The Dynamics of Mixed Stands of Alnus Rubra and Pseudotsuga Menziesii: Extension of Size--Density Analysis to Species Mixture

1. The monoculture self-thining concept was expanded to polycultures by describing a self-thinning surface in a mixed-species size--density space. The size--density relationships of pure and mixed populations of Alnus rubra and pseudotsuga menziesii in the Pacific Northwest, USA were investigated. 2. For polycultures of A. rubra and P. menziesii, the average size--density surface showed a near-planar region for stands consisting principally of A. rubra. Thus, A. rubra dominates stand dynamics in this region. At high proportions of P. menziesii, the surface showed a curvilinear increase towards the relative density level of pure P. menziesii stands. The curvilinear shape of the self-thinning surface cautions against using a single self-thinning line for mixed stands where the species proportion changes over time. 3. The complexity of the size--density relationship indicates the need for a more thorough exploration of the underlying population dynamics. The growth model allowed investigation of mixed-stand development by identifying stand density, relative dominance of one species, and species proportions as important factors driving stand development along the self-thinning surface. 4. Simulations of the growth model showed the need to investigate the development of younger populations, the spatial pattern of mixtures, and the effects of dominance of one species on population development.