Restoring the Cullen Nature Preserve to its Historic Oak Savanna Habitat
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PART TWO Oaks and Oak Ecosystems |
PART TWO: Oaks and Oak Ecosystems
Oak Savanna - A Disappearing and Rare Landscape
Currently in Minnesota and throughout the Midwest, oak savannas are an extremely rare plant community with less than 0.5% remaining. Globally, oak savannas in northern latitudes (temperate zone) are one of the world’s most endangered ecosystems. Prior to European settlement, much of the land in Minnetonka was managed and maintained as oak savanna. Native Americans played an integral role, managing these open landscapes to provide grazing animals forage and ultimately nutritious food sources (game, fruit, and seed) that sustained the Native American community.
Currently in Minnesota and throughout the Midwest, oak savannas are an extremely rare plant community with less than 0.5% remaining. Globally, oak savannas in northern latitudes (temperate zone) are one of the world’s most endangered ecosystems. Prior to European settlement, much of the land in Minnetonka was managed and maintained as oak savanna. Native Americans played an integral role, managing these open landscapes to provide grazing animals forage and ultimately nutritious food sources (game, fruit, and seed) that sustained the Native American community.
What is an Oak Savanna?
Oak savannas are landscapes characterized by widely spaced oak trees and a prairie-like ground layer vegetation. These landscapes were historically maintained as open grasslands through human-facilitated means including fire, and natural disturbances such as grazing by bison and elk. The primary tree species in oak savannas— bur and white oaks—are long lived and considered keystone species because they support an extraordinary amount of wildlife, more than any other tree species.
Oak savannas are landscapes characterized by widely spaced oak trees and a prairie-like ground layer vegetation. These landscapes were historically maintained as open grasslands through human-facilitated means including fire, and natural disturbances such as grazing by bison and elk. The primary tree species in oak savannas— bur and white oaks—are long lived and considered keystone species because they support an extraordinary amount of wildlife, more than any other tree species.
Oak savannas are extremely diverse plant communities hosting prairie and woodland flowering plants, grasses, and sedges, in addition to unique plants that grow only in savannas. The 12-acre oak savanna restoration at Cullen Nature Preserve will provide the community with the first present-day model, and foster an appreciation for Minnetonka’s natural heritage.
Oak savannas provide unique habitat for rare and declining bird species such as the red-headed woodpecker. Return of the red-headed woodpecker, a species near threatened, is a special target for the restoration efforts. Bill Cullen, son of Ann Cullen, recalls that the red-headed woodpecker was once common on the property.
Red-headed Woodpecker
"Regenerating oak savannas restores Minnetonka’s natural heritage because oak savanna was the predominant ecological community in Minnetonka prior to European settlement. Oak savannas are more resilient to over-browsing, invasive species, heat, drought, and wind, and can withstand extremes in wet and dry conditions."
2021 Natural Resources Master Plan
2021 Natural Resources Master Plan
How Do Oak Savannas Differ From Forests?
Tree canopy cover in oak savannas can range from sparse (5% tree cover) to more tightly spaced pockets with intermittent canopy openings (50% tree cover). Even at 50% tree cover, the canopy openings still allow enough sunlight to reach the ground to foster the growth of prairie-like vegetation. This dense prairie-like vegetation is a critical component of an oak savanna because it:
Tree canopy cover in oak savannas can range from sparse (5% tree cover) to more tightly spaced pockets with intermittent canopy openings (50% tree cover). Even at 50% tree cover, the canopy openings still allow enough sunlight to reach the ground to foster the growth of prairie-like vegetation. This dense prairie-like vegetation is a critical component of an oak savanna because it:
- stabilizes soil to reduce erosion,
- facilitates the infiltration of rainwater,
- provides habitat for pollinators, birds, and wildlife,
- competes with invasive plants for light and nutrients, helping to reduce the invasibility of the site, and
- historically, provided forage for grazing animals such as bison or elk.
Oaks - A Keystone Species of Oak Savannas and Oak Woodlands
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In Minnetonka, white and bur oaks are the backbone of oak ecosystems. If you drive through the community today, you don’t have to look very hard to find legacy oak trees growing in homeowner yards or parks and open space. These oaks (genus Quercus) are keystone species and not only define oak ecosystems, but support a high diversity of other organisms through food web and habitat interactions. If you remove a keystone species from a system, the structure of the system will quickly fall apart. Without oak regeneration or management to remove invading trees affecting white and bur oak health, the legacy oak trees will eventually die and become extirpated from the system.
Oaks are very long lived and adaptable, surviving fluctuating temperatures and periods of droughts. These majestic trees can live to seven hundred years or more and throughout their lifespan support hundreds of butterfly and moth larvae (caterpillars), that in turn provide food for songbirds and their hungry chicks, and cavities for birds such as woodpeckers and owls to nest in. |
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The Preserve’s oak trees are majestic giants and several are over 170 years old—some of the oldest oak trees in Minnetonka! For this reason, all restoration decisions will be driven by the oak savanna plant community target goals and best management practices to preserve the health of the legacy oak trees present on the property. |
Saving the Legacy Bur and White Oaks
No ecological restoration has occurred at the Cullen Nature Preserve for over 150 years. With the absence of fire and grazing animals maintaining an open landscape and facilitating the growth of prairie-like vegetation, the Preserve (and all of Minnetonka’s remaining savannas) have become severely degraded. Degradation of savannas can result from a combination of factors including:
- the removal of key natural inputs (fire and grazing),
- the establishment and growth of non-oak native trees creating a closed canopy forest,
- the loss of dense ground layer prairie-like vegetation from tree canopy shading, and
- the invasion by invasive plants.
Do Red Oaks Belong in an Oak Savanna or Oak Woodland?
Shade-tolerant oaks such as red oak (Quercus rubra) also invade the bur and white oak ecosystems. Historically, red oaks made up a small percentage of the total number of trees in white and bur oak ecosystems. Red oak acorns were the primary food source of passenger pigeons. When fire was excluded from landscapes and passenger pigeons extirpated, red oak began to increase in abundance in the bur and white oak ecosystems.
Shade-tolerant oaks such as red oak (Quercus rubra) also invade the bur and white oak ecosystems. Historically, red oaks made up a small percentage of the total number of trees in white and bur oak ecosystems. Red oak acorns were the primary food source of passenger pigeons. When fire was excluded from landscapes and passenger pigeons extirpated, red oak began to increase in abundance in the bur and white oak ecosystems.
Diseased Red Oaks Growing in a Former White and Bur Oak Ecosystem Marked for Removal
Red oak is more shade tolerant than bur and white oaks but it is also very susceptible to, and is the primary host of, oak wilt disease. This fungal disease can spread by vectors such as insects and wind, but can also spread in overstocked oak ecosystems from red oak to white and bur oak through their connected root systems. In an oak savanna, single oak trees are typically widely spaced apart or grew in small groves or groupings, limiting the root system connections and exchange of nutrients (and diseases).
Oak Ecosystem Invasion by Fire-intolerant Native Trees - Mesophication
In upland terrain, bur and white oak trees were once the most abundant tree species in Minnetonka’s oak savannas and oak woodlands. Today, with widespread fire exclusion, the remaining and highly degraded oak ecosystems lack the former diversity of plants, insects, birds, and mammals. Many still host legacy oak trees, but they have largely become densely forested by invasive plants and invading native, shade- and fire-intolerant tree species. This process of invasion by fire-intolerant trees when fire exclusion occurs is termed mesophication.
For example, ironwood, elm, ash, black cherry, maple, and basswood now grow in these former oak savannas and oak woodlands. With the invasion and growth of these fire-intolerant tree species, the oak ecosystems are radically altered, becoming overstocked with trees, and if left unmanaged and without the frequent input of fire, become cool, moist, densely shaded, and highly susceptible to invasion by invasive species.
In upland terrain, bur and white oak trees were once the most abundant tree species in Minnetonka’s oak savannas and oak woodlands. Today, with widespread fire exclusion, the remaining and highly degraded oak ecosystems lack the former diversity of plants, insects, birds, and mammals. Many still host legacy oak trees, but they have largely become densely forested by invasive plants and invading native, shade- and fire-intolerant tree species. This process of invasion by fire-intolerant trees when fire exclusion occurs is termed mesophication.
For example, ironwood, elm, ash, black cherry, maple, and basswood now grow in these former oak savannas and oak woodlands. With the invasion and growth of these fire-intolerant tree species, the oak ecosystems are radically altered, becoming overstocked with trees, and if left unmanaged and without the frequent input of fire, become cool, moist, densely shaded, and highly susceptible to invasion by invasive species.
A Former Oak Savanna Now Invaded by Mesophytic Trees (and Buckthorn)