Globally endangered temperate oak savannas are being restored in south-central Iowa


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By Sibylla Brown

Controlled burns restore oak savannah, leading eventually to a healthier understory and canopy. The restoration process can take 10 years. (Courtesy of Sibylla Brown)Landowners in south-central Iowa are setting their woodlands on fire to restore globally endangered oak savannas. Temperate-zone North American oak savannas are one of the world’s most endangered ecosystems. Once stretching from Minnesota south to the Texas Hill Country, presettlement oak savannas, the transition between the eastern deciduous forests and the tallgrass prairie, encompassed 30 million acres. One of the top 10 ecoregions for diversity of reptiles, birds, butterflies and vascular plants, this two-tiered community consists of open-canopy deciduous trees over an herbaceous ground layer of grasses, sedges and wildflowers. It contains many globally and federally endangered plant and animal species and numerous unique plant communities.

Oak savannas evolved after the retreat of the last glaciation when aboriginal Americans began using controlled burns. Fire was the Indians’ primary technology. It has been well documented in books such as Omer C. Stewart’s “Forgotten Fires” that they burned their lands annually, sometimes twice a year. Fire was used to drive game, facilitate hunting, renew grasses for pasturage, clear brush and increase seed production. The open woodlands also facilitated travel.

Most of these savannas have been destroyed by clearing, plowing or overgrazing. Now only 0.02 percent remains. Since European settlement in the early 20th century, fire suppression has turned the unmanaged remnants into closed-canopy forests with dense understories of invasive brush and pole timber. However, unlike the many Midwest prairies that have been lost to agriculture or development and can only be reconstructed, savanna remnants can be restored. Woodlands that have not been plowed will sustain portions of their native plant diversity. All they require to release the suppressed plants is sunlight and fire.

The European settlers believed that fire would destroy their timber. Many people still associate woodland fires with the conflagrations of western U.S. wildfires and believe that fire destroys woodlands. Just the opposite is true—fire is essential to savanna ecology. The thick bark and deep roots of oaks, the dominant savanna trees, makes them fire resistant. Fire will only destroy what doesn’t belong.

Many southern Iowa landowners agree and are restoring thousands of acres of overstocked woodlands with thinning and fire. The earliest restoration, Timberhill, in Decatur County was begun in 1993. Working with their district forester, the Timberhill landowners began their restoration by thinning the overstocked oak and hickory woodland. In 1995 they implemented annual prescribed burns to control regrowth and invasive brush. Stimulated by fire and sunlight, the Timberhill woodlands soon exhibited a dense cover of native grasses and sedges. This cover not only controlled erosion but improved soil fertility. Soon uncommon flowering plants such as white prairie clover, leadplant and false foxgloves filled in the understory. Without any interseeding this 200-acre restoration now supports over 450 vascular plants. From early spring until fall frost there is always something blooming in the Timberhill woodlands.

Some conservationists believe the best way to preserve woodlands is to put a fence around them and keep people out. That given enough time the processes of ecological succession will take over and heal the land. While they are passionate about the need to save the songbirds’ winter habitats in Central and South America, they are unaware of the importance of temperate North American woodlands to songbird survival. It is here where they were born and raised that they return each spring to propagate their species. A constant food source is necessary for them to successfully breed and raise their young. In savanna restorations this is provided throughout the growing season by insects feeding on the understory plants. In 2006 a study at Timberhill found the breeding bird population to be strikingly different between the managed woodlands and adjacent unmanaged land. Thirty species of birds were found to be breeding in the thinned and annually burned Timberhill plots, whereas only four species (all cavity nesters) were noted breeding in the unmanaged plots. The open woodlands are also preferred habitat of the red-headed woodpecker whose population is seriously threatened by habitat decline.

Savanna remnants can easily be identified by the presence of wolf trees, the widely spreading oaks that were once the only trees in a particular site. Their lower branches will probably be dead from overcrowding and lack of sunlight, but the size and breadth of wolf trees makes them stand out even in heavily overstocked woodlands. The ground layer will usually be dominated by sedges. Savanna remnants can also be identified by studying historic documents such as the original General Land Office survey notes and USDA aerial photographs.

To restore savanna, most woodlands need only sunlight and fire. This is best achieved by a combination of understory thinning and annual prescribed fire. Annual fires are much less destructive than periodic fires. With lower fuel loads, annual fires scud over the surface of the ground, leaving much invertebrate habitat intact, whereas periodic fire incinerates, parboils and kills because of the heavy fuel buildup. Fire alone can be used to restore oak savannas, but it takes much longer.

In southern Iowa we have learned that the best practice is to burn a restoration for three years before thinning. Fire will clear much of the invasive brush and some of the weed trees, making thinning easier. The next step is to clear any remaining brush and trees growing under the crown of the wolf trees. We have observed that savanna restoration proceeds in three stages. In stage one the understory fills in with common native forbs such as woodland sunflowers and tick trefoil. In stage two the woodland sunflower and tick trefoil gradually decline and are replaced by sedges, grasses and moss. In stage three the common forbs disappear and conservative forbs take their place. This restoration proceeds slowly and can take 10 years.

For the landowner doing restoration the most difficult task is determining the correct canopy density. Is enough sunlight reaching the understory to restore plant diversity? Interseeding of savanna indicator species is often recommended, but this may lead to unexpected consequences. An inventory of plants taken before proceeding with restoration followed by annual monitoring enables one to track progress and measure the changes in floristic quality. Listen to the plants and they will tell you when you’ve thinned enough. Given enough time and persistence, each site will develop its own unique character.

Since 2003 U.S. Fish and Wildlife Service (USFWS) private lands biologist Gregg Pattison has been assisting southern Iowa landowners to restore their woodlands. In addition to providing cost-share funds for thinning he provides technical assistance, particularly for prescribed burns. He has worked with landowners to demonstrate how to conduct a prescribed burn, construct firebreaks and help them identify highly restorable savannas. The Fish and Wildlife Service has also sponsored Southern Iowa Oak Savanna Alliance (SIOSA), an organization that provides a forum for people interested in restoring and preserving oak savannas. SIOSA sponsors fire training and restoration workshops for landowners.

In cooperation with the USFWS Iowa private lands program, 4,000 acres in 12 counties are currently being restored and six county conservation boards have set aside oak savanna demonstration plots. Woodlands have been restored in other states throughout the range of presettlement oak savannas. In her 1985 survey, “The Extent and Status of Midwest Oak Savanna,” Victoria A. Nuzzo found 113 sites totaling 2,607 hectares of high-quality savanna, but ongoing restoration efforts in southern Iowa and other states proves the extent of highly restorable savannas to be much greater.



Submitted by Anonymous (not verified) on
This article could easily be used as a basis for discusion in Iowa science classrooms.

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