Summer 2004

Frequent Fire Miles
Fire plays a critical role in maintaining high-quality prairie

How many fires does it take to maintain a prairie? In 2001, Michael Jones and I, working for The Morton Arboretum in Lisle, Illinois, looked into the past to find out.

We resurveyed 62 sites identified by the Illinois Natural Areas Inventory (INAI) in 1976 as the highest-quality prairie remnants in the Chicago region.

Measuring the rich plant life of Somme Prairie. Photo by Marlin Bowles.

Our first discovery was that 22.6 percent of these high-quality prairies have been either developed or completely overgrown with brush since the 1976 survey. While only one of the highest-quality (grade-A) prairies was lost, 35 percent of the still-remarkable grade-B sites disappeared.

We measured plant species richness (the number of native species), composition (the relative abundance of different plant species), and structure (the ratio of woody vegetation to grasses) at the 62 sites, ranking Somme Prairie Nature Preserve, in Cook County, and Spring Bluff Forest Preserve, in Lake County, richest among them.

Reviewing site management histories kept by land managers, we found that fire played a large — yet not large enough — role in maintaining these high-priority sites. Of the 39 sites with fire management records, about 60 percent had been burned four times or more over the last 20 years. We found that this burn rate prevented loss of native species richness in grade-A sites, and actually improved species richness in many grade-B sites.

However, we also found that much more frequent burning is required to maintain the structure and composition of prairies. For example, it may be necessary to burn more than 10 times in 20 years in grade-A prairies to prevent an increase in woody vegetation and loss of native grasses. Wetter prairies may need to burn even more frequently.

Unfortunately, 80 percent of the sites with burn records were burned less than 8 times in 20 years. Formerly dominant prairie grasses and forbs have declined, and species that are considered invasive are increasing in abundance. The study also found an increase in nonnative species in almost all prairies, and this increase was not affected by fire. Other factors, such as increased deer populations, may be enhancing the shift in vegetation.

As the data used for this study were collected only twice in 25 years, we offer the findings as projections to be confirmed with further monitoring.

— Marlin Bowles

Bowles and Jones are ecologists with The Morton Arboretum and Christopher Burke Engineering, respectively.