Marker variation was measured in 12 Acer platanoides L. and 9 Betula pendula Roth populations using 11 and 18 allozyme loci, respectively. Both species grow in the boreal–temperate zone. Insect-pollinated A. platanoides with limited seed dispersal has a scattered occurrence, while wind-pollinated B. pendula with light wind-dispersed seeds has a more continuous distribution. The average expected heterozygosity was 0.132 for A. platanoides and 0.141 for B. pendula. Genetic differentiation among A. platanoides populations (Fst = 0.099) was higher than that of B. pendula (Fst = 0.032). Expected heterozygosities in marginal and central populations of A. platanoides were at approximately the same level. Contrary to the expectation, differentiation was higher among central populations.

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The genetic structure of 33 natural Quercus robur stands in Finland was studied using 13 allozyme loci to analyze the effects of fragmentation in a wind-pollinated tree species. The present fragmented and discontinuous distribution of oak is a result of both short-term human impact and long-term climatic and geological change, including post-glacial land uplift. In accordance with general expectations, genetic diversity in small populations was lower than that in large populations, and differentiation among small populations was higher than that among large populations. Heterozygote deficiency was more pronounced in large populations, which is proposed to be a Wahlund effect created by either spatial sub-structuring or the existence of synchronized flowering lineages. Also genetic differentiation was higher and diversity lower in Finland than the estimates reported for Central Europe. There were differences in the genetic structure on sites of different geological age. We suggest that on most geologically old sites drift has a prominent effect whereas on younger sites also founder effects may be important.

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Studies on the amount of genetic variation in marginal populations and differentiation between them are essential for assessment of best gene conservation strategies and sampling schemes. Thirteen marginal populations of Ulmus laevis in southern Finland and one in Estonia were investigated for genetic variation in 20 allozyme loci. Population differentiation among Finnish stands was high, F st = 0.290, and mean genetic diversity low, H e = 0.088. The differentiation follows the isolation-by-distance structure within the core of the distribution area (lake Vanajavesi). Fairly high frequency of recurrent genotypes was observed, but this did not have an influence on the genetic parameters. The observed genetic structure is consistent with the central-marginal hypothesis. In the light of the results, the Finnish gene conservation strategy for U. laevis seems to be on a sound basis.