Functional convergence in tropical forest canopy physiology.

MEINZER F.C, G. GOLDSTEIN, S.J. BUCCI, S.A. JAMES, L.S. SANTIAGO AND F.G. SCHOLZ.

Montreal

Simposio; IX International Congress of Ecology.; 2005

Plant functional traits may scale with simple physical and structural features in an apparently universal manner across a broad range of species. Such functional convergence is presumed to have its origin in evolutionary constraints that limit combinations of structural and functional traits. Using data gathered from tropical woody species native to Panama, Brazil and Hawaii, we present examples of species-independent scaling of hydraulic architecture, whole-plant water transport, leaf water relations and photosynthetic gas exchange with features such as plant size, and sapwood density and hydraulic capacitance. Among all species studied, total daily water transport followed a common sigmoid trajectory with increasing basal stem diameter. Multiple functional traits covaried with wood density in a similar manner across species. Leaf-specific hydraulic conductivity, maximum rates of photosynthesis and stomatal conductance, minimum leaf water potential and leaf osmotic potential at zero turgor all declined with increasing wood density, suggesting that in diffuse-porous species wood density constrains hydraulic function to specific operating ranges. Maximum sap velocity declined linearly with increasing sapwood capacitance, and whole-tree hydraulic resistance and capacitance covaried in an inverse manner, leading to similar time constants for dynamic changes in canopy transpiration in response to changes in environmental variables.  These findings illustrate the constraints that plant anatomical, structural and physical attributes place on physiological functioning from leaf to whole tree.  Despite their species richness, tropical forests thus exhibit considerable physiological convergence among species.