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Forests in Flux

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Published on: Jun. 2, 1996

Last revision: Oct. 21, 2010

What does a perfect forest look like? Most of us, if asked that question, would probably think of an open woodland with towering, 300-year-old trees. When we talk about forest conservation, that is the forest we want to save from destruction.

Forests begin with a few plants and go through the process of "succession," ultimately arriving - ideally - in a climax or old-growth state. But disturbances like fires, tornadoes or diseases crash the party, interrupting succession and creating something far different from the perfect forest with the 300-year-old giant trees.

During most of the past century, ecologists have known and talked about forests as communities continually changing toward a "climax" condition. This climax forest was thought to be as perfect as a forest gets. It fully occupied all the sites and niches available for occupation and made maximum use of environmental resources. A climax forest was considered to be at the peak of productivity, which it maintained in a condition of a dynamic equilibrium, or steady state.

The Climax Forest

Under ideal conditions, each successive collection of plants and animals would move into a forest and dominate available space for a few years or a few centuries and be replaced by a later succession of organisms. Plants in each stage would change environmental conditions to the point that another group of plants was better able to grow there, before giving way to these new plants.

Changing environmental conditions include shading the ground, accumulation of organic matter and moisture and different forest animals and microorganisms. Succession would continue toward a climax forest. A full array of species would be present in this climax forest - more species diversity than in any of the pre-climax or successional forests. The climax forest would be relatively stable. Sure drought, flood or wind would come along from time to time, but their effects would be temporary. The forest would soon recover.

Human disturbances were thought to have more serious and long lasting effect. Cutting, burning, land clearing, use of herbicides, pollution and erosion are more frequent and severe; they are outside the range of tolerance of the natural forest and would scar the forest forever. But if a forest was unaltered by human presence, it would eventually develop into a dynamic equilibrium - a climax forest.

The Disturbed Forest

Other ecologists have a somewhat different view of a forest. Rather than succeeding toward a climax steady state, forests are continually set back to an earlier successional stage. All forests exist in a state of perpetual disturbance.

In this view of a forest, succession may proceed toward the climax forest, but the true climax is seldom, if ever, achieved. Disturbances to forests are so many in number and so frequent in the forests of the midwest that there may be no such thing as a perfect, stable forest.

Which forests did the first explorers and settlers in Missouri see? Probably more disturbed forest than climax forest. The same natural disturbances are present today. Some of these disturbances are quite obvious, while others are much more subtle.

Flood, Erosion, Siltation

In 1993 and 1995 thousands of acres of forests were under water along the major rivers.

Less obvious are the thousands of less severe flooding events that occur after smaller, more localized thunderstorms. Ice dams, beaver dams, rivers changing course and man's activities also cause localized flooding, erosion and siltation.

The impact of a flood on a forest can be minimal or major. Floods carry floating wood which batters, scars and otherwise physically disturbs growing vegetation.

Flood waters deposit gravel, sand and silt as waters slow down and remove various particles and solutions. Flood waters may warm or cool the soil or displace soil air, affecting growth of plants. Flood waters typically contribute seed to a regenerating forest. Flooding rivers can change channels, leaving behind sloughs, marshes and poorly drained land. The mere presence of water or saturated soil has an effect on germination of seeds and growth and development of plants.

Some species can tolerate high levels of moisture for long periods of time, while others will disappear after a few days. The effects of flooding can range all the way from total destruction of a forest, reverting it to the earliest stages of succession, to minor changes in species composition or the death of individual trees.

Fire

The effects of fire on forest are well known. Much was written about the Yellowstone fires of 1988 and the numerous fires of the western United States of 1994. Missouri has its share of fires today, and has had fires for the past thousands of years.

Natural fires in Missouri are rare - Missouri forests experience less than one lightning-caused fire per million acres, compared to almost 40 in parts of Florida, California and the mountains of Idaho and over 60 in areas in Arizona and New Mexico.

Indians reportedly burned the forests of what is now Missouri 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.

Wind

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 by uprooting them or breaking their trunks or branches. Usually, the understory and ground layer are relatively undamaged. The removal of the larger trees in a forest releases understory vegetation and may speed up or slow down succession.

Wind disperses seeds over long distances and, when coupled with the creation of open growing space, may greatly change the species composition of a forest. Commonly seen in Ozark forests, old uprooted trees of years gone by will leave mounds of bare soil in the opening created by the fallen tree that provide good germination and growing space for pioneering species. The long term indirect effects of wind include mixing the soil, allowing buried seeds to germinate, abrasion between branches of adjacent trees and a buildup of woody fuel for a future severe fire.

Ice storms

Missouri averages 5 to 15 ice storms each year. Although most are serious only for highway travelers, ice occasionally plays havoc with trees. Like wind, ice can break branches and trunks and release smaller under-story plants below. Some species tolerate ice more than others, so ice storms can have a great effect on species composition.

Drought

Periodic drought directly affects trees by weakening or killing some species and releasing others. The skirmish line between a glade and its associated dry forest community gradually moves in favor of the forest in good years, and in favor of the glade in times of drought. Periods of drought alone would maintain some dry woodlands and glades, but usually predisposes a community to fire.

Mammals, Insects and Disease

Animals can influence the species composition of a forest. Large grazing and browsing animals, such as deer, beavers and domestic livestock, can have serious direct impact on a forest. Smaller mammals seem to favor seeds. Beavers build dams and can cause localized flooding, further changing the mix of tree types in a forest.

A handful of imported insects and diseases have had a tremendous impact on the forests of the Eastern United States. The chestnut blight totally wiped out the American chestnut from millions of acres and is currently endangering the Ozark chinquapin. Dutch elm disease greatly reduced the abundance of elms in bottomland forests throughout Missouri. The gypsy moth isn't here in force, yet, but it will probably affect our forests.

Other natural disturbances

Several other types of natural disturbances, which may or may not operate in Missouri, affect forest land throughout the United States. These include landslides, lightning, avalanches, glaciers and volcanos. You don't see glaciers in Missouri today, but they had a profound effect on the topography of North Missouri about a million years ago and on species composition of forests as little as 10,000 years ago.

The 1980 volcanic eruption of Mount St. Helens resulted in the removal of the top 600 feet of the mountain in a few minutes. This dispels the notion that human caused disturbances are always more severe than natural ones. Effects of lightning are frequent but small in size; landslides may be infrequent, small in size, but severe.

The climax or perpetual disturbance forest - which do we have in Missouri? Perhaps some of each, but little of the former and much of the latter. Ecologists studying climax forests in Michigan and New Hampshire in the 1940s called the climax forest "a phantom, always moving ahead in the future and becoming visible for only relatively brief periods on small areas."

 

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