Background
Non-native, invasive crack willow trees are among the largest trees along northern Minnesota waterways, having colonized miles of stream banks around the Duluth area. Named for their brittle branches that break easily, these multi-stemmed trees frequently drop heavy limbs into the water. This debris creates large, tangled dams that may block fish movement and cause stream beds to accumulate sand and silt, which degrades vital spawning and aquatic invertebrate habitats. Their expansive root systems can also make streams less responsive to floods and channel-shaping events.
However, managing crack willow poses a unique challenge because the trees may also provide ecological benefits. Their massive root systems can stabilize vulnerable banks against erosion, and their sweeping broad canopies offer crucial shade that may help maintain the cool water temperatures necessary for native trout survival as well as providing leaves that form the base of the stream food web. To resolve these conflicting factors, researchers are studying both the biological and physical stream changes caused by the willow invasions to determine if targeted management can reduce the harms without losing the benefits. The team will also investigate how the trees respond to the in-stream root removal treatment.
Project Goals
- Evaluate Stream Condition: Measure the impacts of crack willow on native fish populations and the aquatic insects they rely on for food as well as the habitats that support them both.
- Assess Channel Stability: Quantify how the tree's dense root networks and debris dams impact stream flow, sediment build-up, and bank erosion.
- Inform Management: Test manual debris-removal methods to give local land managers clear guidelines on how to balance the tree's benefits with its potential ecological harms.
Project Outcomes
The anticipated outcomes are a comprehensive assessment of crack willow impacts on fish and aquatic insect communities, paired with an evaluation of how their root networks alter stream channels and substrate. The key anticipated result is providing local stream managers with science-based, practical guidelines on whether, where, and how to manage crack willow to optimize coldwater stream health.
Research Projects
NRRI researchers are leading and collaborating on two crack willow projects with University of Minnesota Duluth researchers and partners:
Riparian crack willow: benefit, harm or both for stream fauna?
Research Team
- Valerie Brady | principal investigator
- Amber Ulseth | co-principal investigator
- Robert Hell | researcher
- Holly Wellard Kelly | researcher
- Kari Pierce | researcher
- Brennan Pederson | researcher
Riparian crack willow: are channel impacts controllable by in-stream management?
Research Team
- Karen Gran | principal investigator
- John Swenson | co-principal investigator
- Andrej Pilipovic | co-principal investigator
Project Partners
- The Natural Resources Research Institute (NRRI) is leading the biological evaluation of fish, aquatic macroinvertebrates, and in-stream habitat responses.
- The University of Minnesota Duluth (UMD) Earth and Environmental Sciences Department is leading the investigation into stream channel morphology, sediment deposition, and response of the Crack Willow trees to treatments.
- The City of Duluth is collaborating as a local land management partner, providing site access and coordinating with the research teams on urban stream locations.
- The Minnesota Department of Natural Resources (DNR) is providing historical fisheries and stream data, assisting with regulatory guidance regarding protected trout streams, and helping evaluate long-term coldwater habitat impacts.
- The South St. Louis County Soil and Water Conservation District (SWCD) will supervise the Conservation Corps team that will remove the crack willow debris and roots from the stream channels. The SSL SWCD also brings specialized local expertise in watershed health and erosion control, assisting the research teams with field site selection, stakeholder outreach, and local land management coordination.
- The Conservation Corps of Minnesota and Iowa will provide a team to remove excessive crack willow fallen debris and tree roots from selected areas of the stream channels to test this control method for improving the condition of streams invaded by crack willow.
- The 1854 Treaty Authority is collaborating to provide vital tribal resource management perspectives, ensuring that the research aligns with inter-tribal efforts to protect and preserve native fish populations and aquatic ecosystems within the 1854 Ceded Territory.
Funding
Funding for these projects was provided by Minnesota Environment and Natural Resources Trust Fund as recommended by the Legislative-Citizen Commission on Minnesota Resources (LCCMR) through the Minnesota Invasive Terrestrial Plants and Pests Center.
