Population density and mating pattern in western larch

Variation in the mating systems of western larch as affected by different stand density was investigated in two adjacent populations (natural and seed tree) in two locations in the interior of British Columbia, Population multilocus outcrossing rates, individual tree multilocus outcrossing rates, and the correlation of outcrossed paternity were estimated from analysis of allozyme variation in embryonic and megagametophytic tissue of wind-pollinated seeds. All population outcrossing rate estimates were significantly different from t = 1, Positive association between population density and outcrossing rate was detected for one location (0.903 versus 0.876) and inverse association was obtained for the second (0.793 versus 0.912). In three populations, the distributions of individual tree outcrossing rates were skewed toward high outcrossing. In the fourth population, which produced a low multilocus estimate of outcrossing, individual tree outcrossing rates were widely distributed. High and significant correlated matings (6% and 10%) were detected in the populations with high densities, while low (0.1% and 2.0%) estimates were detected in populations with low density. It appears that the effects of population density observed in this study and reported in previous studies can be overridden by the presence of barriers that limit pollen movement.