Research project on the invasion patterns and impacts of invasive plants in New England riparian forests is about to kick off!

By Chad Hammer

            As the morning sky is filled with the elegant song of our feathered friends and the buds on the tips of trees are starting to explode, I think that it is safe to say that Spring is finally here, and we are free from the grip of old man winter.  Spring is a great time to witness the behavior of flora and fauna as the plants are starting to come alive again, migratory birds are making their way back, wildlife is active, and giving birth to their young. It is also a very interesting time to observe the interactions between native and non-native plants, particularly woody shrubs. Plants are often in a covert, high stakes competition for resources and survival with their neighbors, where coming out of dormancy and leafing-out the quickest is an unfortunate advantage for many non-native species. This can be seen right about now as most of the multiflora rose, barberry, and honeysuckle is already developing leaves here in southern New Hampshire, while most of our native shrubs are just starting to wake up. This is one of the many traits that non-native plants possess that allow them to quickly outcompete our native species and form dense stands that cause local extinction to native plants and may also cause changes to ecosystem functioning and processes. Humans depend on properly functioning ecosystems to provide us with many benefits that most do not think about or may not even be aware of.  These are called ecosystem service benefits.

Japanese barberry            That’s where my research comes in. With the guidance of my advisor Dr. John Gunn, I will be assessing the impacts of terrestrial invasive plants in New England’s riparian forests (forest found along a river or a stream). Native riparian vegetation is very important to the health and functioning of these ecosystems and allow these habitats to provide us with many benefits such as water filtration, nutrient and runoff removal from urban and agricultural areas, carbon storage, bank stability, erosion and flood control, and providing fish and wildlife habitat. Riparian forests are very unique areas, where the vegetation and streams depend on one another. One of the many examples of this is how large native trees provide shade to streams, which maintain cool water temperatures and dissolved oxygen levels; this is vital for brook trout and aquatic insects. The shade also benefits the terrestrial environment with increased humidity and maintaining soil moisture. This is necessary for the many amphibians and rare plants that are found in riparian habitats. Even fallen dead trees continue to provide energy inputs, increase food web dynamics, and increase habitat complexity. This is vital to stream health and aquatic life. These dead trees and large woody debris create debris dams that allow pools of water to form. These pools are important refuge for stream fish and also facilitate processes that allow clean water. These are just a few of the ecosystem services and processes of riparian forests that benefit humans and other organisms, but all of them depend on properly functioning ecosystems, which may be impacted by changes to plant communities.

            Unfortunately, riparian habitats are one of the most disturbed ecosystems in the world and are highly modified for transportation, flow regulation, and drainage purposes. These modifications often introduce invasive plants into riparian habitats, and also create open areas with newly available resources and nutrients. These areas are easily colonized by highly competitive invasive plants. Once established, these invaders quickly outcompete native plants and form dense monocultures, which can reduce local biodiversity, reduce regeneration of native trees, and negatively impact forest structure and ecosystem processes. These invasive plants are further spread throughout the watershed by land use practices and natural events such as hurricanes, flowing water, and wind & animal dispersal.

            My research has three main objectives. The first objective is to quantify the impacts from terrestrial invasive plants to ecosystem services, native riparian vegetation, bank and soil behavior, and stream physical habitat. To do this, my field technician and I will be conducting approximately 100 forest surveys and stream habitat assessments along 70 km of headwater streams in northern New Hampshire in the town of Lancaster (Coos County). Lancaster has a population of 3,507 with 70.4 people per square mile and is often referred to as the gateway to the Great North Woods Region. We will be looking at a suite of response variables such as large woody debris production, tree canopy cover and shading ability, plant species diversity, plant community composition, forest structure, soil characteristics, run off potential, bank stability, erosion, sedimentation, and water quality. Our surveys will take place in a gradient moving from urban/agricultural land-use to more interior forests of the White Mountain National Forest. We will be working in headwater streams of the Israel and Connecticut Rivers and include Caleb Brook, Bone Brook, Redman Brook, Bunnell Brook, Garland Brook, and Great Brook.

Irene cleanup            The second objective is to analyze invasion patterns of invasive plants and assess if they are using streams as corridors to invade less disturbed interior forests of the White Mountains.  I will use ArcGIS to perform analyses to look at invasive plant community’s proximity to roads, culverts, dams, old logging roads, timber harvests, agriculture fields, and land cover types to analyze patterns and relationships to landscape-scale factors and disturbance. This data will be useful to the U.S. Forest Service as they have limited knowledge of potential invasive plants in the northwest portion of the White Mountain National Forest that I will be working in, as they most likely conducted surveys along roads and trailheads, but nothing along rivers and streams. In addition, the upper Connecticut Cooperative Invasive Species Management Area (UCCISMA) and Trout Unlimited (TU) has been conducting only roadside surveys to gather point location data for invasive plant occurrences in the upper Connecticut River watershed. I will be adding location data from the stream surveys conducted for my research to the UCCISMA and TU database.  

            The third objective is to assess the effects of restoring native plant communities in riparian forests and their ability to resist invasive plants. More specifically, does planting native species within a riparian buffer increase resistance to invasive plants, particularly Japanese knotweed following a major natural disturbance? To address this question, my technician and I will conduct vegetation surveys at sites previously replanted with native vegetation by the White River Partnership (WRP) as well as adjacent non-planted sites (paired sites). Much work has been done in the White River Watershed, VT, to restore riparian habitats and their native plant communities, especially after Hurricane Irene caused substantial damage and severe flooding to the area in August of 2011. Hurricane Irene left many areas bare from intense flooding and massive erosion; these areas with newly available niche space and resources, combined with an abundance of moving water greatly facilitated the establishment of invasive species. These study sites are all located in the White River Watershed near the town of Royalton Vermont (Windsor County) and were restored by the WRP after Hurricane Irene. 

Irene damage            Through preliminary roadside surveys, Japanese knotweed (Fallopia japonica) is by far the most abundant invasive species present in our study area although common buckthorn, glossy buckthorn, common reed, false spirea, garlic mustard, goutweed, honeysuckle, Japanese barberry, Japanese knotweed, multiflora rose, oriental bittersweet, purple loosestrife, and wild parsnip are some of the other species that we will be on the lookout for. Field work begins soon (May 21st), so stay tuned for our findings as we explore how these invaders may impact these important ecosystems!

knotweed in Broad Brook
High density (and tall!) Japanese knotweed extending more than 30 feet from the banks of the Broad Brook in Sharon, Vermont.