Competition affects shoot morphology, growth duration, and relative growth rates of Douglas-fir saplings

Saplings of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. menziesii) were grown with and without competition from tanoak (Lithocarpus densiflorus (Hook. & Arn.) Rehd.) at two forest sites in southwestern Oregon. Competition reduced the following morphological parameters on Douglas-fir shoots: number and size of buds; lengths of shoots, internodes, and needles; number of internodes; and biomass of foliage and wood. Differences in the length and biomass of Douglas-fir shoots were attributed to combined reductions in internode number and internode length (distance between needles), with internode number accounting for most (72-87%) of the variation. The effects of competition from previous years on Douglas-fir growth, as manifested in 1986 bud size or stem circumference, explained 65 and 92% of the variation in 1987 growth of shoots and stem basal area, respectively. The current-year (1987) effects of competition explained an additional 6 and 3%. During the 1986 and 1987 growing seasons, competition limited both the rates and duration of growth of Douglas-fir basal area; for height, only the rates of growth were reduced.