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

<jats:title>Abstract</jats:title><jats:p>A novel framework is presented for the analysis of ecophysiological field measurements and modelling. The hypothesis ‘<jats:italic>leaves minimise the summed unit costs of transpiration and carboxylation</jats:italic>’ predicts leaf‐internal/ambient <jats:styled-content style="fixed-case">CO</jats:styled-content><jats:sub>2</jats:sub> ratios (<jats:italic>c</jats:italic><jats:sub><jats:italic>i</jats:italic></jats:sub><jats:italic>/c</jats:italic><jats:sub><jats:italic>a</jats:italic></jats:sub>) and slopes of maximum carboxylation rate (<jats:italic>V</jats:italic><jats:sub>cmax</jats:sub>) or leaf nitrogen (<jats:italic>N</jats:italic><jats:sub>area</jats:sub>) <jats:italic>vs</jats:italic>. stomatal conductance. Analysis of data on woody species from contrasting climates (cold‐hot, dry‐wet) yielded steeper slopes and lower mean <jats:italic>c</jats:italic><jats:sub><jats:italic>i</jats:italic></jats:sub><jats:italic>/c</jats:italic><jats:sub><jats:italic>a</jats:italic></jats:sub> ratios at the dry or cold sites than at the wet or hot sites. High atmospheric vapour pressure deficit implies low <jats:italic>c</jats:italic><jats:sub><jats:italic>i</jats:italic></jats:sub><jats:italic>/c</jats:italic><jats:sub><jats:italic>a</jats:italic></jats:sub> in dry climates. High water viscosity (more costly transport) and low photorespiration (less costly photosynthesis) imply low <jats:italic>c</jats:italic><jats:sub><jats:italic>i</jats:italic></jats:sub><jats:italic>/c</jats:italic><jats:sub><jats:italic>a</jats:italic></jats:sub> in cold climates. Observed site‐mean <jats:italic>c</jats:italic><jats:sub><jats:italic>i</jats:italic></jats:sub><jats:italic>/c</jats:italic><jats:sub><jats:italic>a</jats:italic></jats:sub> shifts are predicted quantitatively for temperature contrasts (by photorespiration plus viscosity effects) and approximately for aridity contrasts. The theory explains the dependency of <jats:italic>c</jats:italic><jats:sub><jats:italic>i</jats:italic></jats:sub><jats:italic>/c</jats:italic><jats:sub><jats:italic>a</jats:italic></jats:sub> ratios on temperature and vapour pressure deficit, and observed relationships of leaf δ<jats:sup>13</jats:sup>C and <jats:italic>N</jats:italic><jats:sub>area</jats:sub> to aridity.</jats:p>