UBC Theses and Dissertations
Salal understory removal effects on the soil water regime and tree transpiration rates in a Douglas-fir forest Kelliher, Francis Maurice
Salal (Gaultheria shallon Pursh.) understory in a 800 tree/ha 31-year-old Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) stand was cut and removed from around one of each of four pairs of adjacent trees, the root zones of which were isolated using plastic sheeting buried to bedrock. The differences in the courses of the average root zone soil water content (Φ) during the growing season were small (maximum difference = 0.03 m³ m⁻³ ) because total evapotranspiration was only slightly higher where salal was present than where it had been removed. Porometer and lysimeter measurements on selected days indicated that salal transpiration was 0.5-1 mm d⁻¹ greater than forest floor evaporation in cut subplots and that Douglas-fir transpiration was 0.2-0.5 mm d⁻¹ higher where salal had been removed. The slight increase in Φ where salal had been removed corresponded to significantly higher soil water potential and Douglas-fir pre-dawn twig xylem water potential at low values of Φ, owing to the steepness of the water retention curve for the gravelly sandy loam soil. This resulted in significantly greater tree diameter growth where salal had been removed than where it remained. Shuttleworth's development of the Penman-Monteith equation for multilayer, partially wet forest canopies was modified for use in the hypostomatous canopies of Douglas-fir and salal. This evapotranspiration theory was combined with standard hourly micrometeorological measurements, transfer resistance functions and canopy and root zone water balance equations to provide calculations of forest evapotranspiration (E) over extended growing season periods. There was generally good agreement between calculated values of E and values determined using Bowen ratio/energy balance, water balance and porometer measurements. The slightly higher values of Φ resulting from understory removal corresponded to significantly higher tree transpiration rates calculated over early (June) and late (August) growing season periods. Most of the difference in calculated tree transpiration occurred during the final one-half of these periods when at low values of Φ slightly lower Φ corresponded to significantly lower ψs where salal remained, leading to a reduction in Douglas-fir transpiration due to stomatal closure. The increase in calculated tree transpiration as a result of understory removal was greatest where understory leaf area index was highest and trees were largest.