Functional Convergence and Constraints in Regulation of Transpiration and Carbon Assimilation in Tropical Forest Canopy Trees

Oregon State University, Corvallis OR

Investigators

Abstract

Lowland tropical forests are notable for their high plant species diversity, particularly tree species. This project is based on the premise that despite the bewildering diversity of tree species in tropical forests, similar adaptive responses occur across a wide range of species. Constraints on the possible combinations of adaptive responses should lead to convergence among basic plant traits related to regulation of carbon assimilation and transpiration, two fundamental processes. Consistent with this, the PIs prior use of the Smithsonian Tropical Research Institute (STRI) canopy crane in Panama to make concurrent measurements of water use at multiple scales from single leaf to whole tree, has revealed a surprising degree of functional convergence in regulation of transpiration among diverse tree species. In the current project, the PIs are building upon the PIs previous observations to determine the extent to which leaf level regulation of transpiration and carbon assimilation are governed by tree architecture and size. The problem of characterizing plant behavior over an adequate range of scale has been particularly acute with large forest canopy trees. The advent of canopy cranes has revolutionized access to all parts of forest canopy trees from their trunks, to intermediate branches, to leaves in the upper crown. The PIs are therefore taking advantage of the unique opportunity provided by the STRI canopy cranes to study large trees as whole organisms. One of our key objectives is to identify appropriate numerical scaling factors that relate tree physiological behavior to tree architectural features such as the ratio of leaf area to sapwood area. These scaling factors will detect functional convergence in regulation of carbon uptake, and the uptake, transport and loss of water among diverse tree species. Development of scaling factors that reveal areas of functional convergence will provide new insights that will greatly simplify interpretation and modeling of tree, stand and ecosystem level responses in these complex, species-rich forests. Furthermore, demonstration that similar functional constraints apply across a wide range of species leading to trade-offs among basic plant traits related to structure and function may serve to move the science of ecology towards new paradigms concerning the nature of differences and similarities among plant species. The PIs expect approaches, results and general conclusions to be applicable to a wide range of natural and managed forests, not just tropical forests. The PIs also expect the results of this research to contribute to the ability to predict the consequences of different management practices for forest productivity and utilization of water resources.