First-year response of Douglas-fir after release from snowbrush ceanothus

Graduation date: 1981The release of Douglas-fir from competition with snowbrush (Ceanothus velutinus Dougl.) has often been suggested as a way to improve conifer yield in the western Cascades. This study investigated this possibility by describing changes in the availability of water and light and the response of Douglas-fir to the changing environment, in the first year following controlled manipulation of competing vegetation. Treatments applied to individual Douglas-fir that occupied intermediate crown positions were: 1) complete release by applying herbicides to all vegetation, 2) release from only shrub competition by basal spraying, and 3) manual cutting of shrubs. The extent of soil moisture conservation during the summer drought was dependent upon the degree of vegetation control. Soil water potential remained near the field capacity level during the entire summer with complete control of competing vegetation. In contrast, the soil water was largely depleted to a depth of 40 centimeters with partial release and to a depth of at least one meter in undisturbed stands. The seasonal water stress of Douglas-fir closely followed the depletion of the lower profile soil water, rapid increases in the stress of control trees occurred once the lower profile water was depleted by competing vegetation. A large amount of standing dead shrub stems were found to effectively replace light normally intercepted by live shrub leaves. While the availability of light was greatly increased in younger stands with low stem biomass, a significant increase in light was not observed well beneath the dead canopy of older stands. Stress following full and sudden exposure with manual release reduced Douglas-fir height growth in the first season after treatment. An increase in height growth was not observed following the other release treatments. Greater relative volume increment and terminal bud volume in chemically released trees in one younger stand did suggest that positive growth responses may take place. Based on these environmental and growth responses, it is hypothesized that the more favorable water and light environments after release will promote increased Douglas-fir growth in the future. Unless other forms of stress are imposed, either later in the stand development or immediately after release such as observed in the manual release treatment, this increased growth will likely improve stand yield