WHAT IS A FOREST TYPE?
 

A forest type gets a name based on the dominant group of tree species.  So, an area of forest where most of the trees and/or most of the volume is "aspen", then the forest type would be named after the aspen trees.  Sometimes non-tree names are used to describe a group of tree species.  The best example would be "northern hardwoods", which represents forest types dominated by sugar maple, red maple, basswood, hemlock, beech, and others.  Trees are chosen for forest type names because trees are the dominant life form in a forest.  A more technical definition of a "forest type" can be found in the "Dictionary of Forestry" (J.Helms, 1998), published by the Society of  American Foresters.  ". . . a category of forest usually defined by its vegetation, particularly its dominant vegetation as based on percentage cover of trees, e.g. spruce-fir, longleaf-slash pine, Douglas-fir."  However, it is important to keep in mind that there are many other life forms that make up a forest ecosystem.  Forest management doesn't focus only on trees.  Entire systems and landscapes are taken into consideration. 

Michigan has 15 forest types, based on summaries made from U.S. Forest Service inventory data.  It is certainly possible to logically re-combine data and definitions to arrive at more or less than 15 forest types.  Forest types are one of the key considerations in prescribing forest management systems.  Due to many variations within forest types, and variations of management systems, there is no single way to manage either one particular forest type, or the forest in general.

Tree species diversity with a Michigan-wide forest type should be looked at carefully.  There are probably no single stands of any forest type that include all the tree species listed as occurring within that forest type.  In fact, with some forest types, many of the species may actually seldom occur together due to very different habitat requirements. 

How could this happen within a single forest type?  Some tree species have wide ecological spectra, meaning they can grow in a wide range of habitats.  Paper birch is a good example, growing commonly in both upland and lowland habitats.  Species associates often have more limited distribution.  In the case of the paper birch forest type, upland associates would include sugar maple, bigtooth aspen, white spruce, and northern red oak.  Lowland associates would include associates such as cedar, balm-of-Gilead, black spruce, and black ash.  However, the upland & lowland associates may seldom mix, but paper birch can dominate the forest in both habitats. 

A Note on Diversity

Composition has to do with species, taxonomy, and biological diversity.  The number of species and how they relate to each other according to taxonomic classes is a reasonably straight-forward concept.  Biological diversity, on the other hand, is a bit more slippery to wrap our minds around.  At first, diversity sounds like a simple species count and relative abundance of each species.  This is certainly a component of the diversity question.  It's also the easiest to identify and study.  However, the diversity of species is only one level of several.

At the most fundamental level of diversity, there is genetic diversity.  How many genes and pieces of genetic information are present in a forest?  The chlorophyll gene, for example, is common throughout most of the plant kingdom (although there are several variants).  Many other attributes or genetic characteristics are also quite common in a forest system.  Vertebrates have far more genetic commonality than genetic difference.  The loss of a species may not represent a loss of genetic diversity, only the loss of particular combination of genetic material.  The raw material will probably remain in the biota.  If you remove the word "the" from the English language, it would make our speech awkward, but it would not eliminate all words with the letters "t", "h", and "e". 

Species diversity is the next level of diversity.  These are the combinations of genes that we are most accustomed to dealing with in the life sciences and from legal perspectives (e.g. endangered species laws).  Yet, the definition of the word species escapes a single, concise, universally-accepted agreement.  Species abundance addresses the issue of how common a particular species is, often in the context of particular region or ecosystem.  There might be 100 species in a suite of characters.  However, 90% of the biomass might consist of only 3 species.  There may be a few species that are very uncommon, or have low abundance.  It is usually the species with low abundance that we are concerned about from the perspective of potential species loss.   It is usually the abundant species that we derive the most of economic base from.  

A collection of individuals of a given species make up a population.   The size, frequency, and distributions of populations are important elements of diversity.  A Canada lynx might be listed as endangered in Michigan, but across its range it is a common animal.  Sometimes populations on the edge of species range will display unique set of genes.  Northern populations of animals tend to have larger size and shorter appendages.  Flowering times of a tree species varies with climate conditions.  The conservation of distinct populations may be important in some cases.

With a given ecosystem, populations interact with other.   There are identifiable associations of species.  These species associations are called communities.   Community diversity is more difficult to measure in the landscape because there are usually a large number of components.  To make identification possible, key species are used to describe a community, such as "northern hardwoods", which is defined by such tree species as sugar maple, beech, and basswood.  However, northern hardwood associations in the western Upper Peninsula lose the beech component.  Community descriptions have an inherent degree of variability across a large geographical region.   This is another element of "diversity".

Lastly, there is something called "ecosystem diversity".  A collection of communities and the association physical factors make up an ecosystem.  Ecosystem diversity is commonly described in terms of biomes, eco-regions, and similar large-scale terms.  However, ecosystems can also be quite small.  The various communities within a rotting log are distinct from the surrounding forest.  That rotting log, or all the rotting logs in a forest, can be considered an ecosystem. 

While the above levels of diversity suggest a strong hierarchy, the classifications are less distinct in the natural world.  A considerable amount of flexibility and "confusion" exists.  Diversity is complex set of concepts, despite our understandable tendency to reduce it to a species level.

Tree Diversity Within Michigan Forest Types

There are many ways to categorize differences within the forest.  One of the more commonly used classifications is that implemented by the U.S. Forest Service in their Forest Inventory and Analysis Unit (FIA).  In Michigan, there are 15 forest types labeled according to their dominate tree species.  Species diversity varies naturally between forest types.  Human influence has also impacted species diversity.  Tree species diversity may, or may not, reflect the diversity of plant forms, but might serve well as a preliminary indicator.  Lastly, the FIA figures are for each forest type on a statewide basis.  Every stand of trees belonging to a particular forest type will not display the same level of diversity as other stands within the forest type.  Geography, stand history, soils, water, and other factors create variability within a particular forest type. 

The forest type with the highest number of tree species is the northern hardwoods, typified by maple, basswood, beech, and yellow birch.  Seventy-one tree species have been recorded within northern hardwood stands across Michigan.   It is the most common forest type in Michigan and continues to become more common as time passes.  The northern hardwood type is generally at the later stages of forest succession. 

The forest types with the least number of tree species is tamarack and black spruce.  These stands tend to be highly dominated by either tamarack or black spruce, with 21 other species occurring.  These stands typically grow on wetland sites where fewer tree species can survive.  However, these two species grow in many different forest types, as we'll examine further down this page.

The simple number of tree species is only one way of looking at diversity.  The idea of "species richness" is important, too.  Richness has to do with how dominant a few species are.  For example, two stands may each have 1,000 trees of 25 tree species.  In one stand, each species may have an equal number of trees, or 40 trees per species.  This would be a species "rich" stand.  The second stand may have 5 species that have 100 trees each, and the remaining 20 species have only 25 trees each.  This stand would less "rich". 

If we look at Michigan's 15 forest types using a tree species richness measure, the most diverse forest type is swamp hardwoods (elm-ash-cottonwood).  The least diverse type would be jack pine.

Yet another way to look at forest tree diversity is the species distribution across different forest types.  For example, sugar maple, beech, and basswood are largely restricted to northern hardwood stands.  They are seldom found in other forest types.  On the other hand, most of the white spruce volume is not found in white spruce stands.  The following table shows how much volume of a particular species is found within it's "typical" forest type.  Species with high percents may be good indicators of a single forest type.  Species with low percents are not very good indicators, but have the ability to survive across a wide spectrum of site conditions and forest associates.  This concept is commonly used to identify understory species, particularly wildflowers, that can be used to indicate certain site conditions and productivity.  Wildflowers are frequently better indicators than trees. 

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This site created and maintained by Bill Cook, MSU Extension Forester for the Upper Peninsula of Michigan, now retired.  He continues to respond to suggestions, questions, and corrections. Contact him at cookwi@msu.edu.