Balancing the costs of carbon gain and water transport: testing a new theoretical framework for plant functional ecology

A novel framework is presented for the analysis of ecophysiological field measurements and modelling. The hypothesis leaves minimise the summed unit costs of transpiration and carboxylation' predicts leaf-internal/ambient CO2 ratios (c(i)/c(a)) and slopes of maximum carboxylation rate (V-cmax) or leaf nitrogen (N-area) vs. stomatal conductance. Analysis of data on woody species from contrasting climates (cold-hot, dry-wet) yielded steeper slopes and lower mean c(i)/c(a) ratios at the dry or cold sites than at the wet or hot sites. High atmospheric vapour pressure deficit implies low c(i)/c(a) in dry climates. High water viscosity (more costly transport) and low photorespiration (less costly photosynthesis) imply low c(i)/c(a) in cold climates. Observed site-mean c(i)/c(a) shifts are predicted quantitatively for temperature contrasts (by photorespiration plus viscosity effects) and approximately for aridity contrasts. The theory explains the dependency of c(i)/c(a) ratios on temperature and vapour pressure deficit, and observed relationships of leaf C-13 and N-area to aridity.