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Northern Research Station
11 Campus Blvd., Suite 200
Newtown Square, PA 19073
(610) 557-4017
(610) 557-4132 TTY/TDD

Sustaining Forests

Native Invasive Species

Research Issue

[photo:] Deer overbrowsing has dramatically reduced the plant diversity of eastern US forests. Hay-scented fern (Dennstaedtia punctilobula) dominates the understory of this forest in Pennsylvania, which would make it difficult for many plant species to recover even if deer populations were eventually brought under control.Recent changes in disturbance and browsing regimes have strongly impacted species composition in forest understories worldwide.  In some cases, these changes have led to large increases in the density and cover of a small number of native understory plant species which may then expand to form interfering vegetation layers.  No matter where they occur, these interfering plant layers share a number of attributes:  1) The understory layer is often more dense with greater vegetation cover and lower diversity than was common prior to their expansion.  2)  This layer may alter or suppress tree regeneration by creating conditions in the understory near ground level that are inimical to seeds and seedlings.  3)  Once formed, this layer is resistant to displacement by other plant species.

Our Research

Our research over the past 40 years has centered on understanding the causes, consequences, and control of such interfering plant layers.  Through long-term vegetation records and experimental approaches, we elucidated that many of these layers in the Allegheny Plateau region become established following overstory mortality and protracted deer overbrowsing.  Additional experimental work on a common interfering species, hay-scented fern (Dennstaedtia punctilobula) has found both light limitation and an enhanced seed/seedling predation regimes beneath its canopy are the primary mechanisms of interference.  Current work is expanding the scope of interfering species to additional species including New York Fern, Beech brush, Striped Maple, and Pin Cherry.  We have also taken the lead on developing treatment prescriptions for the control of interfering layers.  The herbicide prescription developed by our unit has become the standard throughout Pennsylvania, New York, and elsewhere in the Northeast.  Our research continually evolves as other species become problematic under the shifting environmental context and new herbicide formulations become available.  Finally, we have research underway to determine if competition from a common, early-successional understory species (Rubus spp.) can provide a viable alternative to the current herbicide prescription for the control of interfering fern layers. 

Expected Outcomes

Results from our herbicide work will continue to inform private, state, and federal forest resource management agencies regarding the efficacy of specific herbicide mix in achieving the desired control over the suite of interfering plant species.  In tandem with this work, we will continue to evaluate the impacts of such herbicide treatments on non-target species, including herbaceous plants, birds, and amphibians.  Finally, assess the potential and viability of plant-plant competition as an alternative to herbicides.   

Research Results

Over 50 publications on the causes, consequences, and control of interfering plant species (Selected list below)

Horsley, S. B. 1991. Using Roundup and Oust to control interfering understories in Allegheny hardwood stands. Pages 281-290 in Proc. Central Hardwood For. Conf.. USDA For. Serv., University Park, PA.

Horsley, S. B. 1993. Mechanisms of Interference between Hay-Scented Fern and Black-Cherry. Canadian Journal of Forest Research 23:2059-2069. 

Ristau, T. E., and S. B. Horsley. 1999. Pin cherry effects on Allegheny hardwood stand development. Canadian Journal of Forest Research 29:73-84. 

Ristau, T. E., and S. B. Horsley. 2006. When is pin cherry (Prunus pensylvanica L.) a problem in Allegheny hardwoods? Northern Journal of Applied Forest Research 23:204-210.

Royo, A. A., and W. P. Carson. 2006. On the formation of dense understory layers in forests worldwide: consequences and implications for forest dynamics, biodiversity, and succession. Canadian Journal of Forest Research 36:1345-1362.

Research Participants

Principal Investigators

  • Alejandro A. Royo, USDA Forest Service- Northern Research Station Research Ecologist
  • Todd Ristau, USDA Forest Service- Northern Research Station Research Ecologist

Research Partners

  • Stephen B. Horsley, USDA Forest Service- Northern Research Station Scientist Emeritus
  • Robert White, USDA Forest Service – Eastern Region Forest Silviculturist, Allegheny National Forest

Last Modified: 01/27/2010