Publisher Summary The study of leaf anatomy and of the mechanisms of the opening and closing of stomatal guard cells leads one to suppose that the stomata constitute the main or even the sole regulating system in leaf transpiration. Meteorologists have developed a wide variety of formulae for estimating evaporation from vegetation that are based entirely on weather variables and take no account at all of the species composition or stomatal properties of the transpiring vegetation. These “potential evaporation” formulae are widely and, to a large degree, successfully used for estimating evaporation from vegetation that is not water-stressed. Transpiration depends on stomatal conductance, net radiation receipt and upon air saturation deficit, temperature, and wind speed. Saturation deficit and wind speed vary through leaf boundary layers, through canopies, and through the atmosphere above the canopies. The sensitivity of saturation deficit to changes in stomatal conductance depends on where the saturation deficit is measured. If all of the stomata on a single leaf change aperture in unison, there may be a substantial change in saturation deficit measured at the leaf surface but a negligible change in saturation deficit measured a centimetre or two away, outside the leaf boundary layer.

Stomatal Control of Transpiration: Scaling Up from Leaf to Region

Studies of heat and mass exchange between leaves and their local environment are central to our understanding of plant-atmosphere interactions. The transfer across aerodynamic leaf boundary layers is generally described by non-dimensional expressions which reflect largely empirical adaptations of engineering models derived for flat plates. This paper reviews studies on leaves, and leaf models with varying degrees of abstraction, in free and forced convection. It discusses implecations of finding for leaf morphology as it affects – and is affected by – the local microclimate. Predictions of transfer from many leaves in plant communities are complicated by physical and physiological feedback mechanisms between leaves and their environment. Some common approaches, and the current challenge of integrating leaf-atmosphere interactions into models of global relevance, are also briefly addressed.

Tansley Review No. 59 Leaf boundary layers

Effects of spatial scale on stomatal control of transpiration

In this book some physical aspects of greenhouse climate are analyzed to show the direct interrelation between microclimate and crop transpiration. The energy balance of a greenhouse crop is shown to provide a sound physical framework to quantify the impact of microclimate on transpiration and to identify the constraints set on climate management by the termodynamic behaviour of the canopy. Before the relationship among microclimate, canopy temperature and transpiration is rendered in mathematical terms, a good deal of experimental work is necessary to establish sub-models for the heat transfer of the foliage, for the radiative transfer within the canopy and for the canopy resistance to vapour transfer. The sub-models are merged in a combination-type equation to obtain the temperature of a greenhouse crop and its transpiration. The resulting estimates are shown to reproduce accurately the temperature and transpiration of a greenhouse tomato crop, as measured at time intervals of a few minutes. To illustrate the practical application of the model thus developed a number of examples are presented. In particular, it is shown that defining the transpiration rate as the criterion for the control of air humidity within a greenhouse would deliver a quantitative framework for that control. That would largely enhance the efficiency of the (expensive) procedures applied at present for the control of humidity in greenhouses.

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Transpiration of greenhouse crops : an aid to climate management

Principles of Environmental Physics: Plants, Animals, and the Atmosphere, 4e, provides a basis for understanding the complex physical interactions of plants and animals with their natural environment. It is the essential reference to provide environmental and ecological scientists and researchers with the physical principles, analytic tools, and data analysis methods they need to solve problems. This book describes the principles by which radiative energy reaches the earth’s surface and reviews the latest knowledge concerning the surface radiation budget. The processes of radiation, convection, conduction, evaporation, and carbon dioxide exchange are analyzed. Many applications of environmental physics principles are reviewed, including the roles of surface albedo and atmospheric aerosols in modifying microclimate and climate, remote sensing of vegetation properties, wind forces on trees and crops, dispersion of pathogens and aerosols, controls of evaporation from vegetation and soil (including implications of changing weather and climate), and interpretation of micrometeorological measurements of carbon dioxide and other trace gas fluxes.

	
		Presents a unique synthesis of micrometeorology and ecology in its widest sense
	
		Deals quantitatively with the impact of weather on living systems but also with the interactions between organisms and the atmosphere that are a central feature of life on earth
	
		Offers numerous worked examples and problems with solutions
	
		Provides many examples of laboratory and field measurements and their interpretation
	
		Includes an up-to-date bibliography and review of recent micrometeorological applications in forestry, ecology, hydrology, and agriculture

Principles of Environmental Physics: Plants, Animals, and the Atmosphere

Experimental determinations of the local heat transfer by forced convection from model leaves heated by a constant energy flux were made in the laboratory under laminar and turbulent flow conditions. The results are expressed in a logarithmic dimensionless plot of the local Nusselt number, Nu
 d
 , against the local Reynolds number, Re
 d
 . For the laminar case, Nu
 d
 was only a linear function of Re
 d
 
1/2 downwind from the leading edge regions, although this relationship departed from that predicted theoretically due to the finite size and thickness of the model. For the turbulent case, a simple relationship between Nu
 d
 and Re
 d
 was found over a wide range of Reynolds numbers. The enhancement of heat transfer in the turbulent case depends primarily on the scale of turbulence rather than on the turbulent intensity. Past workers have discussed their results in relation to a factor β, defined as the ratio between the heat transfer predicted by the Polhausen equation, and that measured. The results suggest that β is not a unique parameter and may not be useful in describing the overall turbulent transfer process.

J. A Clark

Papers

Heat transport coefficients for constant energy flux models of broad leaves