Neighbor Effects in Mast Year Seedlings of Acer Saccharum

The size and age distributions of Acer saccharum Marsh, seedlings were studied in a mature hardwood forest in southeastern Ontario between 26 June and 26 September 1986. Ninety‐one percent of the seedlings surveyed were produced in 1984 which was a mast year for this population of sugar maple. Mean height within quadrats was negatively correlated with the density of two‐year‐old seedlings in the first half of the sampling period, suggesting that competition may be an important factor affecting seedling size. A significant positive correlation in the second half of the sampling period, however, suggested a temporal shift in the pattern of seedling growth in which the shorter, more suppressed seedlings under the higher densities had increased their relative height in response to earlier competition. The sum of the heights of pairs of nearest neighboring seedlings sampled over the survey period was negatively correlated with the distance between them. The sum of the biomasses of pairs of nearest neighboring seedlings collected in October 1986, however, was positively correlated with the distance between them. Variation in the height of two‐year‐old seedlings was unaffected by light intensity but was to some extent accounted for by soil variables, and total percent cover of other species. The density of the two‐year‐old seedlings, however, accounted for the largest percentage of the variation in their mean height. The data suggest that competition between the seedlings derived from mast seeding in 1984 may represent an important component of the process of natural selection affecting this population.