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Influence of Ceanothus velutinus and Associated Forbs on the Water Stress and Stemwood Production of Douglas-Fir

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Stem dimensions for two age groups of Douglas-fir growing in the central Cascade Mountains of western Oregon were related to water stress and the amount of interference from dense Ceanothus velutinus and forbs 8 years earlier. In 1978, three regimes were established in four 5-year-old and four 10-year-old stands by means of controls (no treatment) and two herbicide treatments to individual trees in each stand: a partial treatment (C. velutinus eliminated) and a complete treatment (both shrubs and forbs eliminated). In the subsequent year, soil water potential during late summer was < - 1.5 MPa at 10-, 40-, and 100-cm depths, where C. velutinus was growing with forbs. In the absence of shrubs and forbs, soil water potential at 100 cm was near field capacity throughout the 1979 growing season. Predawn stem water potential and Douglas-fir during late summer was significantly lower for trees competing with C. velutinus and forbs than for trees without competitors in the complete treatment, or for trees competing with forbs in the partial treatment, in the four 5-year-old stands and in two of the 10-year-old stands. By 1986, Douglas-fir stems were 2 to 6 cm larger in basal diameter and 1 to 2 m taller in the absence of competitors. Interference from C. velutinus and forbs had a greater effect on stem size of 5-year-old than 10-year-old trees. The correlation between growth and water stress suggests that interspecific competition for soil water during summer drought is a factor limiting stemwood production. For. Sci. 34(2):333-343.

Keywords: Pseudotsuga menziesii; interference; water potential

Document Type: Journal Article

Affiliations: Associate Professor, Virginia Polytechnic and State University, Blacksburg, VA

Publication date: June 1, 1988

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