Deer and invasive plants in suburban forests: assessing variation in deer pressure and herbivory

Fragmented suburban forests of the northeastern US are challenged by abundant white-tailed deer and nonindigenous plant invasions. Deer browsing/grazing pressure varies among sites, potentially affecting herbivory on nonindigenous plants and their invasion success. We aimed to identify a useful deer pressure indicator for suburban forests and then use it to relate deer pressure to grazing on and abundance of two herbaceous invaders, <i>Microstegum vimineum</i> and <i>Alliaria petiolata</i>. We compared three indicators: fecal pellet accumulation rate, deer browse on indigenous woody plants, and indigenous shrub layer cover. The pellet method produced estimates generally far below the region’s known deer density. Browse rates and shrub layer cover were negatively correlated, and correlations of the three indicators with evidence of deer pressure from a subsequent 6.5-year exclosure experiment supported shrub layer cover as the better choice. Using that measure in 10 forests, we detected a weak pattern of more grazed stands under greater deer pressure, but few plants per stand were grazed; any negative influence of deer on these species was limited to individuals, without population effects. <i>Alliaria petiolata</i> abundance was unrelated to deer pressure, but <i>M. vimineum</i> abundance was greater in forests with more deer pressure, suggesting facilitation of its invasion.