Tall trees help identify old-growth forests, but so do numerous openings created by fallen trees. The downed logs don't go to waste; they slowly release their nutrients to the forest floor, feeding an explosion of new growth.
There aren’t many acres of old-growth forest left in Missouri. No remaining forest in Missouri is "pristine" in the sense of never having been affected by humans.
Fires by Native Americans and subsequent fires and open-range livestock grazing by European settlers have touched virtually every forested acre in the state.
However, there are forests that escaped the wave of harvesting and land clearing that swept though Missouri in the early 1900s. A few have experienced relatively little human impact since that time. Some tracts were left alone because they were hard to reach or because they had poor quality timber. Others simply had a string of owners who didn’t wish to harvest timber. Most such tracts are now publicly owned or otherwise protected from human disturbance.
The total area of old-growth forest remaining in Missouri depends upon how we define what is or is not "old growth." The oldest trees must be at least 100 years old, and 150 to 200 years is more commonly used as an age limit. But an old-growth forest is not defined simply by the age of its oldest trees. For example, even though they may contain some old trees, forests in which some trees have been regularly harvested are not considered old growth.
Old-growth forests require periods without severe disturbances, such as fire or drought. The absence of disturbances for long periods not only creates a forest with old trees, it also creates other characteristics associated with old-growth forests: gaps in the forest canopy where large trees have fallen, young trees growing either in the canopy gaps or in the shade of older trees, large standing dead trees and large fallen logs.
In Missouri there are about 62,000 acres of forest with some trees that are at least 130 years old. Fewer than 8,000 acres would be considered good examples of relatively undisturbed old-growth forest. Only about 800 acres would be considered excellent examples of old growth.
That works out to less than five hundredths of one percent of Missouri’s 14 million acres of forestland. These remnant old - growth forests are scattered throughout Missouri in tracts that range from about 15 to 300 acres in size. (See sidebar for places to visit old-growth forests in Missouri.)
We often envision old-growth forests being chock-full of giant trees. That’s not the case, though. There are typically fewer than five large, old trees on a given acre of Missouri old-growth forest, but due to their size they are some of the most memorable.
For every large, old tree in an old-growth forest there are hundreds of smaller, young trees. In fact, on dry, rocky, south-facing slopes even the oldest trees may remain small in diameter and short in stature due to the harsh growing conditions. The best opportunities to find any remaining undiscovered patches of old forest are on dry, south-facing slopes that are too steep to log and too dry to produce large timber.
Compared to the bustle of our daily lives, a forest - especially an old-growth forest - seems like a place where things never change much from year to year. However, old-growth forests are surprisingly dynamic. One characteristic always associated with old forests is the presence of scattered large gaps in the treetops. These openings can be 50 feet or more in diameter. The gaps are created when large trees fall over.
Trees can be weakened by old age, insects, disease and competition with other trees, but wind, ice or snow typically bring them down. Often a falling tree knocks down neighboring trees on its way to the ground. Nothing in the forest commands your attention as quickly and completely as the sound of a large tree crashing down on a wet, windy day. It immediately alters your perspective about how rapidly these forests can change and the physical forces that are involved.
The light that reaches the forest floor through gaps in the canopy promotes the growth of new trees. Initially there may be hundreds of small trees in a gap, but most will eventually be crowded out as they get bigger. Only a few trees will grow fast enough to reach the top levels of the forest canopy. It may take 50 years or more for the new trees to fill a large gap and reach the height of the surrounding trees. This is the process that allows old-growth forests to continuously regenerate themselves. It is also the reason that old-growth forests have many more small, young trees than large, old ones.
One thing that old-growth forests have in abundance is dead wood. It is easiest to see this in the winter when the leaves are off the trees. A single large tree can add more than two tons of wood to the forest floor when it falls. Fallen trees decompose slowly and retain moisture during dry spells.
Downed logs are beneficial to at least 35 species of birds, bats, snakes, salamanders, skinks and frogs.
Not all the dead wood is on the ground. In Missouri old-growth forests about one out of every 10 standing trees is dead. Some standing dead trees (called snags) are completely intact; others are just stubs left standing after their tops have broken out. They remain standing for years.
Snags and the cavities they contain provide room and board for a host of other wildlife species. The habitat created by snags and fallen logs is not unique to old-growth forests, but old-growth forests typically have more of it than younger forests do, and the large snags and large fallen logs in old-growth forests persist for many decades as they gradually decompose.
In time, high winds, heavy snow and ice damage all leave their mark on old-growth forests. Such disturbances are unavoidable over a century or two. Severe weather can rapidly change the appearance of an old-growth forest and add large volumes of dead wood to the forest floor.
A 1994 ice storm at Kirksville in northern Missouri severely damaged about 7 percent of the trees in the nearby Dark Hollow old-growth tract. Overnight that one event increased the accumulated dead wood on the forest floor by 25 percent. For many years to come that forest will be less prone to ice or wind damage because the most susceptible trees are already down, and the hardy ones survived. In time the forest will fill in the canopy gaps with new branches and new trees as it renews itself once again
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