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Restoring Oak Forest in the Baraboo Hills

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The Nature Conservancy has worked to protect the vast forests in the Baraboo Hills for more than 50 years. While the forests are still in relatively good condition, the oak trees are slowly declining. recently caught up with Baraboo Hills Project Coordinator Ann Calhoun to talk about why oaks are important and the Conservancy’s effort to maintain and restore them.

What’s special about the Baraboo Hills?

Ann Calhoun:

Mostly forested, the Baraboo Hills are a sanctuary for birds and other wildlife in a southern Wisconsin landscape largely dominated by agriculture. The Hills are home to a diverse mix of habitats, ranging from cool damp valleys with relict hemlock stands to dry rocky glades with stunted oak and hickory trees. This variety of forest types provides suitable habitat for a wide diversity of plants and wildlife – especially birds. More than half (about 135) of the 226 bird species that breed in Wisconsin use the Baraboo Hills as a nesting area, and as many as 250 different species either breed in the Hills or pass through during migration.

How have these forests changed over time?

Ann Calhoun:

One of the biggest changes is that, in the parts of the Hills where oak species were once dominant, we are losing many of the oak trees that remain. Now these oak forests are transitioning to maple, basswood, cherry, ironwood and other species, which shade and crowd out the oaks, preventing them from naturally regenerating.

Why are the oaks not regenerating?

Ann Calhoun:

Oaks need lots of light. Some portions of the Hills – particularly the dry southern and western slopes – have a long history of fire disturbance, which kept the forests more open, allowing sun to penetrate to the forest floor where acorns could germinate and grow. Fires started by Native Americans were a major source of disturbance, but big wind events and grazing animals also kept the forests more open. Today, pressure from deer browse, unsustainable logging, encroachment of buckthorn and other invasive species, drought and forest pests also play a role.

Is acorn production, or lack of it, an issue?

Ann Calhoun:

Yes. The heavy mast years, when oaks produce lots of acorns, are critical. It’s been found that a relatively small percentage of the oak trees in a forest produce the majority of the acorn crop in a given year. As the mature oak trees that bear the bulk of the acorns die and diminish in number, the remaining oaks may not be producing enough acorns to keep ahead of the squirrels, turkeys and other wildlife that eat them, so fewer acorns remain to plant themselves in the ground and grow into trees.

Why is it important that we maintain the oaks in the Hills?

Ann Calhoun:

Acorns are a major food source for turkeys and lots of other wildlife. Some insects only live and/or feed on oaks when they are leafing out, and there are many species of Neotropical migratory birds, including the cerulean warbler, that depend on these insects for food. Oaks are an iconic tree here in the Midwest with their big, wide-spreading branches and open, park-like understory. Few of these oak woodlands remain in Wisconsin, or the world for that matter, and those that do remain are fairly degraded and are declining quickly.

What is the Conservancy’s plan for maintaining and restoring oaks in the Hills?

Ann Calhoun:

Our forest management and efforts to restore oak forest will be based on sound science and a recently completed, detailed forest inventory of trees, shrubs and understory vegetation. We will begin restoration efforts within the larger Baxter’s Hollow and Hemlock Draw preserves, in areas that still have a good number of healthy, acorn-producing oaks. Bird experts have been conducting bird surveys in the same area where the forest inventory was completed. We are analyzing the bird and forest inventory data and will use it to develop a list of the key elements necessary to maintain and improve the health of oak forests in the Hills with a keen eye toward important bird habitat needs. This information will help guide our forest management and monitoring efforts.

This summer, we’ll begin work on 70 acres in the southern portion of Hemlock Draw Preserve where we have a good component of white oak remaining on the forested slopes and a little bit of natural oak regeneration already occurring. We’ll cut ironwood and small maple trees, along with invasive shrubs like multiflora rose and honeysuckle, to give large, mature oaks more “breathing room.” This will allow more sunlight to reach the forest floor, improving conditions for acorn germination and oak sapling growth. Farther down the road, management techniques will also include prescribed fire and removal of some larger, shade-tolerant trees depending on how the oaks respond to the initial treatments.

What would happen if we left things as they are and didn’t do any of the restoration work you’re talking about?

Ann Calhoun:

The oak trees would eventually decline to the point where there would be very few of them in the forest – they would mostly persist on forest edges or in small gaps where they could get just enough light to hang on. In the areas once dominated by oak, we would have a forest dominated by maples and other shade-tolerant tree species, which are native to the Hills but have expanded and out-competed the oaks in the absence of disturbance, such as fire. As the forest floor has become more shaded, we have already seen the diversity of grasses and wildflowers diminish. Ultimately, we’d lose some of the bird species and other wildlife that depend on oaks for food, insects and nesting habitat.

Our window of opportunity to restore oaks in the Hills is closing. Of the thousands of acres we own, there are only a handful of places where there is any natural oak regeneration, but we are discovering areas where there are opportunities to restore oak if we act soon. So now is the time to set the stage for the remaining oaks to thrive and produce the next generation of oak trees, which are a vital component of the forests Nature Conservancy members have worked so hard to protect.

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