Canadian County OSU Extension Service

Fire Effects in Native Plant Communities

If you have been watching the news lately you probably have heard about the burn bans across the state as a result of the dry conditions, high winds, and the amount of dead plant material on the ground. This is furthermore evidenced by the black patches of burnt grass that you see along roadsides, the smoke you may occasionally see on the horizon, and the news which is always quick to inform us of danger and damage of the fire.

Oklahoma lies in the Great Plains and therefore has had a history with wildfires, as they were what maintained the landscape long before cities were established. 

However, today most of Oklahoma’s native prairies, shrublands, and forests are out of balance because of fire suppression. A good example of this is the invasion of eastern redcedar and ashe juniper into prairies, shrublands, and forests throughout Oklahoma. Redcedar invasion is one readily visible indicator of poor land management and ecosystem dysfunction. Another example is densification of forests in central and eastern Oklahoma. Historically, periodic fires thinned the forests and helped maintain open woodland or savannah-like conditions.

Since natural fires no longer happen as frequently, prescribed fire is an ecological driver that can be used to restore ecosystems and landscapes to their historical diversity and productivity. It is essential for maintaining wildlife habitat including endangered species, water quality and yield, livestock production, timber production, parasite and disease control, wildland fuels/wildfire risk reduction, and ecosystem function. Prescribed fire also can be used to enhance the appearance of forests, shrublands, and prairies. It can also be used to create savannas, a mixture of scattered trees and native prairie. Fire also controls woody plant invasion into prairies.

Much of a plant’s adaptation to fire is determined by its growth form, bud location, or bark thickness. Another effect is related to the timing of the fire relative to the plant’s growth cycle. These effects are further confounded by the interaction of previous management, previous climatic patterns (i.e. drought), and previous intensity and duration of herbivory.

For example, bunchgrasses like little bluestem accumulate dead material above the root crown and the center of the plant dies over time. After a fire, it sometimes appears that the plant was killed when in fact the center of the plant was already dead. In contrast, rhizomatous grasses, such as big bluestem, have growing points below the soil surface and do not accumulate fuel next to the root crown.

Woody plants are adapted to fire by location of buds or protective bark. Most woody plants resprout if top growth or apical buds are killed. Once apical dominance is lost, dormant basal buds below the soil surface begin growth. Some woody plants such as eastern redcedar lack basal buds and do not resprout. The absence of the resprouting adaptation suggests that eastern redcedar did not develop an evolutionary adaptation to fire. Many woody plants have thick bark and are adapted to intense fire. Eastern cottonwood, post oak, and shortleaf pine are examples of fire tolerant woody plants. Shortleaf pine is one of the most fire adapted coniferous plants and one of the few conifer species that resprout after being top-killed. Woody plants greater than 8 inches in diameter at 4.5 feet above the ground generally do not resprout. Some woody plants like sumac (Rhus spp.) have rhizomes and basal buds that are an adaptation to fire and herbivory.

The frequency, intensity, and season of the year when a fire occurs are second only to precipitation’s influence on vegetation. Fire frequency in Oklahoma, prior to settlement, ranged from once per year to once in 30 years. The return interval of fire depended primarily on fuel load (a function of precipitation, soil type, and herbivory), topography, and the location of fuel breaks such as rivers, streams, and rock outcrops. Fuel breaks were also created by bison grazing patterns, prairie dog towns, and previous fire set by Native Americans. Fire frequency increases when continuous herbaceous fuels and unbroken landscapes allow fire to cover large areas.

Fire intensity affects plant response to fire and is often used in the management of woody species. The bark of older trees and shrubs commonly insulates the plant from the heat of low-intensity fires, but smaller stems and seedlings are killed. Therefore low-intensity fires in wooded areas will cause the vegetation to begin to shift toward a savanna appearance. High intensity fire, however, can top-kill the larger trees. Woody plants that are capable of resprouting usually do so vigorously following fire. Higher intensity fires in wooded areas may shift the vegetation toward a sprout thicket if mature trees are top killed. The response of woody plants to fire is primarily a function of species, size class, topographical position on the landscape, and fire history.

The timing of the burn affects plants differently depending on a variety of factors including season of growth and stage of growth relative to the fire event. Plants can generally be grouped into two basic categories: cool-season plants that grow during the winter and/or spring and mature in the spring and warm-season plants that start growth during the early summer and mature in the fall. Plants are most susceptible to the effects of fire when the plants are actively growing. Cool-season plants are more susceptible in late winter into spring and warm season plants are most susceptible during the late summer into early fall before dormancy. Annual plants, which have a life span of one year, are most susceptible to fire when they are actively growing and before they have dropped their seed.

None of this information justifies burning during a burn ban, it is only to shed some light on the ecological effects that are occurring the next time you hear about a fire. 

Until next time - STOP, LOOK, and ENJOY!

~ Casey

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