Regulation of branch-level gas exchange of boreal trees: roles of shoot water potential and vapor pressure difference

Abstract

Effects of shoot water potential (Psi) and leaf-to-atmosphere vapor pressure difference (VPD) on gas exchange of jack pine (Pinus banksiana Lamb.), black spruce (Picea mariana (Mill.) B.S.P.), and aspen (Populus tremuloides Michx.) were investigated at the northern edge of the boreal forest in Manitoba, Canada. Laboratory measurements on cut branches showed that net photosynthesis (A(n)) and mesophyll conductance (g(m)) of jack pine and g(m) of black spruce did not respond to Psi until a threshold Psi was reached below which they decreased linearly. Photosynthesis of black spruce decreased slowly with decreasing Psi above the threshold and declined more rapidly thereafter. The threshold Psi was lower in black spruce than in jack pine. However, stomatal conductance (g(s)) of black spruce decreased continuously with decreasing Psi, whereas g(s) of jack pine showed a threshold response. Mesophyll limitations were primarily responsible for the decline in A(n) at low Psi for jack pine and black spruce in the middle of the growing season, but stomatal limitations became more important later in the season. Field measurements on in situ branches on warm sunny days showed that both conifer species maintained Psi above the corresponding threshold and there was no evidence of Psi limitation on A(n) of jack pine, black spruce or aspen. Vapor pressure difference was important in regulating gas exchange in all three species. An empirical model was used to quantify the g(s) response to VPD. When parameterized with laboratory data for the conifers, the model also fit the corresponding field data. When parameterized with field data, the model showed that stomata of aspen were the most sensitive of the three species to VPD, and stomata of black spruce were the least sensitive. For jack pine and aspen, stomata of foliage in the upper canopy were significantly more sensitive than stomata of foliage in the lower canopy. Vapor pressure difference had a greater impact on A(n) of aspen than on A(n) of the conifers as a result of aspen's greater stomatal sensitivity to VPD and greater slope of the relationship between A(n) and intercellular CO(2) concentration (C(i)). During the 1994 growing season, VPD averaged 1.0 kPa, corresponding to ratios of C(i) to ambient CO(2) of 0.77, 0.71 and 0.81 for jack pine, black spruce and aspen, respectively. We conclude that increases in VPD at the leaf surface in response to climate change should affect the absolute CO(2) and H(2)O fluxes per unit leaf area of the aspen component of a boreal forest landscape more than those of the conifer component.

Sensitivity of mean canopy stomatal conductance to vapor pressure deficit in a flooded Taxodium distichum L. forest: hydraulic and non-hydraulic effects

“…An exponential rise to a maximum is the form commonly used to describe stomatal response to light (Jarvis 1976). Many forms of non-linear functions have been used to describe the decrease in G S with increases in D (see Lohammar et al 1980;Massman and Kaufmann 1991;McNaughton and Jarvis 1991;Granier and Loustau 1994;McCaughey and Iacobelli 1994;Monteith 1995;Dang et al 1997;Martin et al 1997). Models use these functions to generate a multiplier, one of several used in multipleconstraint functions, to reduce maximum g S or G S to estimates of actual conductance (see Schulze et al 1994); maximum conductance is determined for each species under non-limiting environmental conditions (e.g., high irradiance without water limitation; Dai et al 1992).…”

Modeling the impact of drought on canopy carbon and water fluxes for a subtropical evergreen coniferous plantation in southern China through parameter optimization using an ensemble Kalman filter

“…Photosynthesis peaks when SWC is near the field capacity and decreases with the departure of SWC from this optimum. When soil or atmospheric water is insufficient, leaf stomatal conductance reduces and correspondingly CO 2 fixation is limited (Dang et al, 1997;Hogg et al, 2000;Grant et al, 2006a). Extensive studies have demonstrated the influence of SWC on C fixation at leaf (Infante et al, 1999;Lawlor, 1995) and canopy (Reichstein et al, 2002;Rambal et al, 2003) levels.…”

A Test of Gas Exchange Measurements on Excised Canopy Branches of Ten Tropical Tree Species

“…But these structures are not available everywhere and there is need for reliable estimates of leaf-level processes in places where it is not practical to build such structures. Researchers have measured gas exchange of canopy leaves by cutting or shooting small branches and re-cutting stems under water to reestablish the xylem water column in the form of a potometer before measurement (Koyama 1981, Reich et al 1995, 1998, Dang et al 1997. Other researchers have measured gas exchange of trees by simply detaching a group of leaves and placing them in a cuvette for immediate measurement (Ginn et al 1991, Gerrish 1992, Samuelson 1998.…”

“…The time scale at which these changes occur is likely to be important in determining the degree of excision-induced effects on gas exchange. However, these responses vary widely across the plant kingdom leaving us with little information regarding the reliability of gas exchange measurements on excised foliage; few studies provide verification of how leaves on excised stems perform (Ginn et al 1991, Dang et al 1997.…”