Forests in Flux
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.
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.
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."