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Wisconsin Natural Resources magazine

Wisconsin Natural Resources magazine

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A natural disturbance: The blowdown of an old-growth hemlock stand at Kemp Station in Oneida County. © JMAR Foto-Werks
A natural disturbance: The blowdown of an old-growth hemlock stand at Kemp Station in Oneida County.
© JMAR Foto-Werks

October 2004

More than big trees

A complex ecological system.

Karl Martin, Natasha Kassulke and Tony Rinaldi


Some people think of old-growth forests as large moss-covered redwood and Douglas fir trees in the Pacific Northwest. But depending on where they are growing, old-growth trees can occur in a great variety of sizes, shapes and ages from short twisted wind-swept mountaintop bristle cone pines that are thousands of years old, to lofty centuries-old hemlock in northern forests.

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In a Wisconsin bog, a stand of black spruce can be hundreds of years old, yet the trees may measure only an inch or two in diameter. Certain short-lived tree species like aspen and jack pine also get relatively old (100 to 150 years), but they are generally not thought of as developing into old-growth.

Old-growth is sometimes thought of as park-like in appearance with towering crowns and a shaded open understory. However, the size and age of individual trees within an old-growth forest are dependent on natural past disturbances that create openings in the canopy where sun reaches the ground and allows new growth. Much of what was old-growth hardwood-hemlock forest in Wisconsin probably had this broken-up appearance with small pockets of new growth developing as the overstory trees died or blew down, singly or in groups.

With such geographical and ecological variety, it is easy to see why a single definition of "old-growth" has not been generally accepted. Common characteristics include the presence of large trees, multi-layered vegetation, canopy gaps, large snags and cavity trees (standing dead trees), and large fallen logs and limbs (collectively called coarse woody debris).

Other terms used include natural appearing, unmanaged look or over mature when talking about old-growth.

While any of these features can occur in young forests, only in old-growth or older forests managed specifically for these characteristics do they occur simultaneously.

Old-growth forests are extremely diverse. Old-growth can have old and young trees growing together in a mixture of species or single species such as red pine. They are often covered by a thick growth of mosses and lichen, which provides a home for insects, birds and small mammals.

Insects and woodpeckers open up the dead wood providing habitat for many other species.

Fallen trees may form pools in streams holding woody debris long enough for much of it to be utilized by insects and animals. These small pools provide fish shelter and attract insects that are an important food source.

Old-growth once described as a "biological desert," unhealthy and unproductive, is now considered a very complex and productive ecological system. With natural disturbances – wind, fire, and insects – at work, old-growth forests are places of rebirth as well as death. "Cradle knolls" and "nurse logs" describe the seedbeds created as trees tip over and expose mineral soil, and where rotting logs provide a microclimate suitable for yellow birch and hemlock seeds to germinate and grow.

Standing cavity trees, large snags and downed logs occur in commercially managed second-growth forests, but are often larger and more abundant in old-growth.

In other regions, certain wildlife and plant species depend on old-growth habitat. That is not the case in the glaciated and relatively young upper Great Lakes landscape, but the diverse habitat provided in old-growth certainly promotes a diverse and somewhat unique wildlife and plant community.

"Old-growth forests in the Great Lakes Region may not contain unique species like spotted owls, but they do contain a unique assemblage of plants and animals," says Gerald Bartelt, chief of the DNR's Wildlife and Forestry Research section.

All forests are dynamic, always changing from one stage to another, influenced by natural forces and humans. Changes can be subtle and occur over long periods, or can happen in seconds from a windstorm or fire.

Natasha Kassulke is the associate editor of Wisconsin Natural Resources magazine.