Data from: Deer-mediated changes in environment compound the direct impacts of herbivory on understory plant communities

1. 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 understory communities by altering environmental conditions.
2. To examine how deer indirectly influence understory 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 understory 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 non-parametric multiplicative regression (NPMR).
3. Excluding deer altered sapling communities and several aspects of the understory 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.
4. The composition of understory communities and the frequencies of various species and groups 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.
5. 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 understory environment, such as light availability, soil compaction and depth 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 understory community change, having important implications for forest composition. Thus, we suggest a reexamination of the common assumption that understory community shifts stem primarily from tissue removal.