Leaf and environmental parameters influencing transpiration: Theory and field measurements

The influence of variations in the boundary air layer thickness on transpirtion due to changes in leaf dimension or wind speed was evaluated at a given stomatal resistance (rs) for various combinations of air temperature (Ta) and total absorbed solar energy expressed as a fraction of full sunlight (Sffs). Predicted transpiration was found to either increase or decrease for increases in leaf size depending on specific combinations of Ta, Sffs, and rs. Major reductions in simulated transpiration with increasing leaf size occurred for shaded, highly reflective, or specially oriented leaves (Sffs=0.1) at relatively high Ta when rs was below a critical value of near 500 s m-1. Increases in Sffs and decreases in Ta lowered this critical resistance to below 50 s m-1 for Sffs=0.7 and Ta=20°C. In contrast, when rs was above this critical value, an increase in leaf dimension (or less wind) resulted in increases in transpiration, especially at high Ta and Sffs. For several combinations of Ta, Sffs, and rs, transpiration was minimal for a specific leaf size. These theoretical results were compared to field measurements on common desert, alpine, and subalpine plants to evaluate the possible interactions of leaf and environmental parameters that may serve to reduce transpiration in xeric habitats.