The functional co-ordination of leaf morphology, nitrogen concentration, and gas exchange in40 wetland species

Abstract

We grew 40 commonly co-occurring species of wetland herbs from eastern North America under uniform conditions to evaluate the overall pattern of interspecific variation in specific leaf mass (SLM), foliar nitrogen content, stomatal conductance (g), and internal leaf CO2 concentration (ci). While the relationship between any two of these traits that influence net photosynthetic rate is constrained to some degree, there is sufficient flexibility to allow the evolution of different but more or less equally effective interrelationships among these central elements of leaf form and function. We use contemporary techniques of structural equation modelling to describe the general nature of such evolutionary diversification in leaf form and function among these wetland plants. Our model essentially extends the Cowan-Farquhar model of stomatal regulation to include relationships between SLM and foliar nitrogen. The model can take two forms, with variables expressed as either per unit leaf area or per unit leaf mass. When variables are expressed on an areal basis, the model predicts that a species with a higher SLM will have a higher foliar nitrogen level. The foliar nitrogen level, in accordance with the Cowan-Farquhar model, in turn determines the dynamics of stomatal regulation in relation to the marginal cost of water loss relative to carbon gain. The dependence of stomatal regulation on foliar nitrogen also determines the maximal rates of stomatal conductance and net photosynthesis. Internal CO2 concentrations within the leaf follow as a necessary consequence of these interrelationships. This areal-based model describes the data for the 35 C3 wetland species well; the same basic model applies to the five C4 species in our sample, except for shifts in the quantitative effects of net photosynthetic rate and stomatal conductance on internal CO2 levels. When the variables are expressed on a mass basis, a slightly different model results, as net photosynthetic rate decreases directly with SLM and is not related to species level variation in either leaf nitrogen concentration or maximal stomatal conductance. Both forms of the model indicate the need to advance our understanding of the ecological and evolutionary basis for variation in SLM, including its association with traits such as leaf demography and canopy architecture as well as environmental characteristics of the habitats where particular species predominate.