Canopy processes: implications for transpiration, interception and splash induced erosion, ultimately for forest management and water resources

Abstract

Some recent experimental and theoretical developments in research related to tropical forest canopies are reviewed. Deuterium tracing studies in India, which rely on the collection of samples of transpired water from leaves in the canopy, have established the importance of ‘tree size’ as an index of whole tree transpiration and the use of leaf area, basal area and sapwood area for scaling transpiration from the tree to plot scale is discussed.

Experimental studies of the interception loss from tropical trees in Indonesia, India and Sri Lanka have established that conventional interception models of the Rutter type, although adequate for use in temperate climates, are entirely inappropriate for use in the tropics. It is now recognised that the failure of this approach is due to neglect of the process of drop size dependent canopy wetting. The use of rainfall simulators and optical disdrometers in the tropics has demonstrated that to achieve the same degree of canopy wetting upwards of ten times the depth of rainfall may be required for high intensity tropical storms (with large drop sizes) as compared with low intensity, frontal rainfall systems, common in temperate climates. Rainfall simulator studies have also demonstrated that the final degree of wetting is also reduced when vegetation is wetted with drops of large size. These studies have also demonstrated that the drop size of secondary drops falling from vegetation is dependent on the vegetation type and is very much greater for large leafed species such as Tectona grandis as compared with species such as Pinus caribaea with smaller needle formed leaves.

The combination, in the tropics, of large drop sizes in the primary rainfall and large drop sizes in secondary drops falling from vegetation helps to explain why in absolute and relative terms interception loss from tropical trees is less than that in temperate climates. The recognition that drop size generation is related to vegetation type has important implications for splash induced erosion and the choice of tree species on soils subject to erosion.

Knowledge of forest canopy processes is now leading to a better appreciation of the hydrological, meteorological and water resource impacts of forests. The impact of trees in Amazonia and the Sahel on climate and trees in the Zambezi basin on water resources is discussed. New spatially distributed modelling methods which are being incorporated within Decision Support Systems for forest and water resource management, which also take account of ecological and socio-economic objectives, are also outlined.