REGULATION OF WATER FLUX THROUGH TRUNKS, BRANCHES, AND LEAVES IN TREES OF A LOWLAND TROPICAL FOREST

We studied regulation of whole-tree water use in individuals of five d iverse canopy tree species growing in a Panamanian seasonal forest. A construction crane equipped with a gondola was used to access the uppe r crowns and points along the branches and trunks of the study trees f or making concurrent measurements of sap flow at the whole-tree and br anch levels, and vapor phase conductances and water status at the leaf level. These measurements were integrated to assess physiological reg ulation of water use from the whole-tree to the single-leaf scale. Who le-tree water use ranged from 379 kg day(-1) in a 35 m-tall Anacardium excelsum tree to 46 kg day(-1) in an 18 m-tall Cecropia longipes tree . The dependence of whole-tree and branch sap velocity and sap flow on sapwood area was essentially identical in the five trees studied. How ever, large differences in transpiration per unit leaf area (E) among individuals and among branches on the same individual were observed. T hese differences were substantially reduced when E was normalized by t he corresponding branch leaf area:sapwood area ratio (LA/SA). Variatio n in stomatal conductance (g(s)) and crown conductance (g(c)), a total vapor phase conductance that includes stomatal and boundary layer com ponents, was closely associated with variation in the leaf area-specif ic total hydraulic conductance of the soil/leaf pathway (G(t)). Vapor phase conductance in all five trees responded similarly to variation i n G(t). Large diurnal variations in G(t) were associated with diurnal variation in exchange of water between the transpiration stream and in ternal stem storage compartments. Differences in stomatal regulation o f transpiration on a leaf area basis appeared to be governed largely b y tree size and hydraulic architectural features rather than physiolog ical differences in the responsiveness of stomata. We suggest that rel iance on measurements gathered at a single scale or inadequate range o f scale may result in misleading conclusions concerning physiological differences in regulation of transpiration.