Illinois Natural History Survey - University of Illinois

The Role of Fire in Maintaining Plant Diversity in Oak Woodland Communities

 Illinois is in an intermediate location between the predominant forests of the East and grasslands of the Great Plains. While prairies covered about 60% of the state around 1820, the vegetation was a shifting mosaic of prairie, savanna, woodland, and forest influenced in large part by the broad-scale use of fire by Native Americans. Conditions following European settlement of the region eventually favored forest development in many areas as fire frequency declined, particularly with fragmentation of the landscape by cropland and other developments. Fire-dependent communities, such as many prairie, savanna, and oak woodland habitats, began to undergo vegetational changes as oak grub sprouts and saplings became established with fire absence.

Today fire is used to maintain many prairie remnants, but it has been less frequently applied in oak woodlands. Many oak woodlands have lost much of their open savannalike structure and diverse assemblage of ground-cover species, and mesic sites appear to be converting to forests of maple and other species. While change always has been a characteristic of the prairie-forest transition zone, overall floristic diversity tends to decline when tree density (and shade) increases in these communities because a rich pool of shade-tolerant replacement species typically is lacking.

Results from research in post oak-dominated flatwoods and barrens indicate there is considerable potential for restoring diversity and stability in these habitats using prescribed fire. Flatwoods are level woodlands with slowly permeable soils, local openings, and micro- depressions; they typically are seasonally moist (early spring) and dry (summer). Barrens are a type of savanna community characterized by scattered trees and a herbaceous prairie flora within generally forested regions. They typically occur on shallow soils in uplands with exposure to the south or southwest. Some flatwoods and barrens have floristic similarities; both are usually xeric habitats during much of the growing season. The harsh environmental conditions in both communities have slowed but not stopped the vegetational changes typical of the post-fire era.

Following experimental dormant-season fires in both flatwoods and barrens, ground-cover species diversity increased at all scales measured (Fig. 1). Population sizes increased for most herbaceous species and the proportion of infrequent species (prone to local extinction) declined. In a comparative study of flatwoods on the 

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An experimental burn in a flatwoods near Mt. Vernon in Jefferson County.

southern till plain, one site with about 20 years of annual fire prior to vegetation sampling had species diversity in sample quadrats more than four times that of unburned sites, and weedy species were absent. However, after three recent fires at another site, a sharp increase in ground-cover diversity included not only characteristic flatwoods species (e.g., numerous sedge and forb species) but also native weeds such as white snakeroot and pokeweed. These latter taxa are not typical of flatwoods; they probably reflect a grazing history and, like the new sedge species, appear to have emerged from the soil seed bank. This site is surrounded by disturbed land, including pasture, and the post-fire results may be influenced by these edge effects.

Surprisingly, in the barrens studied, prairie grasses at a fire-treatment site and a fire-free control site are in parallel decline over time despite two burns. Many studies in prairies indicate that fire increases the abundance of prairie grasses. In contrast, some panic grasses (Panicum spp.) typically found in open woodlands increased dramatically following each fire. The prairie grasses may be continuing a decline due to the increased shade from past forest encroachment. While the experimental fires significantly reduced the density of trees (in both barrens and flatwoods), the changes were limited to small-diameter stems and shading was not greatly reduced. More intensive use of fire, or a combination of cutting and burning, likely will be necessary to reverse trends for prairie grasses in barrens.

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Figure 1. Changes in ground-cover species density among sample quadrats in post oak-dominated barrens and flatwoods following applications of prescribed fires. Season and year of fires are indicated by arrows (1993 burn in flatwoods was a winter burn and resulted in little response). Error bars are standard error (very small with flatwoods data).

While we still lack adequate data on effects of fire frequency and season on total biodiversity, the opportunity remains in the short term for the recovery of at least a portion of the diverse ground-cover characteristic of oak woodland and savanna communities. However, with continuing fire absence at most sites, vegetational changes soon may yield only depauperate and unsustainable examples of oak woodlands throughout much of the prairie-forest transition zone.

John B. Taft, Center for Biodiversity



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