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Lysimeter measurements of salal understory evapotranspiration and forest soil evaporation after salal removal in a Douglas-fir plantation Osberg, Peter Martin


Two weighing lysimeters were constructed in a 23-year-old Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) stand. The salal (Gaultheria shallon Pursh.) understory was mechanically removed from a 30 x 40 m plot and one lysimeter was located at the center of this plot to measure daily forest soil evaporation (E[sub s]). For 8 consecutive days, in August 1984, E[sub s] ranged between 0.08 and 0.34 mm d⁻¹ which was 15-18% of the daily total stand evapotranspiration. In July 1985, after removing salal from the second lysimeter, which was positioned under a less dense tree canopy, E[sub s] was 0.34 - 0.41 mm d⁻¹ which was 17-21% of the daily whole stand evapotranspiration. Forest floor diffusive resistances (r[sub co]), computed by rearranging the Penman-Monteith equation, were found to range between 900 and 3500 s m⁻¹. The measured daily evapotranspiration rate from the understory (E[sub t]) with salal present was 0.60 - 0.84 mm d⁻¹, which was equivalent to 30-42% of the daily stand evapotranspiration. The dependence on windspeed of the bulk aerodynamic resistance (s m⁻¹) to vapour transport (r[sub A]) between the salal canopy and measurement height (0.3 m above the top of the salal canopy) was found to be r[sub A] = 25.2 ū[sup – 0.554], where ū is the mean windspeed (m s⁻¹) at measurement height. Approximately 70-75% of r[sub A] was estimated to be attributable to the leaf boundary-layer resistance. The evaporation rate from wet foliage was found to be 5 times greater than the transpiration rate from dry foliage exposed to similar meteorological conditions. Analysis of the relative importance of net available energy flux density and advective processes in determining the latent heat flux density from the understory yielded a range of Q values between 0.15 and 0.22. The vapour pressure deficit below the tree canopy was found to be largely determined by the vapour pressure deficit above the tree canopy, with very little local adjustment.

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