Evidence that soil depth and clay content control the post-disturbance regeneration of balsam fir and paper birch under heavy browsing from deer

Abstract

It is not well understood why successful regeneration of balsam fir and paper birch was observed in the south-central portion of Anticosti Island, despite a high-density deer population reputed to severely browse the seedlings of these 2 species. The area where this regeneration occurred was severely affected by a hemlock looper epidemic in the early 1970s and was specifically confined to one geologic formation (Chicotte). We tested whether or not the occurrence of balsam fir and paper birch coincided with a certain range of soil properties (pH, exchangeable cations, soil depth, clay content, and forest floor thickness). Out of 49 plots surveyed, balsam fir and paper birch respectively occurred on 30 and 25 plots, while black or white spruce occurred on all plots. There was co-occurrence of balsam fir and paper birch in 15 plots in which the relative abundance of paper birch was low. Multivariate regression trees (MRT) indicated that balsam fir occurred on shallow soils, whereas paper birch occurred on deep soils. On shallow soils, MRT indicated better regeneration of balsam fir in soils with low clay content. Results suggest 2 concurrent mechanisms related to site fertility leading to the regeneration of balsam fir and paper birch. The first involves low fertility conditions that stimulate balsam fir to produce higher concentrations of anti-herbivore compounds. The second mechanism involves increased tolerance of birch saplings to repeated deer browsing on the deeper and more fertile soils. Future research should strive to confirm these mechanisms and understand why they were efficient on the Chicotte formation but not elsewhere on the island.