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When stomata are open, the trade-off between water loss through transpiration and CO2 uptake via photosynthesis is a result of the evolutionary history of land plants from green algae. To supply the up to 1000-fold higher flux of water leaving the leaf compared with CO2 molecules entering the leaf during leaf gas exchange (Nobel 2009), plant hydraulics has evolved to be highly efficient. The intimate connection between plant water transport and photosynthesis has been explained elegantly with a theory of stomata themselves (Cowan and Farquhar 1977) and expanded to connect plant hydraulics and photosynthesis directly through an economic approach (Katul et al. 2009). This theory is well supported by data including correlations between plant hydraulic conductance and stomatal conductance (Meinzer and Grantz 1990), mesophyll conductance to CO2 (Peguero-Pina et al. 2012) and the quantum yield of photosystem II (Brodribb and Feild 2000). These studies illustrate that the connections between plant hydraulics and photosynthesis include both biochemical and gas exchange components of photosynthesis. However, the main plant physiological regulation over the photosynthesis and transpiration compromise, stomatal conductance, is still without a full mechanistic basis from genes to environmental responses. As a result, our predictive understanding of plant hydraulic and photosynthetic responses to anthropogenic changes to climate and historical disturbance regimes is still hindered even in the most recent soil–plant–atmosphere models (Berry et al. 2010). Modern models of stomatal conductance are, very often, still based on more than three decades old concepts of multiplicative constraints (Jarvis 1976) and semi-mechanistic parameterizations (Ball et al. 1987, Leuning 1995, Buckley et al. 2012). Such models require sufficient empirical data for appropriate calibration and thus cannot independently predict stomatal controls over water and carbon exchange without parameterization. Empirical estimates of stomatal conductance are divided into two types, gas exchange and sap flux. The benefits of gas exchange measurements are that they include both transpiration and photosynthesis, and they permit manipulation of some environmental conditions including light, temperature, CO2, wind speed and humidity (Long and Bernacchi 2003); however, the disadvantage is that the measurements disturb the environment of the leaf, sample a small area of the canopy and are limited in time. In contrast, sap flux measurements are relatively continuous, sample a relatively large area of the xylem pipes supporting the canopy and do not disturb the environment of the leaves. The disadvantages of sap flux measurements include potentially biased measurements based on xylem anatomy and sensor type (Bush et al. 2010, Steppe et al. 2010), consideration of capacitance between the measurement point somewhere in the stem and the loss of water from the leaves (Phillips et al. 1997, Meinzer et al. 2003, Zweifel et al. 2007), and estimation of the environment of the whole canopy (Ewers et al. 2007). Both methods, neither of which are direct measurements of stomatal conductance, also require some type of scaling from the area of water loss estimated (i.e. part of the stem sapwood system or a subset of leaves) to the area of water loss under investigation (i.e. whole canopy, stands, watersheds or landscapes). Given the known issues in sap flux-based estimates of stomatal conductance (Ewers and Oren 2000) and still incomplete knowledge concerning environmental regulation over stomatal conductance Commentary

Understanding stomatal conductance responses to long-term environmental changes: a Bayesian framework that combines patterns and processes