Prior to European settlement, fires were a natural part of the North American landscape. Some habitat types, such as prairies, evolved to depend on fire to maintain ecosystem health and vigor. Prairies likely burned every one to five years.

Following European settlement, habitat fragmentation and fire suppression reduced the number of naturally occurring fires and there was a decline in the number of fires set by Native Americans. Without fire, woody vegetation such as trees and shrubs begin to encroach on prairies. If allowed to establish and grow, these woody plants will begin to shade native prairie plants. This can lead to an increase in soil moisture and decrease available light, making it harder for prairie plants to establish and easier for woody species to thrive and spread.

To maintain fire-dependent ecosystems, land managers now use prescribed fire as a tool to mimic the fires that historically occurred on the landscape. At the Arboretum, prairies account for approximately 140 acres (57 hectares) of the total 1,200 acres (487 hectares) in Madison, and prairie units are burned every two to four years.

While prescribed fires are often conducted in the spring or fall when desirable vegetation is not actively growing (dormant season), most naturally occurring historical fires occurred in the summer (growing season) when lightning strikes are more common. In recent years, land managers and researchers in the Midwest have introduced prescribed fires in the summer. This expands the available time each year to conduct prescribed fires (burn window) and may have positive effects for setting back undesirable vegetation, such as woody species in prairies, while benefitting additional desirable species.

In 2018, Arboretum research and land care staff started a project in Wingra Overlook Prairie with the goal of assessing the efficacy of dormant season versus growing season prescribed fires for controlling woody stem encroachment in the prairie.

Wingra Overlook Prairie is a 5.14-acre (2.08-hectare) prairie located northwest of Arboretum Drive across from Longenecker Horticultural Gardens. In 2018, 30 one-square-meter plots were established in the western half of the prairie. In each plot, all woody stems were marked with a numbered metal tag and identified to species.

The study area was then divided into two burn units, each containing 15 one-square-meter plots. The spring burn unit (section B) was burned on April 15, 2019 (dormant season). The summer burn unit (section A) was burned on August 30, 2019 (growing season). Within five weeks prior to each fire, the height and basal diameter of each tagged woody stem was measured.

A total of seven woody species were found in Wingra Overlook Prairie: American elm (Ulmus americana), black cherry (Prunus serotina), gray dogwood (Cornus racemosa), honeysuckle (Lonicera spp.), multiflora rose (Rosa multiflora), quaking aspen (Populus tremuloides), and smooth sumac (Rhus glabra). The summer and spring burn units differed some in species composition, with the spring burn unit having a species richness of six (no American elm) and the summer burn unit having a species richness of five (no multiflora rose or quaking aspen).

Gray dogwood had the highest frequency and density in both burn units, occurring in 60 percent of the summer burn plots and 67 percent of the spring burn plots. Smooth sumac had the next highest frequency, occurring in 53 percent of the summer burn plots and 33 percent of the spring burn plots, though mean density of honeysuckle was higher than smooth sumac in the summer plots (see figure 1 for data). In both burn units, gray dogwood also had the highest mean basal diameter of all species (see figure 2). Overall, the summer burn unit had more large diameter stems (greater than 20 millimeters) than the spring burn unit (see figure 3).

For each burn unit, photo monitoring was conducted following the burn. Monitoring points were visited weekly during the growing season and periodically throughout the dormant season. The photos were compiled into videos to follow regrowth in Wingra Overlook Prairie after a dormant season (spring) burn and growing season (summer) burn.

Spring burn

Summer burn

In 2020, these plots will be remeasured to assess mortality of woody stems following the 2019 prescribed fires. Comparisons will be made by species and size class to determine if differences exist between dormant season and growing season prescribed fires for controlling woody stem encroachment. This will help inform prescribed fire management at the Arboretum.

—Christy Lowney, research specialist