Restoring the Cullen Nature Preserve to its Historic Oak Savanna Habitat
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PART FOUR The Restoration |
PART FOUR: The Oak Savanna and Oak Woodland Restoration
Restoration Goals
Oak ecosystems are diverse because the dense matrix of ground layer perennial vegetation, comprised of grasses, sedges, and flowering plants, amounts to hundreds of plant species. This plant diversity and the multitude of insects that rely on this plant diversity, in turn provide habitat and food for birds and mammals, leading to robust, interconnected food webs.
The oak savanna and oak woodland restoration at Cullen Nature Preserve is designed to:
Oak ecosystems are diverse because the dense matrix of ground layer perennial vegetation, comprised of grasses, sedges, and flowering plants, amounts to hundreds of plant species. This plant diversity and the multitude of insects that rely on this plant diversity, in turn provide habitat and food for birds and mammals, leading to robust, interconnected food webs.
The oak savanna and oak woodland restoration at Cullen Nature Preserve is designed to:
- enhance the health of the legacy oak trees,
- reestablish and increase plant diversity,
- foster the regeneration of oaks to produce different age classes,
- provide a restored, living model to showcase to the public, and
- increase the awareness of the need for ecological restoration and appreciation of these rare ecosystems.
Restoration Methods - A New Approach
This 12-acre restoration will look very different from the forests that one might observe while visiting a Minnetonka park. The Cullen restoration will include the implementation of new restoration approaches that are not currently or regularly employed in the management of our parks' natural resources including:
A Shock to the System
The former savanna at the Cullen Nature Preserve is now an altered and unstable system. The legacy oaks are on life support. If no intervention were to occur, the community would lose this rare savanna ecosystem and the majestic oaks that provide the critical backbone of the plant community. To reverse the trend of degradation, bold restoration management actions are required, actions that will ‘shock’ the system back to its former stable and resilient state.
To achieve these oak savanna tree cover targets at Cullen Nature Preserve, harvesting of all invasive plants and many non-oak trees will occur to create openings in the canopy. This will produce a park-like visitor experience with extensive and scenic views through the understory of the oaks, and outstanding bird and pollinator habitat.
This 12-acre restoration will look very different from the forests that one might observe while visiting a Minnetonka park. The Cullen restoration will include the implementation of new restoration approaches that are not currently or regularly employed in the management of our parks' natural resources including:
- extensive harvesting of undesirable woody biomass,
- seeding to establish a diverse native vegetation ground layer, and
- regular use of prescribed fire.
A Shock to the System
The former savanna at the Cullen Nature Preserve is now an altered and unstable system. The legacy oaks are on life support. If no intervention were to occur, the community would lose this rare savanna ecosystem and the majestic oaks that provide the critical backbone of the plant community. To reverse the trend of degradation, bold restoration management actions are required, actions that will ‘shock’ the system back to its former stable and resilient state.
To achieve these oak savanna tree cover targets at Cullen Nature Preserve, harvesting of all invasive plants and many non-oak trees will occur to create openings in the canopy. This will produce a park-like visitor experience with extensive and scenic views through the understory of the oaks, and outstanding bird and pollinator habitat.
What to Expect - Restoration Activities
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Invasive Plant Removal and Opening of the Tree Canopy
During the winter months and while the ground is frozen, large tree harvesting and invasive plant management equipment will be utilized to remove the undesirable plants and open up the canopy. Prairie and Savanna Grass Seed Next, native prairie and savanna grass seed mixes will be sown to start establishing the prairie-like vegetation. Weed and Invasive Plant Management After establishing native grasses and flowering plants, and allowing existing plants to respond to the increase in sunlight, contractors, during the summer, will monitor and manage any undesirable plants. This may include mowing down the vegetation and returning in autumn to herbicide any remaining invasive plants such as buckthorn regrowth. Flowering Plant Seed After the burn, and while the ground is still bare, native flowering plants will be sown, continuing to establish diversity of the prairie-like vegetation. |
2021 City of Minnetonka Natural Resources Master Plan
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Prescribed Fire - A Primary Management Tool of Prairies and Oak Ecosystems
Once dense vegetation is established that can provide enough fuel, the restoration area will be burned. Burning will help control small invasive plant seedlings and temporarily provide bare ground for sowing more seed.
Historically, prairies, oak savannas, and oak woodlands were managed and maintained by natural and human disturbances. Grazing animals such as bison and elk played an important role in sustaining the open character and dense ground layer vegetation of these ecosystems. The most important human management tool was the regular use of fire. In Minnesota, Native Americans used fire for various purposes such as:
- providing nutritious forage (fresh grass) for grazing animals (game),
- improving the nutritional quality and seed or fruit set of foraged plants, and
- reducing underbrush to improve the ease of travel and enhance hunting sightlines.
Oak savannas are fire-dependent ecosystems
Fire Frequency Influence on Prairies and Oak Ecosystems
Historical evidence including written accounts by early Euro-American explorers and settlers, and burn scars on oak trees, demonstrates that oak ecosystems and prairies were burned with relative frequency. The frequency and intensity of the fires would result in an ever changing but stable oscillation between prairie, oak savanna, and oak woodland. For example, a ten or twenty year fire interval would allow oak seedlings to become large enough, and develop thick enough bark to withstand the next fire and likely result in an oak savanna. Historically, more frequent fires such as every two or three years, would sustain a prairie.
Historical evidence including written accounts by early Euro-American explorers and settlers, and burn scars on oak trees, demonstrates that oak ecosystems and prairies were burned with relative frequency. The frequency and intensity of the fires would result in an ever changing but stable oscillation between prairie, oak savanna, and oak woodland. For example, a ten or twenty year fire interval would allow oak seedlings to become large enough, and develop thick enough bark to withstand the next fire and likely result in an oak savanna. Historically, more frequent fires such as every two or three years, would sustain a prairie.
Oak leaf litter, as opposed to the leaf litter of other mesophytic tree species, is an excellent fuel source for fires. Oak leaves tend to be larger, drier, thicker, and more resistant to decay. When oak leaves dry, they curl, are held uncompressed off the ground, and form gaps of air, providing an oxygen source for the fire to move across the landscape more effectively.
Fire, Leaf Litter, and Fuel
Like grazing, historically, fire was an important component of the oak savanna ecosystem that prevented the savanna from transitioning to a closed-canopy forest. Bur and white oaks have evolved with fire and are, therefore, well adapted to survive fire due to thick insulating bark. Oak seedlings even have adaptations that help to assist with survival during fire, callusing over and resprouting using reserves from their extensive taproot. Oak leaf litter, as opposed to the leaf litter of other mesophytic tree species, is an excellent fuel source for fires. Oak leaves tend to be larger, drier, thicker, and more resistant to decay. When oak leaves dry, they curl, are held uncompressed off the ground, and form gaps of air, providing an oxygen source for the fire to move across the landscape more effectively.
Mesophytic tree leaf litter is thin, decomposes quickly or is quickly consumed by non-native earthworms, and mats to the ground and holds moisture longer than oak leaf litter making it a poor fuel for carrying fire through the landscape. By harvesting mesophytic trees and restoring a part of Cullen Nature Preserve to oak savanna, fire can again be introduced as a management tool to effectively maintain the site, and help prevent the recurring invasion of mesophytic trees and invasive plant species.
Like grazing, historically, fire was an important component of the oak savanna ecosystem that prevented the savanna from transitioning to a closed-canopy forest. Bur and white oaks have evolved with fire and are, therefore, well adapted to survive fire due to thick insulating bark. Oak seedlings even have adaptations that help to assist with survival during fire, callusing over and resprouting using reserves from their extensive taproot. Oak leaf litter, as opposed to the leaf litter of other mesophytic tree species, is an excellent fuel source for fires. Oak leaves tend to be larger, drier, thicker, and more resistant to decay. When oak leaves dry, they curl, are held uncompressed off the ground, and form gaps of air, providing an oxygen source for the fire to move across the landscape more effectively.
Mesophytic tree leaf litter is thin, decomposes quickly or is quickly consumed by non-native earthworms, and mats to the ground and holds moisture longer than oak leaf litter making it a poor fuel for carrying fire through the landscape. By harvesting mesophytic trees and restoring a part of Cullen Nature Preserve to oak savanna, fire can again be introduced as a management tool to effectively maintain the site, and help prevent the recurring invasion of mesophytic trees and invasive plant species.
The dense, shaded conditions of a sugar maple 'Big Woods' forest. These low diversity forests lack the dense ground layer vegetation of oak ecosystems. In addition, maple leaf litter decomposes quickly or is readily consumed by non-native earthworms, resulting in bare ground that is susceptible to erosion as shown in the photo above.
Looking Forward Toward a Resilient Future
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After Cullen Nature Preserve and Bird Sanctuary receives restoration actions including large tree harvesting, prescribed burning, and seeding to reestablish ground layer vegetation, this “shock to the system” will reverse the more than one hundred and fifty years of degradation and mesophication, and begin transitioning the site into a resilient and biodiverse oak savanna.
Management of the site will be ongoing and we will continue to perform restoration actions in response to both invasive species pressure and to ensure plant species richness and diversity. In the coming years and decades, and with continued monitoring, these rich and diverse plant communities will gradually become more resilient and stable, and ultimately serve as a refuge for a rare and common birds, mammals, and pollinators. We envision that Preserve visitors will have awe-inspiring experiences and will revel in the beauty of these rare plant communities. |
Part 3: Climate < Restoration
REFERENCES AND FURTHER READING
Abrams, M. D. (2005). Prescribing fire in eastern oak forests: is time running out?. Northern Journal of Applied Forestry, 22(3), 190-196.
Arthur, M. A., Alexander, H. D., Dey, D. C., Schweitzer, C. J., & Loftis, D. L. (2012). Refining the oak‐fire hypothesis for management of oak-dominated forests of the eastern United States. Journal of Forestry, 110(5), 257-266.
Babl, E., Alexander, H. D., Siegert, C. M., & Willis, J. L. (2020). Could canopy, bark, and leaf litter traits of encroaching non-oak species influence future flammability of upland oak forests?. Forest Ecology and Management, 458, 117731.
Bowles, M. L., Jacobs, K. A., & Mengler, J. L. (2007). Long-term changes in an oak forest's woody understory and herb layer with repeated burning. The Journal of the Torrey Botanical Society, 134(2), 223-237.
Frelich, L. E., & Reich, P. B. (2010). Will environmental changes reinforce the impact of global warming on the prairie–forest border of central North America?. Frontiers in Ecology and the Environment, 8(7), 371-378.
Frelich, L. E., Reich, P. B., & Peterson, D. W. (2017). The changing role of fire in mediating the relationships among oaks, grasslands, mesic temperate forests, and boreal forests in the Lake States. Journal of Sustainable Forestry, 36(5), 421-432.
Kreye, J. K., Varner, J. M., Hamby, G. W., & Kane, J. M. (2018). Mesophytic litter dampens flammability in fire‐excluded pyrophytic oak–hickory woodlands. Ecosphere, 9(1), e02078.
McDaniel, J. K., Alexander, H. D., Siegert, C. M., & Lashley, M. A. (2021). Shifting tree species composition of upland oak forests alters leaf litter structure, moisture, and flammability. Forest ecology and Management, 482, 118860.
McEwan, R. W., Dyer, J. M., & Pederson, N. (2011). Multiple interacting ecosystem drivers: toward an encompassing hypothesis of oak forest dynamics across eastern North America. Ecography, 34(2), 244-256.
Nowacki, G. J., & Abrams, M. D. (2008). The demise of fire and “mesophication” of forests in the eastern United States. BioScience, 58(2), 123-138.
Thomas-Van Gundy, M. A., & Nowacki, G. J. (2016). Landscape-fire relationships inferred from bearing trees in Minnesota. Gen. Tech. Rep. NRS-GTR-160. Newtown Square, PA: US Department of Agriculture, Forest Service, Northern Research Station. 32 p., 160, 1-32.
Varner, J. M., Kane, J. M., Kreye, J. K., & Engber, E. (2015). The flammability of forest and woodland litter: a synthesis. Current Forestry Reports, 1(2), 91-99.
Abrams, M. D. (2005). Prescribing fire in eastern oak forests: is time running out?. Northern Journal of Applied Forestry, 22(3), 190-196.
Arthur, M. A., Alexander, H. D., Dey, D. C., Schweitzer, C. J., & Loftis, D. L. (2012). Refining the oak‐fire hypothesis for management of oak-dominated forests of the eastern United States. Journal of Forestry, 110(5), 257-266.
Babl, E., Alexander, H. D., Siegert, C. M., & Willis, J. L. (2020). Could canopy, bark, and leaf litter traits of encroaching non-oak species influence future flammability of upland oak forests?. Forest Ecology and Management, 458, 117731.
Bowles, M. L., Jacobs, K. A., & Mengler, J. L. (2007). Long-term changes in an oak forest's woody understory and herb layer with repeated burning. The Journal of the Torrey Botanical Society, 134(2), 223-237.
Frelich, L. E., & Reich, P. B. (2010). Will environmental changes reinforce the impact of global warming on the prairie–forest border of central North America?. Frontiers in Ecology and the Environment, 8(7), 371-378.
Frelich, L. E., Reich, P. B., & Peterson, D. W. (2017). The changing role of fire in mediating the relationships among oaks, grasslands, mesic temperate forests, and boreal forests in the Lake States. Journal of Sustainable Forestry, 36(5), 421-432.
Kreye, J. K., Varner, J. M., Hamby, G. W., & Kane, J. M. (2018). Mesophytic litter dampens flammability in fire‐excluded pyrophytic oak–hickory woodlands. Ecosphere, 9(1), e02078.
McDaniel, J. K., Alexander, H. D., Siegert, C. M., & Lashley, M. A. (2021). Shifting tree species composition of upland oak forests alters leaf litter structure, moisture, and flammability. Forest ecology and Management, 482, 118860.
McEwan, R. W., Dyer, J. M., & Pederson, N. (2011). Multiple interacting ecosystem drivers: toward an encompassing hypothesis of oak forest dynamics across eastern North America. Ecography, 34(2), 244-256.
Nowacki, G. J., & Abrams, M. D. (2008). The demise of fire and “mesophication” of forests in the eastern United States. BioScience, 58(2), 123-138.
Thomas-Van Gundy, M. A., & Nowacki, G. J. (2016). Landscape-fire relationships inferred from bearing trees in Minnesota. Gen. Tech. Rep. NRS-GTR-160. Newtown Square, PA: US Department of Agriculture, Forest Service, Northern Research Station. 32 p., 160, 1-32.
Varner, J. M., Kane, J. M., Kreye, J. K., & Engber, E. (2015). The flammability of forest and woodland litter: a synthesis. Current Forestry Reports, 1(2), 91-99.