Divergent nonlinear responses of the boreal forest field layer along an experimental gradient of deer densities

Abstract

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.

Compositional and functional trajectories of herbaceous communities after deer density control in clear-cut boreal forests

“…Our study used the infrastructure of an experiment on deer density control established in 2001 95 on sites with both intact and clearcut forested areas (Tremblay et al 2006). As plant responses in the 96 forests were slow (Hidding et al 2012), we chose to focus on clearcut areas in the current study.…”

Independent Effects of Invasive Shrubs and Deer Herbivory on Plant Community Dynamics

“…While several studies reported deer herbivory did not affect total herbaceous cover [8][9][10][11], these studies did not separately examine cover of forbs, graminoids, and ferns. As deer density and resulting herbivory intensity increases, the relative dominance of graminoids and ferns also increases [12][13][14] while decreasing the relative dominance of forbs [15,16]. Deer herbivory can accelerate the proliferation of invasive species, especially in conjunction with canopy disturbances that increase light to understory plants [17].…”

Long‐term biological legacies of herbivore density in a landscape‐scale experiment: forest understoreys reflect past deer density treatments for at least 20 years

“…Specifically, higher densities of deer caused forests to be increasingly dominated by unpalatable tree species and understoreys to be increasingly dominated by ferns and grasses. In a similar, though shorter-duration, experiment in boreal forests of Quebec, Canada, Tremblay, Huot & Potvin (2006) recommended managing deer at densities < 7.5-15 deer km À2 for the conservation of browse-sensitive plants. While these studies come to similar conclusions about deer density thresholds, once these thresholds have been crossed, can forest communities recover?…”

“…While these studies come to similar conclusions about deer density thresholds, once these thresholds have been crossed, can forest communities recover? Based in part on guidance from these studies (Tilghman 1989;Horsley, Stout & deCalesta 2003;Tremblay, Huot & Potvin 2006, 2007, in some areas, deer are beginning to be managed at more sustainable levels (see e.g. Royo et al 2010;Tanentzap et al 2011).…”