Summary
We examined whether particular years of mast seed production in sugar maple (Acer saccharum) are associated with increased likelihood of cohort recruitment success into the sapling stage over three decades of heavy browsing pressure from white-tailed deer in a mature woodland population in southeastern Ontario, Canada. The population was sampled in 2014 for seedling and sapling stages (≤6 cm in stem diameter) to obtain an age frequency distribution spanning about 80 y and including survivors of seed cohorts produced in two known mast years at the study site (2013 and 1984) and in other mast years known to have occurred within the broader region (but not confirmed for the study site). The age frequency distribution is roughly bimodal with zero to very few individuals recorded for ages 9 through 29 y, corresponding with the known time period (early 1980's to late 2000's) of regional overabundance for white-tailed deer in eastern Ontario and at the study site in particular. The 1 y old seedlings (from the 2013 mast year) and the survivors of putative mast year cohorts from 2006 and 2000, however, are especially conspicuous, with less striking recruitment success indicated for the older confirmed mast year cohort from 1984 (which had more years of accumulated impact from mortality risks). Our results suggest seed masting in sugar maple can bolster cohort recruitment success that otherwise would virtually (or completely) fail when severe impact from deer browsing is combined with other typical early life-stage mortality risks, e.g., from drought, neighborhood competition, and persistent overhead canopy shade.
Methodology
Study site
The study was conducted in 2014 within a 10 ha woodland bordered by Darling Farm Road and Opinicon Road, at Queen's University Biology Station (QUBS; 44°32′19.9″N, 76°22′03.2″W) in eastern Ontario. The woodland is known locally as “The Sugarbush” and was used for maple syrup production in the first half of the last century, but the site has been free of major human disturbances since 1976 when the property was acquired by Queen's University as part of QUBS (Raleigh Robertson, pers. comm.). The understory is deeply shaded in the summer by a mostly closed canopy of large Acer saccharum (sugar maple) trees, with less abundant canopy species including Fraxinus americana L. and Carya cordiformis (Wang.) K. Koch. and with Ostrya virginiana (Mill.) K. Koch. as the most abundant sub-canopy tree. The same population was used in an earlier study of a 1984 mast year cohort of sugar maple seedlings (Taylor and Aarssen, 1989).
Field Sampling
Because most sugar maple seedlings become established beneath or near parent trees, 25 of the largest canopy ('target') trees within the population were selected for sampling. The shape of the study site is long and relatively narrow; therefore, target trees were selected toward the approximate center of the site, along the long axis, avoiding trees whose canopies were within 10 m of a road or hiking trail. For each target tree, the furthest extent of the canopy at the north, south, east, and west corners was located using a hand compass and visually projected onto the forest floor and marked with a flag. A four-sided polygon was created using a meter tape connecting these four corners, and then a 1.0 m by 0.5 m quadrat was placed randomly at 1, 2, or 3 m intervals along the outside of the perimeter. If quadrat positioning was prevented by the presence of a large sapling or tree, or a boulder, the quadrat was placed on the inside of the perimeter line.
Within each 1.0 m by 0.5 m quadrat, all sugar maple individuals ≤ 50 cm tall were aged by counting annual terminal bud scale scars. In addition percent ground cover (by visual estimation) of other woody species (≤50 cm tall) was recorded, as well as percent cover of sedge (Carex) species and total percent cover of other herbaceous species. A total of 346 quadrats were surveyed within the polygons of the selected trees between 14 July and 5 September, 2014.
A larger sample plot was used for aging larger sugar maple saplings. Beginning at the first sampled target tree at one end of the long axis of the site, a transect line was drawn from the base to the next closest target tree, and continuing to the next closest target tree, and the next in turn until all target trees were similarly connected by transect lines. Along these successive transect lines, 5 m by 10 m plots were laid out using a meter tape with the corner of each plot placed at randomly chosen positions between 5 and 10 m along the line between pairs of target trees and with the 5 m side of the plot directly along the transect line. A coin toss decided which side of the line each plot would lay.
Within each 5 m by 10 m plot, all of the sugar maple saplings taller than 50 cm with a stem diameter of 6 cm or less (at 10 cm above ground), were harvested as stem sections cut at ground level and at 10 cm above the ground. The upper limit of 6 cm stem diameter was chosen so that the upper age limit would be about 70–80 y (based on preliminary sampling), therefore ensuring that the age distribution of samples would include an approximately equal range on either side of the 1984 mast year cohort (30 y old in 2014). A total of 35 of these larger plots were surveyed, with 149 saplings collected, labeled and brought back to the lab for aging.