Ungulate impacts on herbaceous-layer plant communities in even-aged and uneven-aged managed forests

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© 2016 Murray et al. Forest management and ungulate herbivory are extant drivers of herbaceous-layer community composition and diversity. We conducted a white-tailed deer (Odocoileus virginianus) exclosure experiment across a managed landscape to determine how deer impacts interact with the type of forest management system in influencing herb-layer (all vascular plants < 0.5 m tall) species richness and composition. Our study took place 3 yr after harvest in a deciduous forest landscape being managed through even-aged (∼4.1 ha openings) and uneven-aged (∼1.4 ha openings) silvicultural systems. We expected the severity of deer impacts on herb layer species richness and composition to vary according to opening position, opening size, and the spatial scale of inference. At forest stand and landscape scales, species richness within silvicultural openings was greater outside compared to inside deer exclosures, and did not differ according to deer access in edges or the forest matrix. However, greater levels of species richness associated with deer access were driven by infrequently occurring forbs, and overall species composition did not differ. Notably, these species were not exotics or ferns. Deer reduced the density of large saplings and blackberry (Rubus spp.) shrubs in the smaller openings characteristic of uneven-aged management stands, but had no effect on sapling density in the larger openings characteristic of even-aged management stands. This result extends the forage maturation hypothesis to silvicultural systems, and is consistent with predictions that plant tolerance and avoidance of herbivory increase with resource availability. Deer may have facilitated the establishment of forbs in recently created silvicultural openings by temporarily slowing sapling regeneration, creating establishment sites through physical disturbance, and seed dispersal via epizoochory and endozoochory. This outcome is contingent upon declining deer visitation rates as woody vegetation matures as well as distance from source populations of exotic species. We conclude that ecological context, such as local ungulate abundance, disturbance, and landscape factors, influence how ungulates interact with forest management systems.

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