Terrace Oaks Park provides a great example of the City's efforts to restore oak savanna habitat and remove invasive species, like common buckthorn. Terrace Oaks Park is home to many towering bur and white oaks but is also heavily invaded by common buckthorn and other weedy trees. These conditions create eroding soils, a lack of understory grasses and wildflowers for wildlife, poor conditions for the regeneration of young oak trees and a dense overgrowth of woody plants that make the park less aesthetically pleasing and usable for park visitors.
In 2015, the City undertook the first large-scale restoration project in a 19 acre section of the Northwest corner of the park, in partnership with Great River Greening and utilizing grant funds from the Minnesota Outdoor Heritage Fund.
The initial restoration area will be expanded in winter of 2018 to 2019. See a map showing the project expansion area (PDF).
Step 1 - Research the site - Extensive research and planning is conducted before initiating a large restoration effort. Historical aerial photos are reviewed to assess changes in tree cover over time. Dakota County GIS is a good resource for historical aerial photos. In 1937, the project area shows light tree cover and open, savanna like conditions. In 2011, aerial photos show a near complete conversion to dense woods. Buckthorn and weedy tree species, like box elder, make of the majority of the tree species that fill in the savanna.
Step 2 – Open up the canopy - Savannas are open habitats with lots of sunlight reaching the understory. To re-create savanna conditions, a feller buncher is used to remove buckthorn and weedy trees. Piles of woody material are stacked and then skidded to a staging area. From there, the woody material is ground into chips and removed from the site. Stumps were treated with herbicide to prevent re-growth.
A feller buncher is used to cut woody material.
Cut material is staged in temporary piles.
Piles of cut material are moved to staging area and ground into wood chips. Chips are transported to a facility and used to produce energy.
Step 3 – Get growing - Native grasses and an annual grass cover crop are seeded immediately in spring following woody removal. Tractors were used to seed flatter areas. Steep areas were seeded with volunteers. Grasses will help stabilize soils to prevent erosion, provide competition for invasive plants like buckthorn and provide fuel for future controlled burns.
Flatter sections are seeded with a tractor.
Steeper sections are seeded by City volunteers.
Step 4 – Keep after the invasives – Invasive plants were present on the site for many years so it's not just a one year process to get rid of them. A multi-year effort to control invasive plants is initiated and utilizes mowing, spot herbicide use and controlled burns. Most of the target invasive species are listed.
Step 5 – Bring fire back to the landscape - Reintroducing controlled burns to the project area is an important part of the restoration process. Controlled burns are a major part of the natural process that kept savannas open. It took several years for the seeded grasses to produce enough vegetation to carry a fire. More on controlled burns.
Contractors conduct a controlled burn in fall at Terrace Oaks.
Step 6 – Bring on the flowers – After invasive species were under control, a diverse wildflower mix was seeded. The seeding occurred after a controlled burn helped prepare the site by removing dead vegetation and leaf litter. The wildflowers from the seed mix will take several years to establish.
Step 7 - Monitor the site - Ongoing management is needed to maintain savanna habitat. Periodic controlled burns will be necessary, as well as continued monitor and control for invasive species issues.