Self-shading, carbon gain and leaf dynamics: a test of alternative optimality models

A simple model of shoot-level carbon gain is presented addressing the optimal number and life span of leaves in relation to alternative optimality criteria: (1) maximizing carbon export from the shoot, or (?) maximizing the rate of leaf production at the shoot tip. Additionally, the processes that cause declining assimilation with leaf age are considered in relation to (1) leaf position on the shoot (e.g., self-shading) versus (2) leaf age per se. Using these alternative scenarios, only a model based on position-dependent assimilation and maximization of leaf production rates resulted in quantitative predictions for all aspects of leaf dynamics on the shoot (i.e., leaf number, life span, and birth rate), while other approaches predicted that one or more parameters would be infinite. This formulation of the model also predicted that leaves should be maintained on the shoot until the diurnal carbon balance declines to zero, in contrast with other scenarios which predict that leaves should be shed while maintaining a positive carbon balance. Predictions of the model were supported by the results of a field study of carbon gain and leaf dynamics in saplings of three species of tropical pioneer trees (Carica papaya, Cecropia obtusifolia, and Hampea nutricia) which differ in the number of leaves per shoot. The results illustrate that in these fast-growing plants, leaf production and height growth may be more appropriate measures of performance than net carbon export from the shoot, and suggest that leaf senescence is primarily a function of the position of a leaf within the canopy, rather than its chronological age.