Growth and form responses to a range of pre-commercial thinning densities in an aerially seeded jack pine stand in northwestern Ontario: 35-year results

We established a pre-commercial thinning trial in 1986 in a 9-year-old, aerially-seeded jack pine stand. Replicated thinning intensities (i.e., no thin control, 1m, 1.5m, 2m, 2.5m, 3m spacings) were randomly assigned within four designated blocks. Since establishment, periodic re-measurements of permanent plots have been conducted that span 35 years since the establishment of the trial. The temporal pattern of self-thinning varied significantly across the treatments, however 35 years after the treatments were applied, tree densities had largely converged ranging from 1170 to 1930 stems ha-1, but the mean tree size and Dbh distributions were significantly different across treatments. This shift in average tree size, however, did come with a trade-off in terms of stem form and quality class, with trees in the wider spacings having larger live crowns, greater stem taper, and more stem crooks and forked tops. In terms of other ecosystem services, thinning resulted in lower standing dead and downed woody debris volumes, but had increased ground lichen cover. Based on our results, the moderate (1.5 – 2m) PCT level in aerially-seeded jack pine stands on dry, outwash sandy sites appears to be a good compromise between tree growth potential and the provision of other non-merchantable ecosystem attributes.