Do Wolves Protect Forests? Investigating the Link Between Wolf Density, Deer Browse, and Plant Recovery

Large ungulate populations can threaten forest regeneration and many rare or declining understory plants, birds, and small mammals. Reintroduction of large predators is often proposed as a remedy to reduce negative ecosystem effects associated with high ungulate populations, but we know little about the effectiveness of this approach. We assessed whether wolves (Canis lupus) can protect forest understory plants from excessive white-tailed deer (Odocoileus virginianus) browse. We planted white oak (Quercus alba), red oak (Quercus rubra), and zigzag goldenrod (Solidago flexicaulis) seedlings across a gradient of wolf density and residence time in northern Wisconsin and recorded deer browse and frequency of 11 common understory plants at each site. We found that wolf density and residence time had negative effects on deer browse intensity, but these effects were generally weak except when understory vegetation was abundant. Additionally, the presence of common understory plants decreased as a function of wolf density, opposite to what we would expect for a wolf-driven trophic cascade. The weak reduction in browse probability that is associated with wolves, particularly when vegetation is scarce, is unlikely to improve forest regeneration and recovery of understory plant communities currently threatened by high deer populations.