Vertical Variation in Stemflow Generation

(1) Stemfiow production rates of Douglas fir were examined in the field in relation to crown volume and canopy architecture. (2) Experimentally controlled precipitation was applied to the tree, and stemflow monitored at four points down the trunk below major whorls. (3) Total stemflow production was less than 2% of total precipitation, with 98% of the stemflow being produced in the upper half of the canopy volume. (4) The results suggest that the sheltered, depressed branches of the lower canopy contribute little to stemflow production, despite having 31% of the branch interception area. density, stand structure, tree species, and precipitation duration and intensity on interception and net rainfall. Interception is usually measured indirectly as the difference between gross (above the canopy) and net (below the canopy) rainfall. The latter is calculated by summing the total throughfall (_ canopy drip) and stemflow (Kittredge 1948; Leonard 1961; Rothacker 1963; Rutter 1963; Geiger 1965; Helvey & Patric 1965; Zinke 1967; Brown & Barker 1970; Orr 1972; Swank, Geobel & Helvey 1972; Tajchman, Lee & Repa 1979). From the established relations it is possible to develop estimates of the budgetary terms in the form of linear regression equations that are of practical value to the forest and water manager. A rather different approach to the analysis of the forest water budget is that of Robins (1974), and Rutter et al. (1971, 1975). In their work empirically-derived, species-specific estimates of canopy storage capacity, canopy drainage and stemflow, and predicted evaporation rates (based on wind speed, atmospheric humidity and the aerodynamic resistance of the canopy) are used to calculate canopy water balance. In both these approaches the individual tree or forest stand forms the unit of investigation, but little account is taken of the hydrological response of different parts of a given tree. The objective of this study is to examine one response variable (stemflow) in a partitioned tree canopy, and to relate stemflow production to canopy storage capacity and canopy morphology.