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Tree species composition in forests can be strongly modulated by high densities of cervid herbivores ultimately leading to local extirpation of species. To establish which cervid densities are compatible with the recruitment of a browse sensitive tree species, seedlings and saplings should be surveyed under variable cervid densities rather than in their presence or absence alone. We studied the growth and survival of different demographic stages of balsam fir (Abies balsamea) on Anticosti island (Québec, Canada) under controlled densities of white-tailed deer (Odocoileus virginianus). In a seven-year experiment using deer enclosures, we followed the life stage and fate of individually tagged balsam fir seedlings/saplings under forest cover and forest logged at the start of the experiment. Almost no regeneration into the sapling stage (>30 cm) was observed under ambient deer densities after 7 years and decreased survival and growth were observed under an experimental deer density of 15 km2 . However, mortality at 615 deer km2 decreased over time and with age and stem height, converging towards mortality observed at 0 deer km2 . Given the relatively high stem density of saplings at 15 deer km2 7 years after the start of the treatment, our data indicate that at this density considerable balsam fir regeneration may occur, although the ultimate contribution of balsam fir to the canopy remains uncertain. The notion that small seedlings are most vulnerable to deer browsing and that balsam fir recruitment rapidly decreases after logging suggests that maintaining low deer densities is most crucial immediately after a stand-initiating disturbance (e.g. logging).

Macrophytes play a keystone role in shallow aquatic ecosystems. In lakes, macrophytes stabilize clear-water conditions with high biodiversity and their decline can cause a shift to a turbid state with lower biodiversity. Various mechanisms have been suggested as triggers of macrophyte collapse. Herbivory by waterfowl and fish seems to be one of the obvious factors, but the response of macrophytes to herbivory is ambiguous. We hypothesized that herbivory alone does not typically cause macrophyte collapse, but that shading from periphyton can enhance the effect of herbivores. Shading of macrophytes is supposed to increase with eutrophication due to changes in the top–down control cascading from fish via macroinvertebrates to periphyton. We elaborated on this idea by fitting a macrophyte growth model with different herbivore grazing and periphyton shading scenarios. In addition, we performed a meta-analysis on existing experimental herbivore exclosure studies with respect to periphyton growth. The model supported our proposed hypothesis and the reviewed field studies appeared to point in the same direction. We suggest that a significant herbivore impact may indicate a reduced resilience of vegetation to eutrophication, making it an early warning signal for an imminent macrophyte collapse leading to a sudden shift of the system to turbid conditions.

Tree species composition in forests can be strongly modulated by high densities of cervid herbivores ultimately leading to local extirpation of species. To establish which cervid densities are compatible with the recruitment of a browse sensitive tree species, seedlings and saplings should be surveyed under variable cervid densities rather than in their presence or absence alone. We studied the growth and survival of different demographic stages of balsam fir (Abies balsamea) on Anticosti island (Québec, Canada) under controlled densities of white-tailed deer (Odocoileus virginianus). In a seven-year experiment using deer enclosures, we followed the life stage and fate of individually tagged balsam fir seedlings/saplings under forest cover and forest logged at the start of the experiment. Almost no regeneration into the sapling stage (>30 cm) was observed under ambient deer densities after 7 years and decreased survival and growth were observed under an experimental deer density of 15 km2 . However, mortality at 615 deer km2 decreased over time and with age and stem height, converging towards mortality observed at 0 deer km2 . Given the relatively high stem density of saplings at 15 deer km2 7 years after the start of the treatment, our data indicate that at this density considerable balsam fir regeneration may occur, although the ultimate contribution of balsam fir to the canopy remains uncertain. The notion that small seedlings are most vulnerable to deer browsing and that balsam fir recruitment rapidly decreases after logging suggests that maintaining low deer densities is most crucial immediately after a stand-initiating disturbance (e.g. logging).