Temporal and spatial variations in stem heat pulse velocities were investigated in Douglas-fir ( Pseudotsuga menziesii (Mirb.) Franco) of various sizes. Heat pulse velocity was compared with actual transpiration rate and highly significant correlations were obtained. Spatial variations were investigated with respect to radial distances past the cambium and at different locations about a stem. Maximum daytime average velocities were noted within a relatively narrow band in the sapwood, but the depth at which this band occurred varied during the study. Velocity measurements at various sectors around the stem of a codominant tree suggested differential rates of water loss from various parts of the crown. Temporal variations were examined for a codominant tree and were briefly compared with those observed in trees of other crown classes. In all cases heat pulse velocity was directly dependent upon evaporative demand, but with increasing soil moisture stress stomatal closure was noted and velocities no longer followed demand. Diurnal velocity patterns in all trees were generally similar, but variations attributed to differences in sapwood area, crown exposure, or both were noted. FOREST SCI. 23:377-390.
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Lassoie et al. (1977) studied this question.