Summary
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.
Methodology
The study site was a 10-hectare woodland located on the property of the Queen's University Biology Station at Lake Opinicon in southeastern Ontario, Canada (44°34'02"N, 76°21'52"W). The area was logged in the last century, but the study site has since been free of any major disturbance from human activity. The site was bordered by the Opinicon Road on the north and a laneway on the east, west, and south, connected to the Opinicon Road at both ends. Sugar maple was the dominant tree species on the site. Less abundant canopy species included Fraxinus americana L., Ostrya virginiana (Mill.) K. Kock, and Carya cordiformis (Wang.) K. Koch.
Most seedlings become established beneath the canopy of parent trees. Accordingly, 14 sugar maple trees within the population were randomly selected as sampling sites. Only trees with diameter at breast height (DBH) greater than 40 cm were chosen. The north, south, east, and west orientation of each of these trees was located with a hand compass, and the limit of the canopy in each compass direction was projected down to the ground by visual estimation and marked with a flag. These four points were connected by a meter tape to delimit a four-sided polygon. Quadrats, 1.0 m x 0.5 m, were placed randomly at one-, two-, or three-meter intervals along the perimeter of the polygon. If the positioning of a randomly placed quadrat was prevented by the presence of trees greater than 0.5 meters high, it was shifted to the next closest interval along the meter tape. A total of 185 quadrats were surveyed within the polygons of the selected trees between 26 June and 26 September 1986.
For each quadrat the following data were collected: age, height, and leaf number of each maple seedling or sapling less than 50 cm high, light intensity; soil magnesium, phosphorus, and potassium concentration, and pH; and percentage cover of herbaceous plants and non-maple tree seedlings.
The seedlings were aged by counting the annual bud scale scars. Light intensity was measured using a light meter (General Electric, Model 214) on cloudless days between 11 AM and 1 PM and expressed as a percentage of full sunlight. Five measurements of light intensity were recorded per quadrat--one at the center and one at each of the four corners. A single soil sample was collected from the center of each quadrat. The soil samples were sent to Agri-Food Laboratories Canada at Guelph for analyses. The percent cover of other species was estimated visually using a grid to partition the quadrat into smaller sections, each comprising 4% of the total quadrat area.
Within each quadrat, four two-year-old maple seedlings were randomly selected. The height and leaf number were recorded for each seedling and for its nearest neighbor. The distance between the neighboring seedlings was also recorded. A total of 638 neighboring seedling pairs were included.
In mid-October, the heights of 100 randomly selected two-year-old sugar maple seedlings and their nearest two-year-old neighbors were recorded as well as the distance between them. The seedlings were dug up, washed free of soil and debris, dried at 100 C to constant final weight, and weighed.