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1 We used repeated measurements of tree growth and population-level and neighbour- hood conditions from three mixed Alnus rubral Pseudotsuga menziesii forests in the Pacific Northwest, USA to investigate why previous results regarding the importance of neighbourhood competition as a determinant of plant growth were inconsistent 2 We propose that relative dominance ot a particular species determines the importance of neighbourhood interactions, and tested whether growth performance of both species at various stand ages agrees with this relative dominance hypothesis. 3 Neighbourhood and relative dominance interaction indices were modified to accommodate two-species mixtures and were incorporated into a growth model predicting relative diameter growth rates. The corrected Akaike Information Criterion (A/Cc) was used to identify the optimum interaction measures and model forms for each species and measurement period. 4 Interaction indices reflecting the sue of a tree relative to the population were the best predictors of growth of the dominant (i e taller) species and neighbourhood interaction indices of the subordinate species 5 Performance of interaction measures as predictors of relative growth rates in our study varied in agreement with the relative dominance hypothesis for both species and on all sites. Results from other studies suggest that the hypothesis may explain growth performances on a species and individual plant level and for a variety of' life forms. 6 These findings suggest that the spatial scale of plant interactions is influenced by the sue structure of plant populations. The relative dominance hypothesis offers a framework to provide insight into the mechanism of' competition, based on the relative performance of competition indices. [ABSTRACT FROM AUTHOR]

1. The monoculture self-thinning concept was expanded to polycultures by de- scribing 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. menziesu. 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. [ABSTRACT FROM AUTHOR]