The Climax Forest

Mature climax Forest, Yellowstone National Park

To some, a climax forest is a nice name for forests that have escaped disturbance by outside forces like storms or diseases or logging long enough to have settled into a condition of maturity and stability. But to others hearing the name, climax forest, makes them cringe.

Forest succession, explains how forests change. Trees and other plants grow, mature, reproduce, age, and die. Soils build and erode. Weather changes daily, and the climate varies over long periods. Animal populations come and go. With time, these sorts of changes bring still other changes that go beyond the size of the trees to involve shifts in the species and mixes of trees, shrubes, grasses and forbs growing in the forest.

The first stage after fire is composed of various annual herbs and grasses. The seeds provide food for birds and small mammals, such as chipmunks. As succession continues, willows and aspen sprouts dominate and form the habitat of the moose Greater Yellowstone’s shrub browser and early-succession-stage animal.

In Yellowstone the vegetational composition and environment baseline of sagebrush, lodgepole pine, and intermediate sites were studied in Yellowstone National Park; lodgepole pine encroaches on sagebrush areas. Young lodgepoles have little influence on the environment or vegetational composition of the sagebrush; however, when lodgepole pine was mature the shade-intolerant species, including sagebrush died out, due to lack of sun, and shade-tolerant species displaced them. In the mixed conifer forest, the environment becomes optimum for conifers.

Aspens grow as an intermediate succession species. It may be set back by fire, but aspens, being resilient suckers, come right back. When succession is not interrupted, aspen groves serve as the nurse habitat for the spruce trees that if left undisturbed, will replace the aspen and evolve into a spruce forest.

Aspen Trees

Forests of all kinds exhibit layering known as stratification, from forest top to ground level, with each layer having different ecological environments. Each of these layers is home to an endemic assemblage of animals, birds, insects, etc. First, the forest top zone, with large amounts of light and air movement, where certain birds and butterflies spend their lives; second, a mid-zone of tall deciduous trees whose broad leaves capture most of the remaining sunlight and require the high humidity of the still air of the mid-zone; third, a shrub zone that develops best where sunlight has penetrated to near the forest floor; finally, an herb zone on the forest floor. The herb zone, where almost no sunlight penetrates, has saprophytic plants (a plant, fungus, or microorganism that lives on dead or decaying organic matter) adapted to this situation. They obtain all their energy requirements from decaying forest litter.

Eventually, the mixed conifer forest becomes taller, shading out most of the shrub community. Once the conifer forest is established and the earlier shrub community eliminated, forest litter accumulates to a foot or more in depth, composed of needles and small twigs from the lower conifer limbs that die due to shading by the upper limbs.

As the successional shrubs drop out, the ungulates population (Elk, Deer, Moose) also decreases, as there is less for them to eat. As the conifers reach mid-heights, the mid-forest stratification zone forms. Regular light maintains the climax forest at a near-steady state ground-fires that eliminate heavy fuel buildup also provide for new conifer seedling development without the earliest successional stages. Fire is necessary for conifer seedlings to survive the first year of growth. Seeds that fall on decaying forest litter never survive beyond a few months because the seedling roots must penetrate the forest litter containing abundant fungi that promptly destroy the new conifer rootlets. Freshly burned forest floors are clear of litter and sterilized by the heat, after fire seedlings and survive.

Browsing Bull Elk

These nurse shrubs provide shade and a zone of higher humidity for the young conifers. At this stage, the ungulates population increases exponentially due to the increased shrub population. Ungulates being browsers find shrub tips a valuable food source. Ungulates migrate up through the different forest zones following the early shrub tip growth. At the shrub successional stage, large numbers of ground squirrels, chipmunks, and other rodents occur. These rodents are preyed upon by those higher in the food chain, owls and such, and their numbers increase with the increased rodent supply as a food source. Predator populations fall in the climax forest, because rodent numbers are small. Wildlife types change as habitat changes. Open areas with grass attract large grazing animals such as bison, elk, deer, and pronghorn antelope, as well as smaller prairie animals including ground squirrels and pocket gophers. However, as brush and trees begin to grow, the type of wildlife changes. The Lewis & Clark expedition nearly died in the climax forest crossing western Montana and Idaho, as there are birds, but no game.

Some animals can live and breed in young forests and others in mature woods and forests. There usually is an overlap of each species. For example, cottontail rabbits can live in fields, shrubby areas, or young woodlots with dense undergrowth. However, when larger trees shade out lower growing brush, rabbits will move out to find safer cover and those that eat them follow.

The lodgepole pine has a relatively short life span and is a temporary successional stage, coming in much like a weed when the climax forest of the area is removed. Recently, massive lodgepole forest destruction has occurred due to devastation by the pine beetle. Not all lodgepole trees are destroyed if the forest has multiple-aged trees. The tree deaths act as a thinning effect so that a few trees have room to grow larger.

Crown Fire, Yellowstone National Park

A climax forest can be reduced to bare land by a hot forest fire and periodically should. The energy released during a major forest fire is equivalent to an atomic bomb explosion. The atomic-bomb-like smoke plume often reaches sixty thousand feet and is called a potboiler fire by foresters. The bare land afterwards is soon into the beginning stages of forest succession. During the first year, a few annual flowering plants occur, and the biennials and perennials have germinated. During the second year, the biennial herb fireweed occurs in large numbers, with a few other herbs present. During the third through tenth years, various shrubs fill the space entirely. Conifer seedlings, which require shade in early age, develop beneath the shrubs.

Ecologists have been debating these ideas for nearly a hundred years, and they continue to study the specific mechanisms of change. But, while nobody seems to disagree about forest succession, many have a problem with assigning the name “climax” to the later stage. Somehow, perception of the climax forest came to mean an endpoint of the successional sequence. It was the final stage; once attained, it maintained itself forever. It came to carry the connotation that climax is inherently good and somehow better than early successional stages and this isn’t necessarily so.

Succession from bare land to climax forest requires several hundred to a thousand years. Forest fire plays an important role as forest health maintainer when mild, and a catastrophic successional setback when excessively hot.

We have learned ungulates move out of climax forests too feed so we must conclude that old growth alone doesn’t make a forest. A healthy ecosystem will have all stages of succession in order to provide for all the creatures we love to see in our forests. This couldn’t be achieved without fire.