Field measurement of transpiration in Douglas-fir.
Transpiration rates of Douglas-fir trees (Pseudotsuga menziesii) were measured in the field, using tritiated water (HTO) as a tracer for water. Sites were located in the Cedar River Watershed near Seattle, Washington and in the Andrews Experimental Forest near Eugene, Oregon. Transpiration rates ranged from 8.4 l day-1 in a small tree to 530 l day-1 in a large old-growth tree on the Oregon site. A relationship between transpiration rate and sapwood cross-sectional area of trees was found which would permit extension of individual tree measurements to forest populations. The HTO measurements, linked to current physical theories of evapotranspiration, permit the computation of actual daily transpiration rates for individual trees or areas of forest. The method allows direct measurements of water loss from forests in situations where lysimeter installation would be impractical.