by Amina Wilson and Chris Nadeau
Invasive shrubs are non-native species that have a significant impact on native vegetation. Forests in 70% of eastern national parks are at imminent risk of failure, in part because invasive shrubs are preventing regeneration of native plants. Biologists with Acadia National Park work to prevent the spread of invasive shrubs, helping to maintain and strengthen the health of Acadia’s forests and other ecosystems.
Currently, glossy buckthorn (Frangula alnus) is the most problematic invasive shrub found in Acadia, requiring a large investment of time and resources from the park’s invasive plant management team. Since glossy buckthorn was introduced to the United States from Eurasia as an ornamental shrub in the mid 1800s, this hardy plant has become severely invasive across the Northeast. Glossy buckthorn easily sprouts in wetlands and young forests, reducing habitat for native shrubs and insects.
In partnership with NPS and Friends of Acadia, Schoodic Institute scientists are evaluating the effectiveness of current glossy buckthorn management approaches and testing alternative management methods through five ongoing experiments (described below).
Although monitoring is in the early stages, planting shrubs after buckthorn removal and critical period cutting to manage mature buckthorn show promising results (see below). Scientists will continue monitoring all five experiments through at least 2026 to expand the data sets and refine the results.
How effective are current glossy buckthorn management practices?
Why
Managing glossy buckthorn in just two locations in the park, Great Meadow and Bass Harbor Marsh, accounted for roughly one third of Acadia’s invasive plant management efforts in 2024. Park staff want to know how effective treatment methods are, and whether native plants recover after glossy buckthorn removal.
How
In 2021, we began monitoring vegetation at six locations in Bass Harbor Marsh where NPS is managing glossy buckthorn. We record percent cover of glossy buckthorn and other plants along 6 transects multiple times per year.
2024 Key Results
Despite three years of repeated herbicide treatments, 63% of sites still had glossy buckthorn present prior to treatments in 2024. The continued return of glossy buckthorn suggests additional management approaches are needed.
Future Work
Future analysis will focus on the recovery of native plants in management sites.
Will restoring native plants suppress glossy buckthorn?
Why
Restoring native species after buckthorn removal could prevent re-establishment of glossy buckthorn, but we don’t know which species to plant or how effective planting will be.
How
We are testing different native plant restoration strategies (restore shrubs vs. grasses), and evaluating if soil disturbance from restoration promotes buckthorn growth. We have established 10 restoration plots in Bass Harbor Marsh and 11 plots in Great Meadow.
2024 Key Results
- A total of 1,512 native plants have been restored since 2023 with an 89% survival rate.
- Preliminary results vary by location. In Bass Harbor Marsh, planting shrubs has reduced glossy buckthorn regeneration by 41%. In Great Meadow, planting shrubs reduced glossy buckthorn seed germination by 57% and seedling survival by 25%.
- Disturbing the soil to plant native plants has not increased glossy buckthorn regeneration as feared.
- Restoration of grasses, sedges, and rushes have not shown a significant impact on glossy buckthorn.
Future Work
Monitoring of restoration plots will continue through 2026 and results will help inform supplemental management practices. Multiple years of data are needed to confirm preliminary results.
Can we manage mature glossy buckthorn without herbicides?
Why
Critical period cutting is a proven method for controlling a related invasive shrub, common buckthorn (Rhamnus cathartica), without pesticides, by cutting the plant’s main stem and then returning periodically to remove any regrowth.
How
We are testing this same method on glossy buckthorn and evaluating how often regrowth needs to be removed to kill the plant.
2024 Key Results
Preliminary results show little mortality but big differences in the amount of regrowth based on cutting — more frequent cutting led to less regrowth.
Future Work
Monitoring will continue in 2025 to understand the long-term efficacy of critical period cutting.
Does glossy buckthorn have leaves for more of the growing season than native shrubs?
Why?
Many invasive shrubs grow leaves earlier in the spring and/or drop leaves later in the fall than native species, which could explain their success.
How?
We are evaluating if this pattern is true for glossy buckthorn in Acadia to incorporate the information into management.
2024 Key Results
- In the spring of 2024, glossy buckthorn put its leaves on 8-14 days later than most native species. Only common winterberry (Ilex verticillata) had a similar leaf-on date to glossy buckthorn.
- In the fall of 2024, glossy buckthorn lost its leaves 10-27 days later than most native species. Only highbush blueberry (Vaccinium corymbosum) had a similar leaf-off date to glossy buckthorn.
- The late fall leaf drop could partially explain glossy buckthorn’s success in Acadia.
Future Work
Monitoring will continue in 2025 to refine each species’ phenological timeline.
If we plant warm-climate shrubs, will their shade affect glossy buckthorn?
Why
Planting shrubs from southern areas that drop their leaves later in the fall than local species could help reduce re-establishment of glossy buckthorn by increasing competition for light late in the season. However, planting southern shrubs has risks (e.g., disease spread).
How
We are simulating planting southern shrubs using shade cloth to understand its effects on glossy buckthorn.
2024 Key Results
It is too early to see results yet, but monitoring will continue in the coming years.
Future Work
Growth of the buckthorn seedlings will continue to be monitored for any differences between shading lengths.
Do transplanted shrubs from south of Acadia drop leaves late in the fall like glossy buckthorn?
Why
Shrub species found more commonly south of Acadia are adapted to a longer growing season and therefore drop their leaves later in the fall. If those plants maintain a late-season leaf drop when transplanted to Acadia, then they could be good candidates for restoration planting.
How
We are working with neighboring residents to monitor the leaf-on and off dates for coastal sweet pepperbush, which is a species often planted in local gardens, but its native range is south of Acadia.
2024 Key Results
Unexpectedly, in the fall of 2024, coastal sweet pepperbush dropped its leaves 27 days earlier than glossy buckthorn and earlier than many other native plants.
Future Work
The fall of 2024 was very dry and could have influenced our results. We will continue to work with local residents in 2025 to monitor leaf-on and leaf-off dates of coastal sweet pepperbush