The functional co-ordination of leaf morphology, nitrogen concentration, and gas exchange in 40 wetland species

We grew 40 commonly co-occurring species of wetland herbs from eastern Nort h America under uniform conditions to evaluate the overall pattern of inter specific variation in specific leaf mass (SLM), foliar nitrogen content, st omatal conductance (g), and internal leaf CO2, concentration (c(i)). While the relationship between any two of these traits that influence net photosy nthetic rate is constrained to some degree, there is sufficient flexibility to allow the evolution of different but more or less equally effective int errelationships among these central elements of leaf form and function. We use contemporary techniques of structural equation modelling to describe th e general nature of such evolutionary diversification in leaf form and func tion among these wetland plants. Our model essentially extends the Cowan-Fa rquhar model of stomatal regulation to include relationships between SLM an d foliar nitrogen. The model can take two forms, with variables expressed a s either per unit leaf area or per unit leaf mass. When variables are expre ssed 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 ac cordance 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 a lso determines the maximal rates of stomatal conductance and net photosynth esis. Internal CO2 concentrations within the leaf follow as a necessary con sequence of these interrelationships. This areal-based model describes the data for the 35 C-3 wetland species well; the same basic model applies to t he five C-4 Species in our sample, except for shifts in the quantitative ef fects of net photosynthetic rate and stomatal conductance on internal CO2 l evels. When the variables are expressed on a mass basis, a slightly differe nt model results, as net photosynthetic rate decreases directly with SLM an d is not related to species level variation in either leaf nitrogen concent ration or maximal stomatal conductance. Both forms of the model indicate th e need to advance our understanding of the ecological and evolutionary basi s 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.