Measurement of Transpiration in Tropical Trees with Tritiated Water

The measurement of transpiration of water by trees in the field with tritiated water as a tracer depends upon a new application of established theory of radionuclide dynamics in steady—state systems. The techniques required are non—destructive to the trees and probably have negligible disturbing effects on transpiration. Average transpiration rates ranging from 1.75 to 372 liters per day per tree were measured by the proposed method on tropical forest trees which ranged from the understory to the canopy in size. Statistical errors range from 12.1% on the largest tree to 6.2% on the smallest for one standard deviation. Non—random sources of error in the method may include (1) loss of tritium from leaves due to rainfall; and (2) possible enrichment of tritiated water in leaves due to differences in vapor pressure and molecular diffusion coefficients between tritiated water and ordinary water. These require further experimental evaluation. The method may be generally applicable to field measurements of transpiration in trees.