Scale and two views of the loss of plant water to the atmosphere

There are two very different ways in which we can analyze and explain the loss of water from plants to the atmosphere. Plant physiologists going back at least to Brown and Escombe in 1900 have emphasized the way in which the behavior of the stomata determines the rate at which a leaf loses water. It now seems obvious that it is the frequency and aperture of pores in an otherwise mainly waterproof surface that will control the rate at which water diffuses from a leaf to the outside atmosphere. But micrometero-logists take a quite different viewpoint, focusing on vegetation as a whole rather than on the single stoma, leaf or plant. Their approach emphasizes that water will be lost by evaporation only if there is latent heat available for this evaporation. This may be from solar radiation received directly by the transpiring leaves or as 'advective' energy, i.e. heat received as solar radiation elsewhere but transported in moving air. The micrometeorologists have developed formulae for the rate of water loss that are based entirely on the weather: wind speed, solar radiation, temperature and so on. They wholly ignore both the species of plants and their physiology, but their models nevertheless prove to be powerful predictors of the evaporation of water from vegetation that is not suffering from drought. Neither approach is right or wrong: which to use depends on the question being asked. Large-scale, climatically based models, for example, are likely to be the most relevant in predicting the evapotranspiration and photosynthesis that might occur in areas of vegetation as a result of global warming and changes in precipitation (Aber & Federer, 1992).

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