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Fire Ecology in Arkansas Ecosystems

Arkansas Natural Heritage Commission - Tuesday, October 08, 2019
by Theo Witsell

Across Arkansas, as in states across the Southeast, many native plants and animals depend on grassland and open woodland habitat. This seems like a paradox to many people, who were taught in school that at the time of Euro-American exploration and settlement, eastern North America was one vast forest from the Atlantic Coast to the Great Plains. We now know that this was not so, but it does take some background to understand. Why were there extensive areas of natural prairie and savanna in a region that gets enough rainfall to produce wall-to-wall forests?

The short answer is that there are always some other factors besides rainfall that tip the balance in favor of open habitat. Usually this is some combination of a soil type (often thin or poor soil that gets very dry in the summer) and some periodic aboveground disturbance process that works together. Fire is the most widespread of these disturbance processes, though grazing and browsing by large herbivores like bison and elk were once important as well.

Many of these ecosystems have their origin in hotter, drier climatic periods in the past, but fire was largely responsible for keeping them here until recent times. Simply put, these ecosystems are fire-prone and their native species, in general, are fire-adapted. The plants are mostly dormant in the late fall and winter and the aboveground vegetation is flammable for an even larger part of the year. Many of the herbaceous plants native to this ecosystem have most of their biomass belowground and can withstand repeated fires, at least during the late summer, fall, and winter.

Historically, when the summer or early fall storms would arrive and lightning would strike on a large expanse of dry grassland, it could burn for miles until it came to a natural firebreak or it was interrupted by a major rainstorm. Woodlands along streams and at the edges of grasslands would burn as well, becoming more open in times of frequent fire and denser in periods without much fire. Native Americans, and later Euro-American and African-American settlers, would also burn the prairies and woodlands to make travel easier, to improve wildlife habitat, and to encourage the fresh shoots of the grasses favored by bison, and later, by cattle.

How Does Fire Work Ecologically?

The most obvious function of fire in prairie and woodland ecosystems is that it suppresses the smaller and less fire tolerant woody plants (shrubs and trees) and encourages herbaceous (non-woody) species of wildflowers and grasses. Fire suppresses woody plants in two ways. First, it stimulates sun-loving grassland plants to form a vigorous sod, which resists the establishment of woody plant seedlings. Second, fire kills the aboveground portions of smaller woody plants, keeping the brush in check (but rarely eliminating it).

The goal when burning a woodland or savanna is not to kill the larger trees. Whether this is a pine system or an oak system, the dominant (largest) trees are, by their nature, fire-tolerant. They have bark thick enough to withstand moderately intense fires that would kill fire-intolerant species, or smaller specimens of their own species.

Following fire, herbaceous plants grow larger, produce more flowers, produce more fruit and seed, and are more robust than the previous year. This is in part due to increased light, but also to the removal of mulch-like leaf litter and thatch from the ground. Fire also increases the available nutrients in the soil by indirectly stimulating microbial activity and releasing small amounts of nutrients from the ash. Fire in the fall or winter may even lengthen the growing season for grassland and woodland plant species (which do best in warm soil) by removing leaf litter and thatch, and exposing a darkened soil surface to the warming rays of the sun. In the absence of fire, the light-colored leaf litter reflects the sun, acting like a blanket, insulating the ground, slowing the soil-warming process, and smothering new seedlings.

Photos:

Top - Baker Prairie Natural Area in Boone County. Photo by Bill Holimon.

Bottom - Prescribed burn conducted during the 2017 Annual Prescribed Fire Workshop. Photo by Dustin Lynch.

 



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