Invasive species trends in eastern national park forests

Seirup, Camilla; Miller, Kate; Weed, Aaron

From the abstract: "The National Park Service Inventory and Monitoring Program (NPS I&M) monitors forest health in a network of permanent plots in 39 eastern national parks. The parks included in this study vary widely in size, land use history, and designation, but all are impacted by invasive exotic plant species and the threat of further expansion and new invasion. Here we present preliminary findings regarding trends in invasive plants in eastern national parks over the last 12 years of monitoring. We used mixed effects models to determine how invasive plant abundance is changing over time. We modeled trends in each park individually, and assessed multiple scales and metrics of invasion, including proportion of plots invaded, frequency of 1m2 quadrats invaded, and average percent cover of invasives. We assessed trends in invasive plants overall, by species, and by the following guilds: trees, shrubs/vines, graminoids, and herbs. Parks vary considerably in their degree of invasion: Acadia National Park is the least invaded park (4.6% of plots invaded) while the most invaded parks are in the mid-Atlantic states (9 parks with 100% plots invaded). Increasing trends in invasive species abundance far outnumbered decreasing trends, and only two parks in our study showed an overall decline in invasive plant abundance over time: Marsh-Billings-Rockefeller National Historic Park (VT), and Prince William Forest Park (VA). Where detected, significant declines of invasive herbaceous or graminoid species tended to coincide with roughly equivalent increases in invasive shrub abundance. Japanese stiltgrass (Microstegium vimineum) emerged as the most aggressive invasive species and is a high management priority, particularly in northern parks where its range is still limited. Invasive shrubs also increased over time in many parks across the region and should be a high management priority because of their impacts to natural forest processes, biodiversity, and because their presence may be linked to increased risk from tick borne diseases. Invasive plants are a growing challenge for many parks in the northeast and this study will help parks prioritize species for management. Our findings suggest that natural resources in parks suffer from deferred management, particularly of invasive species, just as park infrastructure suffers from deferred maintenance. Reversing the widespread increase of invasive plants we have documented will require a long-term commitment from the NPS similar to the commitment being made to address the costs of deferred maintenance."