Joint influence of deer management and an invasive grass on tree seedling establishment

Background/Question/Methods

Protected areas typically face a variety of threats simultaneously. One challenge for resource managers is to understand how the interaction of these threats can influence the outcome of conservation actions.

Mid-Atlantic forests often face both a high deer population density and invasion by exotic plant species. Deer browse can a have a number of impacts on forest vegetation, including a reduction in tree recruitment. In particular, deer browse will kill seedlings, preventing new trees from becoming established. Invasive plants can also negatively impact native vegetation through mechanisms such as shading or allelopathy. This has the potential to compound problems with tree recruitment in areas with high deer density.

Catoctin Mountain Park, near Thurmont MD, has experienced a reduction in tree recruitment due to high deer density for many years. In response, the park began a deer culling program in the fall of 2009 which has reduced the deer population. However, during this time, Microstegium vimineum, an exotic grass, spread throughout much of the park.

Using data from a long-term, plot-based forest monitoring program we assessed the effectiveness of deer management to improve seedling establishment. We also assessed if areas of the park invaded by Microstegium responded differently. 

Results/Conclusions

Deer management has resulted in a significant increase in the number of seedlings. Prior to deer management (2006-2009) seedling density was ~625/ha, compared to ~4900/ha after management began (2010-2013). During the same time periods, however, cover of Microstegium increased from 11.8% of the park to 17.2% of the park. Based on a negative binomial mixed model, Microstegium cover was a significant positive predictor of an increase in seedling density in a plot (p<0.0001). It is unclear if Microstegium cover provides a benefit to seedlings, or if they are both responding to other factors such as soil quality or microclimate. Regardless of the mechanism involved, the spread of Microstegium did not prevent seedling establishment after deer culling began.