We propose to examine the hypothesis that plant resources are allocated to maximize carbon gain and productivity under the constraints imposed by the seasonality of moisture and irradiance. Lifespan, mass per area, nitrogen concentration, area per plant mass, area-based construction costs, and assimilation capacity of leaves will be examined as determinants of productivity (relative growth rate) during the dry season and wet season in Panama. These features have been hypothesized to be mutually supporting, interdependent, and casually interrelated. Variation in leaf features will be documented over light gradients within the canopies of mature evergreen tropical species, and among seedlings of these species in an experimental common garden with a factorial design of contrasting levels of light and water. These species encompass considerable variation in leaf longevity, while producing new leaves in seasons of contrasting water and light availability. As such they allow the examination of specific predictions about how functional characters vary in response to variation in the opportunity for carbon gain. Studies will be conducted at the Smithsonian Tropical Research Institute (STRI) using a construction crane for access to the canopy of a tropical dry forest. Garden treatments and canopy trees will be followed for three years. These will be the first comprehensive data for within-canopy physiological variation, and the first comparison of seedlings and canopy trees for leaf longevity and function in tropical species. These studies will demonstrate the potential for selection to produce coordinated responses among leaf characters to short-term and long-term variation in resources. If functional relationships among these features prove to be tightly coordinated, such physiological syndromes will provide an empirical link between leaf and whole-plant processes of growth and reproduction. The canopies of tropical trees provide some of the steepest resource gradients known with respect to light, humidity, leaf water, and possibly nutrient, and thus are a model system to assess the relationships among functional characteristics. Because of the great concern over tropical deforestation, there is a pressing need to understand physiological syndromes associated with species capable of rapid regeneration.
