Ten years before Monteith ( 1965) derived the now widely used Penman–Monteith equation, Raschke ( 1958) recognized the effect of stomatal resistance on the sensitivity of latent heat flux to wind speed and pointed out the counter-intuitive finding that, whereas transpiration rates normally increase with wind speed, they may decrease with increasing wind speed at high stomatal resistances. An additional complication arises from the effect of stomatal control, which modifies the coupling between latent and sensible heat flux. This web of interactions poses a challenge to the intuitive understanding of leaf temperature and energy balance adjustments to environmental forcing. Consequently, temperature sensitivity of transpiration is likely dependent on leaf surface and boundary layer properties. In fact, the main impact of air temperature on transpiration rate is through its effect on sensible heat flux, which in turn is very sensitive to leaf boundary layer conductance. However, the underlying diffusion equation does not feature air temperature but vapour concentration in the air, which is only very weakly dependent on air temperature. For example, the sensitivity of evaporation and transpiration rate to air temperature is often represented by the increasing slope of the saturation vapour pressure curve with increasing temperature (Raschke 1958, 1960 Monteith 1965 Priestley & Taylor 1972). The elimination of leaf or surface temperature from the calculation permits analytical estimates of evaporation or transpiration rates for given atmospheric and surface conditions but, at the same time, obscures important feedbacks. the Penman–Monteith equation (Monteith 1965)). Since the seminal work by Penman ( 1948) that skillfully combined aerodynamic considerations and Bowen's energy balance considerations (Bowen 1926) with simplifying assumptions to eliminate leaf temperature from latent heat flux calculation, common treatments of leaf or canopy gas and energy exchange do not explicitly consider leaf temperatures (e.g. These unintuitive feedbacks between wind, leaf size and water use efficiency call for re-evaluation of the role of wind in plant water relations and potential re-interpretation of temporal and geographic trends in leaf sizes.
However, there is indication that the effect of long-term trends in wind speed on leaf gas exchange may be compensated for by the concurrent reduction in mean leaf sizes. Our leaf-scale analysis suggests that the observed global decrease in near-surface wind speeds could have reduced WUE at a magnitude similar to the increase in WUE attributed to global rise in atmospheric CO 2 concentrations. We provide theoretical and experimental evidence that leaf water use efficiency (WUE, carbon uptake per water transpired) commonly increases with increasing wind speed, thus improving plants' ability to conserve water during photosynthesis. However, evidence suggests that increasing wind speed enhances carbon dioxide (CO 2) uptake while reducing transpiration because of more efficient convective cooling (under high solar radiation loads). A widespread perception is that, with increasing wind speed, transpiration from plant leaves increases.
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