In the past, plant physiological studies have contributed substantial understanding of the behaviour of plants with respect to hydrological processes in vegetation and the eects of de®cits and surpluses of water on plants. In this paper progress in some important current ®elds of plant physiological research, with particular relevance to ecohydrology, are examined.
The control of water loss by plants, through manipulation of stomatal opening and amounts of foliage, is examined. Models that describe the control of stomatal behaviour by a combination of hydraulic and chemical signalling combined with transpiration ¯ux have emerged in recent years. There is, however, a need for detailed testing of such models in ®eld situations, particularly with large woody vegetation. Forest transpiration is examined in detail. In traditional broadleaves and conifers in temperate regions transpiration is modest and similar between forests. A number of factors are considered to explain the situation. A strong negative association of stomatal conductance with air humidity de®cit is thought to relate to the low overall transpiration. High transpiration rates are associated with large stomatal conductances, which show little reduction in association with increased air humidity de®cits. Soil moisture de®cits have less impact in forests with inherently low transpiration.
There is growing interest in the mechanisms of competition for soil water between plants growing together. Studies have been aided recently by the feasibility of direct measurements of root hydraulic conductivity. Further understanding of patterns of water uptake by roots has emerged, with new approaches involving sap ¯ow and stable isotope techniques.
Physiologists have provided considerable insight into controls of water loss at the individual leaf or plant level. At the larger scale of the vegetation stand, the leaf control may be modi®ed considerably depending on the structure of the vegetation, which in¯uences coupling with the atmosphere. In tall, rough vegetation, such as forests and woodlands, the coupling is strong, so that the stomatal control of transpiration at the leaf level is maintained at the canopy level.
In recent studies with forest vegetation, declines in transpiration have been observed to occur after a few to several decades from establishment of the forest. These reductions in transpiration can have signi®cant in¯uences on stream¯ow. In parallel, physiological studies have provided explanations both in terms of declining leaf area but also due to reductions in stomatal conductance, as a consequence of reductions in hydraulic conductivity of woody tissues.