The fact that herbivores and predators exert top-down effects to alter community composition and dynamics at lower trophic levels is no longer controversial, yet we still lack evidence of the full nature, extent, and longer-term effects of these impacts. Here, we use results from a set of replicated experiments on the local impacts of white-tailed deer to evaluate the extent to which such impacts could account for half-century shifts in forest plant communities across the upper Midwest, USA. We measured species' responses to deer at four sites using 10–20 year-old deer exclosures. Among common species, eight were more abundant outside the exclosures, seven were commoner inside, and 16 had similar abundances in- and outside. Deer herbivory greatly increased the abundance of ferns and graminoids and doubled the abundance of exotic plants. In contrast, deer greatly reduced tree regeneration, shrub cover (100–200 fold in two species), plant height, plant reproduction, and the abundance of forbs. None of 36 focal species increased in reproduction or grew taller in the presence of deer, contrary to expectations. We compared these results to data on 50-year regional shifts in species abundances across 62 sites. The effects of herbivory by white-tailed deer accurately account for many of the long-term regional shifts observed in species' abundances (R2 = 0.41). These results support the conjecture that deer impacts have driven many of the regional shifts in forest understory cover and composition observed in recent decades. Our ability to link results from shorter-term, local experiments to regional long-term studies of ecological change strengthens the inferences we can draw from both approaches.

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Summary In forests of eastern North America, white‐tailed deer (Odocoileus virginianus) can directly affect, via herbivory, the presence, abundance and reproductive success of many plant species. In addition, deer indirectly influence understorey communities by altering environmental conditions. To examine how deer indirectly influence understorey plants via environmental modification, we sampled vegetation and environmental variables in‐ and outside deer exclosures (10–20 years old) located in temperate forests in northern Wisconsin and the Upper Peninsula of Michigan, USA. We assessed how excluding deer affected understorey community composition and structure, the soil and light environment, and relationships between direct and indirect effects, using non‐metric multidimensional scaling (NMDS), mixed linear models and nonparametric multiplicative regression (NPMR). Excluding deer altered sapling communities and several aspects of the understorey environment. Excluding deer from plots with lower overstory basal area increased sapling abundance, decreasing the amount of light available to groundlayer plants. Exclusion also reduced soil compaction and the thickness of the soil E horizon. The composition of understorey communities covaried in apparent response to the environmental factors affected by exclusion. In several common species and groups, E horizon thickness, compaction, openness, and/or total (sapling and overstory) basal area were significant predictors of plant frequency. Complementary analyses revealed that deer exclusion also altered the frequency distributions of several species and groups across environmental space. Synthesis. Deer alter many facets of the understorey environment, such as light availability, soil compaction and thickness of the soil E horizon, which, in turn, appear to mediate variation in plant communities. Those environmental modifications likely compound direct impacts of herbivory as drivers of understorey community change. Our results provide evidence that deer effects on the environment have important implications for forest composition. Thus, we suggest a re‐examination of the common assumption that understorey community shifts stem primarily from tissue removal.

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Abstract, the fourth and fifth sentences should read: "Among common species, 12 were more abundant outside the exclosures, 14 were commoner inside, and 66 had similar abundances in-and outside. Deer herbivory greatly increased the abundance of graminoids and nearly doubled the abundance of exotic plants." The ninth sentence should read: "The effects of herbivory by white-tailed deer account for many of the long-term regional shifts observed in species' abundances (R 2 = 0.25)."In the "Data on 50-year shifts in abundance" section, the first three sentences should read: "To test whether the differences in species abundance that we observed in-vs. outside the exclosures were related to the long-term (1950s to 2001) regional shifts in abundance measured by Wiegmann and Waller [33] over 62 sites, we first calculated the change in abundance across the fence line as the arithmetic difference of each taxon's frequency in the control browsed plots minus its frequency inside the exclosure plots. We then calculated each taxon's change in abundance over the last 50 years as the arithmetic difference between its frequency in 2000 minus its frequency in the 1950s. We excluded rare species that occurred less than 5 times in both data sets, leaving 53 species for the analysis."In the "Data analyses" section, the last sentence of the first paragraph reads: "We restricted these tests to species that occurred in at least 50 quadrats overall and had expected frequencies greater than five in each treatment." Because of the lower abundances in the corrected dataset, the authors no longer put the 50 quadrat restriction on the analysis and the text should now read: "We restricted these tests to species that had expected frequencies of greater than five in each treatment."Because the authors no longer use the 50 quadrat restriction, the first sentence of the "Species' responses to deer" section should read: "Of the 256 species encountered in the exclosure and browsed plots, 97 occurred commonly enough (expected value of 5) to analyze differences in abundance in-and outside the fences using χ 2 tests."Some of the results have therefore changed, and the remainder of the "Species' responses to deer" section should read: "Twelve species were more abundant in browsed plots while fourteen were more abundant in exclosures ( Table 2). The other 66 species did not differ in abundance between treatments. Species that increased in the presence of deer included three native forbs (Coptis trifolia: 4.5x, Rubus pubescens: 4x, and Symphyotrichum lateriflorum: 2x), two non-native forbs (Hieracium aurantiacum: 4.5x and Lapsana communis: 2x), a fern (Pteridium aquilinum: 7x), two native (Schizachne purpurascens: 2.8x and Carex pensylvanica: 2.5x) and one non-native graminoids (Poa compresa: 5.5x), and three woody species (Ostrya virginiana:PLOS ONE | https://doi.org/10.1371/journal.pone