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Many factors interact to determine genetic structure within populations including adult density, the mating system, colonization history, natural selection, and the mechanism and spatial patterns of gene dispersal. We examined spatial genetic structure within colonizing populations of Quercus rubra seedlings and Pinus strobus juveniles and adults in an aspen-white pine forest in northern Michigan, USA. A 20-year spatially explicit demographic study of the forest enables us to interpret the results in light of recent colonization of the site for both species. We assayed 217 Q. rubra seedlings and 171 P. strobus individuals at 11 polymorphic loci using nine allozyme systems. Plant genotypes and locations were used in an analysis of spatial genetic structure. Q. rubra and P. strobus showed similar observed levels of heterozygosity, but Q. rubra seedlings have less heterozygosity than expected. Q. rubra seedlings show spatial genetic clumping of individuals on a scale to 25 m and levels of genetic relatedness expected from the clumped dispersion of half-siblings. In contrast, P. strobus has low levels of genetic relatedness at the smallest distance class and positive spatial genetic structure at scales < 10 m within the plot. The low density of adult Q. rubra outside the study plot and limited, spatially clumped rodent dispersal of acorns is likely responsible for the observed pattern of spatial genetic structure and the observed heterozygote deficit (i.e. a Wahlund effect). We attribute weaker patterns observed in P. strobus to the longer dispersal distance of seeds and the historical overlap of seed shadows from adults outside of the plot coupled with the overlap of seed shadows from younger, more recently established reproductive adults. The study demonstrates the utility of long-term demographic data in interpreting mechanisms responsible for generating contemporary patterns of genetic structure within populations.

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The spatial patterning of and competition between established large‐toothed aspen (Populus grandidentata) and invading white pine (Pinus strobus) were examined in a 65‐yr‐old forest system at the AuSable Trails Institute of Environmental Studies, Kalkaska County, Michigan. Density, diameter, and distance measures were recorded for tree species on two 40 × 40 m grids containing 64 quadrats each 5 × 5 m. Several indices of dispersion were used to assess pattern and regression analysis of nearest neighbor distance to tree diameter provided an index of competition. The results indicate strong aggregation for both aspen and pine on a large scale, but regularity of pattern for pines in relation to their nearest neighbor. This regularity appears to be due to competition with established aspens and suggests that the spatial patterning of the aspens acts to control the spacing and subsequent development of the invading white pines.

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