Tohoku Regional Breeding Of®ce, Forest Tree Breeding Centre, Takizawa, Iwate 020-0173, Japan and àHokkaido Regional Breeding Of®ce, Forest Tree Breeding Centre, Ebetsu, Hokkaido 069-0836, JapanTo examine the eects of forest cutting on within-population genetic structure, the genetic structure and variability of two Japanese beech (Fagus crenata Blume) stands with contrasting histories in relation to cutting were investigated. Six hundred and sixty beech trees, covering two hectares in total, were mapped and genetically analysed using nine isozyme loci encoding eight enzyme systems. The proportion of polymorphic loci, the average number of alleles per locus, the eective number of alleles per locus, the expected heterozygosity and the observed heterozygosity were 78, 3.3, 1.31, 0.200 and 0.189, respectively, in a secondary stand (designated AK) cut during the 1920s. Corresponding ®gures were 78, 3.3, 1.33, 0.203 and 0.193, respectively, in a primary stand designated KU. The inbreeding coecient and the grand mean of the number of alleles in common (NAC) were 0.055 and 1.684 in AK, and 0.042 and 1.649 in KU, respectively. The genetic variability was slightly but signi®cantly lower in AK. The genetic structure of the two stands was strikingly dierent. The proportions of positively signi®cant Moran's I and SND values found in the shortest distance class were 0.86 and 0.38 for AK, and 0.14 and 0.29 for KU, respectively. Furthermore, signi®cant linkage disequilibrium was observed in AK, but none at all in KU. To examine which, if any, dierences in the genetic structure would be likely to in¯uence succeeding generations, we simulated a self-thinning process. The simulation suggested that reduced genetic variability and linkage disequilibrium would have signi®cant in¯uence in the AK stand for several generations.Keywords: Fagus crenata, founder eect, isozyme variability, linkage disequilibrium, self-thinning, spatial autocorrelation. IntroductionForest tree species are genetically more diverse than most other life forms. Generally, the autosomal genetic variation of such species is largely found in the withinpopulation component, and factors such as range of geographical distribution, mating system and seed dispersal all in¯uence the genetic variability retained in the species (Hamrick et al., 1992). As a tool to study how genetic variation is distributed within populations, spatial autocorrelation techniques (Sokal & Oden, 1978a,b) have played an important role in many studies of genetic structure in forest tree species. These techniques are eective for identifying the major genetic processes involved in the generation of genetic structures in speci®c populations (Sokal & Jacquez, 1991; Sokal et al., 1997). Simulation studies and forest tree population studies have revealed that mating system and seed dispersal have considerable in¯uence on within-population genetic structure (Sokal & Wartenberg, 1983). Coniferous species (in which pollination and seed dispersal occurs mainly by wind, and outcrossing rates are high) h...