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 occupancy
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.