Succession & Forest Change

Succession: The gradual supplanting of one community of plants by another.

Note 1- The sequence of communities is called a sere, or seral stage.
Note 2- A sere whose first stage is open water is termed a hydrosere, one whose first stage is dry ground, a xerosere.
Note 3- Succession is primary (by pioneers) on sites that have not previously borne vegetation, secondary after the whole or part of the original vegetation has been supplanted, allogenic when the causes of succession are external to and independent of the community (e.g. accretion of soil by wind or water, or a change of climate), and autogenic when the developing vegetation is itself the cause.

-Society of American Foresters, 1998

Succession is one of the most important concepts in natural resource management. The fact that "nature" is always changing is critical in appreciating management systems and natural processes.

Succession is predictable if enough is known about a specific site and most of the factors that influence succession at that place and in that time. A series of vegetation types in a given area is a "successional pathway" or "sere". A single vegetation type within a sere is called a "seral stage". In Michigan, a forester or ecologist will usually be able make fairly accurate predictions of succession.

With forests, trees are the dominant life form and it is these associations of trees that give rise to the names of forest types. These types are often names of individual seres within a successional path. For instance, an aspen stand may be taken over by red maple and balsam fir. Decades later, sugar maple may become dominant. On richer soils, the number of potential successional pathways increases. On an infertile, dry, sandy soil, jack pine might be the only forest type that will occur. On a wet soil with microtopography, northern white-cedar might maintain itself for centuries.

The progressive change in forest types has a huge impact on the complement of wildlife species and understory plant species that live there. The forest type will also influence soil development, erosion potential, soil pH, organic matter volume, water retention, water quality, and similar forest characteristics. Forest types also have visual components that influence the way people perceive forests.

Successional change is not abrupt, but quite gradual. These changes will be modified by climate change. Some plants and animals are specific to a particular vegetation or forest type and are rarely present in earlier and later seres. Most plants and animals are

Case Study: Kirtland's warbler is small migratory bird closely and nearly exclusively associated with young, moderately open stands of jack pine while the birds are in the north for the breeding and summer season. Early logging, subsequent fires, and agricultural failure almost drove Kirtland's warbler to extinction. Through active management of jack pine age structure and stand size, the warbler has made a successful comeback. Managing jack pine succession was a critical element in bringing this bird back from the brink of extinction.

more general in their habitat strategy, often finding habitat needs in a variety of vegetation types. Most plants and animals can also get by through using their "second" or "third" choices of preferred habitat. Plants and animals that are very specific and narrow in their habitat needs are often indicator species of a particular condition or vegetation type. If these species occur in low number in few places, they are usually on either the federal or state endangered / threatened species list.

There are biological (biotic) factors and non-biological (abiotic) factors that drive succession.

Biological factors usually involve plants, but sometimes animals. In forests, trees are generally the primary biotic driver. However, keystone species, such as white-tailed deer, can dramatically affect forest processes. To understand how trees cause succession, you have to know about the habitat requirements for various tree species. The most important requirement, in terms of succession, is soil characteristics and a tree's tolerance of shade.

Animals influence succession in a number of ways, too. A major insect epidemic that kills trees, will usually setback succession to an earlier stage. High populations of white-tailed deer over a decade or more will selectively remove some species from a forest type. Crippling the successful regeneration of most (or all) tree species will have major impacts on the succession of plant communities.

Case Study Abiotic Factors: American beech distribution dramatically stops in the Upper Peninsula where soil types change from richer glacial deposits in the east to low fertility soils derived from granitic bedrock in the west. Most hickories, many oaks, sassafras, and sycamore are central hardwood tree species that only grow in southern Michigan's milder climate. On the other hand, the pines and spruces are more adapted to the colder climates and soils of northern Michigan.

Abiotic factors are such things as soil types, moisture levels (swamp vs. upland), weather, and climate. Red pine / red oak forest types grow on sandier, well-drained soils. Cedar, black spruce, and tamarack types typically grow in swamps. Weather impacts succession in the form of windstorms, droughts, late spring frosts, etc. Climate differs from weather in terms of time and geography. Climate change generally occurs over very long periods of time and across large regions. Weather is more variable from year to year and has more localized impacts.

Fire is a particularly strong abiotic factor in succession. Some of our forest types have adapted to regular wildfires. Minnesota forests bordering the prairie are almost entirely comprised of forest types adapted to frequent fires. Frequent fire and a drier climate have resulted in a forest with fewer tree species and forest types. In the Upper Peninsula of Michigan, many forest types have developed where wildfire is often less frequent and average rainfall is higher.

A young aspen stand lacks much height but has many stems per acre. Because aspen is intolerant of shade, we know something catastrophic occurred about 10-15 years ago, maybe a harvest or maybe a windstorm or fire. In any case, the aspen is regenerating well. There is often a diversity of other tree species, such as black cherry, oak, and paper birch. The high number of stems provides good breeding and escape cover for animals such as rabbits and grouse. Deer heavily browse the young trees. The vigorous young trees actively transpire large quantities of water and produce much more oxygen than they use. Beavers and broad-winged hawks prefer this forest type.

As the aspen ages, the trees will thin themselves out and the forest will become taller and less dense. Grouse love the more mature flower buds but will prefer to raise their young elsewhere. The aging aspen will provide uses for red-eyed vireos, woodpeckers, and maybe some owls. Mature aspen allow a fair amount of light to reach the forest floor, so there is still an actively growing understory. On sandy soils, hazel may be common, whose nuts are important food for many animals. On heavier soils, there may be buckthorn, Juneberry, and viburnums. It's also likely that the seedlings from more shade tolerant tree species have begun to grow. They will make up the next forest type.

On sandier and drier soils, the next forest type might be a mix of white or red pine, oak, and red maple. On heavier soils, the new generation might be sugar maple, balsam fir, and white spruce. There are about 35 tree species in Michigan aspen associations, second in diversity to only northern hardwoods. Left unmanaged, over a number of decades, the aspen will eventually die out. The next type might be a white pine-red oak association, or a northern hardwood stand. In wet soils, cedar might become dominant.

Activity Suggestion: Research when and where a recent forest disturbance occurred near you. Was it natural? Was it human-caused? What happened? Visit the site if you can. What tree species are growing in the disturbed area that are not growing in the adjacent undisturbed area? What tree species are more abundant in the disturbed area? Why is this?

A successional path spans a long time, from the human perspective. Over the course of time, it's quite likely the stand will experience some form of disturbance. Disturbances occur from natural causes, such as wind, fire, pest infestation, or it can come in the form of timber harvest. In either case, the course of succession is altered. With forest management, the manipulation of succession is intentional, with a set of goals in mind, ideally within the context of a greater landscape.

Disturbance is essential to the regeneration of many tree species. Jack pine and paper birch were largely dependent upon wildfire for regeneration. Successful fire suppression programs have created a management dilemma for these species that forest scientists needed to overcome. Northern hardwood (sugar maple, beech, basswood, and others) stands need small scale disturbances to create "holes" in the forest canopy to regenerate many species and maintain higher levels of species diversity. Selection harvest and thinning complements this natural process to produce more forest outputs in a shorter period of time.

We typically think of succession in terms of current climate conditions and a time frame of just a century or two. Climate is one of the main drivers of succession but is not constant over the millennia. Climate change studies have demonstrated a progression of widely different successional regimes.

The climate of Michigan has varied considerably since the continental glaciers receded 10,000 - 12,000 years ago. For centuries during the recession, the climate was cool and moist. Boreal species and plant community types extended into Indiana and Illinois. Relict populations of boreal types can still be found scattered across the normally more temperate climates.

During the current post-glacial period, climates have varied considerably from we experience now. There have been cooler moister periods where our forests had much stronger boreal characteristics. There have also been warmer drier periods where much of Michigan was prairie. That's part of the reason why Michigan still has a few prairie remnants.

These changes occur over centuries. Current levels of climate changes are happening over decades. This rapid increase in change will likely disrupt many forest dynamics and the species that depend upon forests, including humans.

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