Long‐term trends indicate that invasive plants are pervasive and increasing in eastern national parks

Abstract

While invasive plant distributions are relatively well known in the eastern United States, temporal changes in species distributions and interactions among species have received little attention. Managers are therefore left to make management decisions without knowing which species pose the greatest threats based on their ability to spread, persist and outcompete other invasive species. To fill this gap, we used the U.S. National Park Service’s Inventory and Monitoring Program data collected from over 1,400 permanent forest plots spanning 12 yr and covering 39 eastern national parks to analyze invasive plant trends. We analyzed trends in abundance at multiple scales, including plot frequency, quadrat frequency, and average quadrat cover. We examined trends overall, by functional group, and by species. We detected considerably more increasing than decreasing trends in invasive plant abundance. In fact, 80% of the parks in our study had at least one significant increasing trend in invasive abundance over time. Where detected, significant negative trends tended to be herbaceous or graminoid species. However, these declines were often countered by roughly equivalent increases in invasive shrubs over the same time period, and we only detected overall declines in invasive abundance in two parks in our study. Present in over 30% of plots and responsible for the steepest and greatest number of significant increases, Japanese stiltgrass (Microstegium vimineum) was the most aggressive invader in our study and is a high management priority. Invasive shrubs, especially Japanese barberry (Berberis thunbergii), Japanese honeysuckle (Lonicera japonica), multiflora rose (Rosa multiflora), and wineberry (Rubus phoenicolasius), also increased across multiple parks, and sometimes at the expense of Japanese stiltgrass. Given the added risks to human health from tick‐borne diseases, invasive shrubs are a high management priority. While these findings provide critical information to managers for species prioritization, they also demonstrate the incredible management challenge that invasive plants pose in protected areas, particularly since we documented few overall declines in invasive abundance. As parks work to overcome deferred maintenance of infrastructure, our findings suggest that deferred management of natural resources, particularly invasive species, requires similar attention and long‐term commitment to reverse these widespread increasing invasive trends.

Tree biodiversity in northern forests shows temporal stability over 35 years at different scales, levels and dimensions

“…For example, there is often no net change on average in local scale species richness in areas that have not undergone substantial land-use change (Dornelas et al, 2014;Vellend et al, 2013). However, when natural vegetation is converted to agriculture, some areas experience declines in species richness (Newbold et al, 2015), while other places experience increases in species richness when non-native species colonizations exceed native species extinctions (Miller et al, 2021;Sax & Gaines, 2003). Disturbances such as forest loss can elevate rates of biodiversity change, but this is true for both losses and gains (Daskalova et al, 2020).…”

Overabundant deer and invasive plants drive widespread regeneration debt in eastern United States national parks

“…Many of the parks experiencing the most severe impacts of chronic deer browse also have the highest and increasing invasive plant loads (Fisichelli & Miller, 2018; Miller et al, 2021). In fact, this study consistently found invasive plant cover to be an important and negatively associated predictor of regeneration abundance, particularly for sapling density and the stocking index.…”

“…Where overabundant deer and invasive shrubs overlap, canopy gaps often result in conversion to invasive shrub thickets that suppress tree regeneration. In MORR, for example, gaps formed from ash dieback and windthrow events have converted to invasive shrub thickets (Miller et al, 2021). Silvicultural practices that create canopy gaps intended to increase forest resilience by promoting diverse and abundant regeneration must therefore be applied with extreme caution in forests with high deer densities and abundant invasive plants (Webster et al, 2018).…”

“…Promoting resilience through canopy gaps is therefore only recommended for parks, or stands within parks, that have low deer and invasive abundance or the capacity to intensively manage invasives in gaps. Parks like BLUE and PRWI, which have low invasive cover (Miller et al, 2021), are classified as insecure regeneration debt, and are dominated by oak–hickory forest, may be best suited for increasing resilience through thinning subcanopy trees (e.g., maples) to increase light to the forest floor (Iverson et al, 2008). However, potential stands in these parks should be surveyed and treated for invasive plants prior to any thinning.…”

Invasive Plants Differentially Impact Forest Invertebrates, Providing Taxon-Specific Benefits by Enhancing Structural Complexity

“…As such, efforts to control invasive plant species must be planned and implemented with consideration of the direct and subsequent impacts of invasion (D'Antonio and Meyerson, 2002;Barney et al, 2013). The distribution of both M. vimineum and A. petiolata is rapidly expanding across national parks in the eastern US (Miller et al, 2020). Due to this spread and potential impacts to native vegetation, control of these species, particularly M. vimineum, is noted as a management priority.…”