The implications of minimum stomatal conductance on modeling water flux in forest canopies

Abstract

[1] Stomatal conductance (g s ) models are widely used at a variety of scales to predict fluxes of mass and energy between vegetation and the atmosphere. Several g s models contain a parameter that specifies the minimum g s estimate (g 0 ). Sensitivity analyses with a canopy flux model (MAESTRA) identified g 0 to have the greatest influence on transpiration estimates (seasonal mean of 40%). A spatial analysis revealed the influence of g 0 to vary (30-80%) with the amount of light absorbed by the foliage and to increase in importance as absorbed light decreased. The parameter g 0 is typically estimated by extrapolating the linear regression fit between observed g s and net photosynthesis (A n ). However, our measurements demonstrate that the g s -A n relationship may become nonlinear at low light levels and thus, extrapolating values from data collected over a range of light conditions resulted in an underestimation of g 0 in Malus domestica when compared to measured values (20.4 vs 49.7 mmol m À2 s À1 respectively). In addition, extrapolation resulted in negative g 0 values for three other woody species. We assert that g 0 can be measured directly with diffusion porometers (as g s when A n ≤ 0), reducing both the time required to characterize g 0 and the potential error from statistical approximation. Incorporating measured g 0 into MAESTRA significantly improved transpiration predictions versus extrapolated values (6% overestimation versus 45% underestimation respectively), demonstrating the benefit in g s models. Diffusion porometer measurements offer a viable means to quantify the g 0 parameter, circumventing errors associated with linear extrapolation of the g s -A n relationship.

A test of an optimal stomatal conductance scheme within the CABLE land surface model

“…Replacing these values with zeros had a large impact on predicted fluxes, particularly under high VPD conditions at the C4-dominated Howard Springs. This result agrees with a recent study by Barnard and Bauerle (2013), who concluded that g 0 was in fact the most sensitive parameter for correctly estimating transpiration fluxes. It is clear that further investigation is needed on the impact of different g 0 assumptions in land surface and ecosystem models.…”

Sensitivity and uncertainty analysis of the carbon and water fluxes at the tree scale inEucalyptusplantations using a metamodeling approach

“…In these low photosynthesis situations, g s is mostly driven by g 0. The high sensitivity to g 0 in the Eucalyptus plantation in this study supports the conclusions of Barnard and Bauerle (2013), who stressed the necessity of measuring g 0 accurately. An estimate of g 0 using linear extrapolation from stomatal conductance model regression underestimated the minimum stomatal conductance by more than 50% compared with direct measurements.…”

“…While high sensitivity to g 1 (the slope between the GPP and TR), as shown by Bauerle and Bowden (2011), was expected, the minimum stomatal conductance (g 0 ) had a greater effect on TR, LUE, and WUE. While g 0 has commonly been assumed to be close to zero with little effect on water fluxes (Caird et al 2007;Zeppel et al 2010), recent studies have shown that g 0 could be higher than previously expected in many ecosystems (Ogle et al 2012) and that its value could change seasonally (Barnard and Bauerle 2013). In a recent study using MAESTRA, it was shown that g 0 had a large effect on TR (Bowden and Bauerle 2008).…”

Evaluating the Community Land Model (CLM4.5) at a coniferous forest site in northwestern United States using flux and carbon-isotope measurements

“…1). Conversely, b bb values reported in the literature are highly variable (1-400 mmol m −2 leaf s −1 in the survey by Barnard and Bauerle, 2013, for a broad range of plant species). In CLM4.0, the default b bb for C 3 plants is significantly smaller than in CLM4.5 (2 vs. 10 mmol m −2 leaf s −1 ) (Oleson et al, 2010).…”