Compromises between water-use efficiency and nitrogen-use efficiency in five species of California evergreens

In five California evergreen trees and shrubs cooccurring in this study but most common in habitats of different moisture availability, leaf nitrogen was a major determinant of photosynthetic capacity. Within each species, stomatal conductance was highly correlated with photosynthetic capacity, resulting in little variation in the concentration of CO₂ in the intercellular spaces. Among species, intercellular CO₂ concentrations varied significantly. Under controlled conditions, the leaves that realized the highest photosynthesis per unit of leaf nitrogen tended to realize the lowest photosynthesis per unit of water transpired. The ratio of photosynthesis to transpiration, an instantaneous measure of intrinsic water-use efficiency, was highest in the species commonly found in the direst habitats and lowest in the species most common in the wettes habitats.