COMPARATIVE-ANALYSIS OF THE GENETIC COMPOSITION OF CANOPY AND JUVENILE SUGAR MAPLE INDIVIDUALS (ACER-SACCHARUM) IN AN OLD-GROWTH FOREST IN SOUTHERN QUEBEC AS RELATED TO ANTHROPOGENIC DISTURBANCE

The unexpected discovery of a relatively undisturbed presettlement for est in southern Quebec, the Muir Forest, allowed us to investigate the potential reproductive effects of recent man-made air pollution upon the genetic structure and variability of sugar maple (Acer saccharum M arsh.) at this site. The genetic structure of the canopy subpopulation consisting of 62 trees over 140 years of age was compared with that o f three understory subpopulations of 90 juvenile individuals, 6 to 15 years old. The genetic structure and variability were analyzed using c ellulose acetate electrophoresis of 14 enzyme systems. The analysis of 20 loci showed an average polymorphism of 60.1%, a mean of 1.83 allel es per locus, a mean of 1.17 effective alleles, and observed and expec ted levels of heterozygosity of 0.136 and 0.148, respectively. There w ere no significant differences among the canopy and the three juvenile subpopulations for these five genetic parameters. Estimated F-is and F-it (Wright's fixation indices) values indicated, respectively, a 7.3 % and a 8.7% average excess of homozygotes. There was a tendency for r are alleles at several loci to be at lower frequencies of absent in so me or all of the juvenile subpopulations. On average, a higher value o f F-is was obtained for the combined juvenile subpopulation than for t he canopy subpopulation, but this difference was statistically not sig nificant. The average genetic differentiation among subpopulations was weak (F-st = 0.015), and only 0.4% of the variability could be attrib uted to differences between the canopy and the combined juvenile subpo pulations. Genetic distances between the canopy and juvenile subpopula tions were very low (e.g., Nei's unbiased genetic distance = 0.000-0.0 03) and not different from those calculated among juvenile subpopulati ons. This study does not support the hypothesis that the reduction in reproductive potential resulting from environmental pollution and from other recent anthropogenic disturbances may involve a differential ga metophytic selection strong enough to reduce the genetic variability a nd (or) modify the genetic structure of progenies generated at the Mui r Forest during the last 2 decades.