Precommercial thinning of trembling aspen in northern Ontario: Part 2 – Interactions with Hypoxylon canker

Hypoxylon canker [Entoleuca mammata (Wahlenberg:Fr) J.D. Rogers and Y.-M. Ju] incidence and mortality were monitored in six northern Ontario trembling aspen (Populus tremuloides Michx.) stands following precommercial thinning. At each of the six sites, a randomized complete block design experiment was established with four replicates of five thinning levels (none, 2-, 3-, 4-, and 5-m spacing). At the time of thinning, three stands were five years old; the remaining were ages 10, 15, and 22. Over the 15- to 17-year observation period, Hypoxylon-related mortality increased to 6–9% by the end of the period, regardless of thinning age or density. Hypoxylon infection incidence (excluding mortality) also increased, to 2–6% in unthinned stands and 8–11% in thinned stands by year 15. No differences among the thinning levels were observed. Prevalence, infection expressed as a percentage of surviving trees, did not differ among thinned and unthinned stands. Mortality attributed to factors other than Hypoxylon was 31–44% in unthinned stands and 7–23% in thinned stands, with no significant differences among thinning levels. Trees dying of other causes were typically small in diameter and of more subordinate crown classes than survivors. In contrast, Hypoxylon-caused mortality was independent of tree size and canopy position. Results suggest that log size and stand yield may be manipulated through density regulation, without concern for interacting impacts associated with Hypoxylon canker. Models are provided for estimating disease losses; predictions can then be factored into the crop-planning process. Key words: trembling aspen, precommercial thinning, Hypoxylon canker, growth and yield, crop planning