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.