Fire Ecology

Fire Effects on Plants

Fire Effects on Plants

Much of a plant’s adaptation to fire is determined by its growth form, bud location, or bark thickness. Another affect is related to the timing of the fire relative to the plant’s growth cycle. These affects are further confounded by the interaction of previous management, previous climatic patterns (e.g. 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. This can be observed by examining burned and unburned plants in the same area. In contrast, rhizomatous grasses, such as big bluestem, have growing points below the soil surface and do not accumulated 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 is 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. Frequent fires generally favor herbaceous over woody plants for a variety of reasons including growth form and location of meristematic tissue. 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. High intensity fire, however, can top-kill the larger trees. Woody plants that are capable of resprouting usually do so vigorously following fire. Low-intensity fires in wooded areas will cause the vegetation to begin to shift toward a savanna appearance. 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.

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