Interference dynamics in mixed red alder/Douglas-fir forests

Graduation date: 2003This study characterized the nature and dynamics of interference in mixed red alder\ud (Alnus rubra Bong.)/Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) stands in the Pacific Northwest, USA. Long-term spatial and tree measurements from the Cascade Head (CH) and H.J. Andrews (HJA) Experimental Forests in western Oregon and Delezene Creek (DC), Washington were utilized to investigate\ud neighborhood and population-level measures of interference. Existing neighborhood and population-level measures of interference were modified to evaluate the intensity and importance of intra- and inter-specific interference. The relationship between relative growth rate and population-level and neighborhood interference were examined over 9 years at the CH and HJA study sites and 38 years at the DC study site. In general, the effects of intra-specific interacted with the effects of inter-specific interference to influence the relative growth rates of red alder and Douglas-fir at all of the sites. Performance of the interference measures as predictors of relative growth rates varied between species and with stand structure. In general, population-level indices were the best predictors of relative growth rates for the species with heights greater than the other interacting species over a given interval of time. In contrast, neighborhood indices were the best predictors of relative growth rates for the species with subordinate or equivalent tree heights to the dominant species over a given interval of time. These results were consistent for both species, all three study sites, and all measurement periods when interference occurred and suggest that the importance of neighborhood interference varies with the competitive status of a species. A conceptual model synthesizes the importance of neighborhood and population-level interference as a function of relative dominance of a species. In addition, the literature suggests that this model may also be appropriate for individuals within a population