Leaf stomatal responses to vapour pressure deficit under current and CO₂‐enriched atmosphere explained by the economics of gas exchange

Abstract

Using the economics of gas exchange, early studies derived an expression of stomatal conductance (g) assuming that water cost per unit carbon is constant as the daily loss of water in transpiration (fe) is minimized for a given gain in photosynthesis (fc). Other studies reached identical results, yet assumed different forms for the underlying functions and defined the daily cost parameter as carbon cost per unit water. We demonstrated that the solution can be recovered when optimization is formulated at time scales commensurate with the response time of g to environmental stimuli. The optimization theory produced three emergent gas exchange responses that are consistent with observed behaviour: (1) the sensitivity of g to vapour pressure deficit (D) is similar to that obtained from a previous synthesis of more than 40 species showing g to scale as 1 -m log(D), where m ∈ [0.5,0.6], (2) the theory is consistent with the onset of an apparent 'feed-forward' mechanism in g, and (3) the emergent non-linear relationship between the ratio of intercellular to atmospheric [CO2] (ci/ca) and D agrees with the results available on this response. We extended the theory to diagnosing experimental results on the sensitivity of g to D under varying ca.

Ecosystem CO2/H2O fluxes are explained by hydraulically limited gas exchange during tree mortality from spruce bark beetles

“…Blue-stain fungi impacts on the hydraulics of Engelmann spruce fit the expectations of a simple plant hydraulic model [Oren et al, 1999] (Figure 3a). Because the slopes of healthy and attacked trees are not different from each other or the 0.5-0.6 region [Katul et al, 2009] (Figure 3a), this indicates that the trees are regulating minimum leaf water potential as hydraulic conductance declines [Ewers et al, 2005;Oren et al, 1999]. At the same time, the blue-stain fungi do not impact leaf photosynthetic biochemistry (Figure 3b) but hydraulic limitation leads to decreased C uptake from gas exchange limitations while trees are dying from attacks.…”

“…Our sap flux data show that the conductance g s,ref was significantly lower for Engelmann spruce attacked by spruce beetle/blue-stain fungus (p < 0.0001) but that the slope of the relationship m to g s,ref was not different for healthy or attacked trees (p = 0.81) or the 0.5-0.6 region (p > 0.61) described by Katul et al [2009] (Figure 3a). There also was no difference in photosynthesis between hydrated branches sampled from healthy or attacked trees (p > 0.18 for all parameters after fitting the logistic sigmoid function (equation (1)); Figure 3b).…”

“…During epidemic I, the ecosystem was fixing carbon at the same rate relative to conductance as it had in the endemic years, and that did not change until the third year of the epidemic (Tables 1 and 2, and Figure 8). We interpret this as during epidemic I, the spruce needles lived in severe drought stress because of beetle-induced hydraulic failure and reduced their stomatal conductance [Brodersen et al, 2006;Kaufmann, 1979] and their photosynthesis [Hubbard et al, 2001], all of which are fundamentally interconnected [Katul et al, 2003;Katul et al, 2009]. The ability of Engelmann spruce to survive for several years after catastrophic hydraulic failure is remarkable.…”

“…Simultaneously, we tested the sensitivity [Oren et al, 1999] relating the slope of the canopy conductance response to vapor pressure deficit VPD (m) to canopy conductance at a reference VPD of 1.0 kPa (g s,ref ). The region 0.5-0.6 (dashed lines) represents most plant taxa [Katul et al, 2009]. (b) Photosynthetic assimilation (A) response to photosynthetic photon flux density (PPFD) from hydrated branches sampled from healthy and attacked trees fit to the logistic sigmoid function (equation (1)).…”

Environmental changes during secondary succession in a tropical dry forest in Mexico

“…Interpolations were obtained from a linear function fitted to data from 10 plots in which both variables were recorded (RH = −3.59 × air temperature + 180, R = 0.78, P < 0.01). The same procedure was used for VPD (VPD = 0.29 × air temperature − 7.06, R = 0.94, P < 0.001), which is a less common but more direct measure of evapotranspiration demand (Katul et al 2009). As these two variables are closely related by definition and in their dependency on temperature, VPD was excluded from the calculations of the PCA axes but included afterwards as an overlay in the ordination diagram to show its relationship to the gradients of environmental variation.…”