Assessment of Prescribed Burning Effects in Paludified Black Spruce Forests in Ontario’s Clay Belt Region

Paludification, the accumulation of poorly decomposed organic matter principally originating from Sphagnum, transforms black spruce forests to forested peatlands in the prolonged absence of fire. High-severity wildfires reverse this process by burning the organic matter layer and thus restart forest succession; in contrast low severity wildfires remove only the tree layer and do not reduce paludification. On the Ontario Clay Belt, a physiogeographic region prone to paludification due to its cold climate and poor drainage, current forest harvest practices (Careful Logging Around Advanced Growth; CLAAG) mimic low severity fires by removing trees but lacking forest floor and soil disturbances caused by fire. Historically, prescribed burning after clear cut (PB) was used as a site preparation technique, and may also be used for controlling paludification as it burns part of the organic soil layer and enhances soil fertility by releasing nutrients. Our retrospective study examines three hypotheses; compared to CLAAG and clear cut, 1) PB has positive effects on soil conditions, 2) controls sphagnum colonisation and 3) results in better growth of black spruce. We sampled 24 sites, using ecological forest classification and harvest records to ensure site equivalency. Results show a significant positive effect of PB on soil decomposition, nutrient contents and pH values. PB reduces Sphagnum establishment and growth. PB significantly increases black spruce growth (terminal shoots), however no difference in mean tree height is observed among treatments. We put forth that these contradictory results could be explained by a time lag in natural regeneration after PB as suggested by forest vertical structure. While not a high severity fire, prescribed burning after clear cut in paludified stands on the Ontario Clay Belt emulates some of its effects. Therefore we conclude that unlike CLAAG, prescribed burning after clear cut facilitates the development of unpaludified stands in a managed landscape