Abstract
Combining sap flux and eddy covariance measurements provides a means to study plant stomatal conductance and the relationship between transpiration and photosynthesis. We measured sap flux using Granier‐type sensors in a northern hardwood‐dominated old growth forest in Michigan, upscaled to canopy transpiration, and calculated canopy conductance. We also measured carbon and water fluxes with the eddy covariance method and derived daytime gross primary production (GPP). The diurnal patterns of sap flux and canopy transpiration were mainly controlled by vapor pressure deficit (D) and photosynthetically active radiation (PAR). Daily sums of sap flux and canopy transpiration had exponential relationships to D that saturated at higher D and had linear relationships to PAR. Sugar maple (Acer saccharum) and yellow birch (Betula alleghaniesis) had higher sap flux per unit of sapwood area than eastern hemlock (Tsuga canadensis), while sugar maple and hemlock had higher canopy transpiration per unit of leaf area than yellow birch. Sugar maple dominated canopy transpiration per ground area. Canopy transpiration averaged 1.57 mm d−1, accounting for 65% of total evapotranspiration in the growing season. Canopy conductance was controlled by both D and PAR, but the day‐to‐day variation in canopy conductance mainly followed a negatively logarithmic relationship with D. By removing the influences of PAR, half‐hourly canopy conductance was also negatively logarithmically correlated with D. Water use efficiency (WUE) had a strong exponential relationship with D on a daily basis and approached a minimum of 4.4 mg g−1. WUE provides an alternative to estimate GPP from measurements of sap flux.
Highlights
[2] Ecosystem and atmospheric models are increasingly coupled to incorporate carbon, water, and energy exchanges between the atmosphere and ecosystems [Foley et al, 2000; Sellers et al, 1997]
Because it is relatively easier to continuously measure water flux of individual trees, through sap flux measurements, than carbon flux from individual trees or from the stand level, and because water use efficiency is a relatively conservative plant property responding to the environment [Tanner and Sinclair, 1983], deriving canopy carbon assimilation from water flux provides a new approach for carbon studies [Moren et al, 2001]
[7] The objectives of this research were (1) to measure sap flux in an old growth hemlock – northern hardwood forest in the Great Lakes region; (2) to upscale sap flux to canopy transpiration, estimate canopy stomatal conductance, and examine transpiration and stomatal responses to environmental conditions; (3) to calculate water use efficiency and its responses to environmental conditions by combining sap flux measurements and eddy covariance measurements; and (4) to discuss the possibility to use the response of water use efficiency to environmental conditions to predict daily carbon assimilation on the basis of transpiration
Summary
[2] Ecosystem and atmospheric models are increasingly coupled to incorporate carbon, water, and energy exchanges between the atmosphere and ecosystems [Foley et al, 2000; Sellers et al, 1997]. Stem sap flux measurements from individual trees provide a method to estimate canopy transpiration [Granier, 1987; Granier and Loustau, 1994] This method allows estimation of canopy stomatal conductance and its response to environmental factors on an hourly timescale [Ewers and Oren, 2000; Kostner et al, 1992; Phillips and Oren, 1998]. Kostner et al [2002] studied transpiration in mature stands of Norway spruce (Picea abies) varying in age from 40– 140 years in Germany, and found water use efficiency, derived from upscaled sap flux and stem growth, declined with stand age. On the basis of sap flux measurements, Phillips et al [2002] found that old (450 years) Douglas fir (Pseudotsuga menziesii) trees in the Pacific Northwest, USA had less crown-averaged stomatal conductance than younger trees because of hydraulic resistance related to tree height. We have not seen studies focused on transpiration in old growth forests in the Great Lakes region
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