis, E decreased with increasing elevation, but this pattern was reversed when E was scaled to the crown using A(1) : A(s) . E-g was controlled largely by A(s) : A(g), and both E-g and G(s) declined from high- to low-elevation sites. Observation-model comparisons of E-g, G(s) and delta C-13 were strongest using the hydraulic model parameterized with precipitation, vapour pressure deficit, A(1) : A(s), height, and A(s) : A(g), supporting the concept that climate, G(s), tree- and stand-structure interact to regulate E-g. Copyright (C) 2008 John Wiley & Sons, Ltd.