Summary1. The density of large herbivores is a major driver of forest ecosystem structure and function in conjunction with episodic disturbances, especially in forests with a regeneration strategy based on shade-tolerant seedlings capable of re-establishing canopy dominance (advance regeneration). Yet, uncertainty about the relationships between forest regeneration, herbivore density and other disturbances makes it difficult to set population goals. Using an innovative controlled browsing experiment, we investigated the relationships between the regeneration dynamics of balsam fir Abies balsamea , the density of white-tailed deer Odocoileus virginianus and timber harvesting. 2. We hypothesize that advance tree regeneration either: (i) recovers approximately linearly as deer density is reduced; (ii) recovers exponentially; or (iii) does not recover because factors other than browsing control advance regeneration. We tested these alternatives through manipulation of deer densities (0, 7·5, 15 deer km − 2 and in situ local densities) and forest cover (clearcut and uncut forest). 3. Balsam fir seedling mortality decreased exponentially with decreasing deer density in clearcut and approximately linearly in uncut forest. Independently of deer density, the recruitment of seedlings in clearcut dropped from 56 ± 5% to 7 ± 1% within 3 years. 4. Seedling growth increased exponentially with decreasing deer density in clearcut whereas no height growth was observed in uncut forest. 5. Overall, the abundance of fir saplings recovered exponentially in clearcut but remained low and independent of deer density in uncut forest. The abundance of spruce Picea spp. saplings was unrelated to deer density and increased with time. 6. Synthesis and applications . Forest disturbance from selective browsing at high deer densities over an extended period of time leads to recruitment failure following a canopy disturbance such as a clearcut. Indirect competitive advantage given to species resistant to browsing can shift forest composition. Nonlinear relationships between fir regeneration and deer densities imply that the level of culling required to reach herbivore densities compatible with natural regeneration of native forest is larger than expected if tree regeneration was proportional to deer density. In the boreal forest of Anticosti Island, local densities < 15 deer km − 2 achieved within 3 years following clearcut are compatible with the maintenance of native forest.

The early responses of the field layer to changes in biotic and abiotic conditions are key determinants of the future composition and structure of forests where sustained heavy browsing pressure has depauperated the shrub understory. We investigated the relationships between white-tailed deer density and field layer plant community dynamics in boreal forests managed for wildlife and timber production. We hypothesized that the growth and reproduction of field layer plants are either: (H(1)) directly proportional to deer density, or (H(2)) related to deer density through nonlinear relationships or (H(3)) through nonlinear relationships with thresholds. We tested these hypotheses using data from a controlled browsing experiment involving a gradient of deer densities (0, 7.5, 15, 27 and 56 deer km(-2)) in interaction with timber harvesting conducted on Anticosti Island, Canada. In recent clearcuts, the dominant responses of the field layer plants were exponential recovery in growth and reproduction with decreasing deer densities. The abundance of browse-tolerant species such as grasses was positively related to deer density, suggesting an apparent competitive gain. These results support the prediction from our second hypothesis, although the presence of ecological thresholds should not be ruled out. Rapid changes in the early successional stages have potentially long-term consequences on successional patterns through processes such as the modulation of germination and early establishment success of seedlings from later successional species. Quantitative data as those presented here are essential for the development of ecosystem management prescriptions. On Anticosti Island, reduction of local deer densities to levels <15-7.5 deer km(-2) in the first 3 years following timber harvesting appears to be compatible with the regeneration dynamics of this system although lower levels of deer densities may be required for the conservation of browse-sensitive plant species.