WATER-BALANCE OF CONIFER LOGS IN EARLY STAGES OF DECOMPOSITION

Seasonal and long-term changes in the water balance of conifer logs du ring the first 8 years of decomposition were studied in an old-growth Pseudotsuga/Tsuga forest in the Oregon Cascade Mountains. Measurements were made of the moisture content of outer bark, inner bark, sapwood, and heartwood and of the flow of water into and out of logs of four s pecies (Abies amabilis, Pseudotsuga menziesii, Thuja plicata, and Tsug a heterophylla). After the logs had decomposed from 1 to 2 years, 38-4 7% of the canopy throughfall landing upon them ran off the surface, 29 -34% leached from the bottom, and 21-30% was absorbed and evaporated. After 8 years of decomposition, water entering and then leaching from logs increased 1.3 times while runoff decreased a similar amount. The proportion of water stored by and evaporated from logs in this study i ndicates that in old growth forests they may intercept 2-5% of the can opy throughfall to the forest floor and that, even in early stages of decomposition, they may affect the hydrological cycle of Pacific North west old-growth forests.