Forests in Flux
to attract game, increase agricultural use or defend themselves. With settlement, fire occurrence increased with increasing numbers of people but the severity of fires decreased with increased levels of fire control.
Forests respond to fire in one of two general ways. Low intensity, repeated fires can create and maintain a thin canopy of fire resistant trees with an understory of fire tolerant grasses and forbs. Years of this type of fire can result in what many have called savanna or woodland.
A second type of response would result from more intense or less frequent fires. This would have a regenerating effect on a forest. After years of good growing conditions without fires to reduce fuel buildups, and with the proper dry wind and fuel conditions, a fire can kill all trees in a forest. Severe fires can destroy ground litter and humus and set the forest back to the earliest successional stage. Fire prepares the soil for germination of fire tolerant species, such as fireweed and shortleaf pine, and actually encourages pine seed to be released from cones.
Another fire disturbance could be human fire protection, resulting in unnatural forests growing in the absence of frequent fires and suffering from the less frequent but more severe fires, which are almost entirely human caused.
Along with fire, wind is the natural major disturbance process in upland forests of Missouri. Wind occasionally comes along in the form of a tornado or severe thunderstorm, knocking down acres or hundreds of acres of trees, returning the site to an early successional stage.
Of the 650 reported tornadoes in the United States each year, an average of 30 are in Missouri, which occur mostly in April, May and June. The average tornado path is 800 feet wide and five miles long, with a maximum of one mile wide and 200 miles long. The average tornado could affect 480 acres; an average tornado year in Missouri could affect 15,000 acres. To look at potential effects another way, in the last 200 years (the average life span of a tree in a "climax" forest), almost 3 million acres could have been damaged by tornadoes.
More frequent are the winds which, helped by saturated soils, blow down individual trees or small groups, most commonly less than an acre. Most of Missouri experiences an average of 50 to 70 days per year with thunderstorms, exceeded only by the southeastern United States. Wind directly affects trees