Analysis of Yield-Density Relationships in Experimental Stands of Douglas-Fir and Red Alder Seedlings
Seedlings of red alder (Alnus rubra Bong.) and Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) were planted into a two-species density matrix composed of five monoculture densities and mixed stands with all possible pairwise combinations of the monoculture densities. Regression equations were fit to the response of mean tree stem volume to the two species' densities generated in this matrix. Regression coefficients quantifying the intensity of competition indicated that alder density had approximately twice the effect of Douglas-fir density on individual tree stem volume of both species. The densities of the two species had a multiplicative effect on mean tree stem volume. In addition, the effects of alder and Douglas-fir densities on tree size were interdependent. The effects of alder density on stem volume varied with Douglas-fir density and declined as Douglas-fir density increased. Similarly, the effects of Douglas-fir density on stem volume varied with alder density. The interdependency between the two species' densities resulted in an unusual pattern in which Douglas-fir individual stem volume increased as Douglas-fir density increased at high densities of alder. For. Sci. 37(2):574-592.
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