Looking Into Forest Past and Future: Eastern Deciduous Forest Succession

Red Cedar trees that have invaded a dry prairie in Eastern Iowa.  Cedars require full sun and will likely be succeeded by Elms, Dogwoods, and Black Cherry which are more shade tolerant.  

Do you ever wonder what a forest used to look like decades ago?  Or do you wonder what a forest will look like in the future?  The key to finding these answers lays in determining what stage of succession the forest is currently in.  Previous posts on Eastern Deciduous Forests of Iowa the concept of succession has been discussed several times.  Simply put, succession is the progression of one group of species to another and to another and again, until a climax community is reached or disturbance starts the process all over again.  Anyone can observe this process in an old abandoned field or even garden.  First herbaceous plants colonize the area, followed by shrubs, then trees, and lastly different types of mature trees.  For those who know what to look for in a forest, a little knowledge can tell you a lot about what a forest was like in the past and what it will be in the future.  

An early succesional stage, or pioneering stage, forest composed of Willow and Cottonwood.

All plant communities start with "pioneer" species.  These are species that first colonize a plot of land and are typically herbaceous like weeds and grasses.  Anywhere from years to decades after herbaceous plants are established, woody plants will begin to colonize.  The first woody plants established require full sunlight, are usually quick growing, and often short lived.  Sumac and Aspen are two good examples of early succession trees that require full sun, are quick growing, and are short lived, usually only living for decades.  Other early succession trees such as Red, White, and Bur Oaks, as well as Red Cedar, also require full sun for establishment but are much longer lived, often living for centuries, and are often slow growing.  Other common early succession trees are Hackberry, Ash, Cottonwood, Willow and Birch.  So if you are hiking and find a grove with some of these trees, you know it is rather young and will become a mid-successional forest in decades to come.

An older pioneering forest composed of primarily of Ash.  There are some smaller younger Oaks growing in the shade of these Ashes.  The Oaks will one day replace the Ashes and this will likely become an Oak-Hickory forest.

Unfortunate for these pioneering plants, they eventually grow large enough to actually shade themselves out.  Pioneer plants require full sunlight and cannot grow in shady environments.  For this reason these plants will not reproduce and instead more shade-tolerant trees will replace them.  Often in early successional forest, if you look at the younger trees growing in the shade you will know the future of the forest.  These smaller younger trees likely will replace the larger trees they grow under.  In Iowa and other parts of the Midwest, Oaks, Walnut, and Hickory trees are more shade tolerant and will eventually replace the pioneering plant community.  Yes, Oaks can be both pioneering and mid-successional depending on the environment.  Longer lived early successional trees such as the Oaks and Red Cedars can also survive into the mid-successional forest.  This middle aged forest also begins to form a more complex structure with a canopy of mature trees, and understory of smaller trees, a shrub layer, and ground cover.  This layering is present in pioneering forests also but becomes more defined with age.  Dogwoods are common shrubs with Ironwood and Elm being common understory trees in these forests.

Again though, lurking in the understory may be trees that will soon become the next successional stage, also called the climax stage.  If you are in an Oak-Hickory forest and find an abundance of small Maple trees growing in the shade you can know that in several decades this will become a Maple forest.  Maples and Basswood are very shade tolerant trees.  If an Oak-Hickory or other type of forest becomes overly shady the Oaks and Hickories will not be able to reproduce being they require sunlight to grow.  Maples and Basswood are quite at home growing in the shade and will slowly grow until the larger trees die or are shaded out.  In Iowa the Maple-Basswood forest is the most stable forest type because these trees are able to continue reproducing themselves, even in their own shade.  So, without any disturbances the Maple-Basswood forest is very stable and will grow indefinitely, and for this reason they are called a "climax community".  Another community of trees will not replace this community until fire, floods, logging, disease, drought, or storms disturb this community, killing off the trees.  Once the community is disturbed we start over with the original pioneering, early successional stage.

Pioneering Stage: Shade intolerant trees 

Mid-successional Stage: Partial shade tolerant trees

Climax Stage: Very shade tolerant trees

Disturbance during any one of these stages starts the process over at pioneering.

This grove of Bigtooth Aspen and Sumac are in the middle of an Oak forest.  It is likely that this area was an Oak forest that was disturbed by logging, which caused the pioneering Aspen and Sumac trees to become established in the full sunlight.

Not all areas will reach a Maple-Basswood climax.  Areas with sandy dry soil will often climax with an Oak forest.  And areas that are frequently disturbed with low burning ground fires will also climax with Oak forests.  Maples and Basswoods are easily killed by fire but Oaks have thick bark adapted to surviving ground fires.  For this reason, Oak forests were the most common type of forest in the uplands prior to settlement in Iowa.  Pre-settlement prairie fires were very common and would have killed trees not adapted to fire but allow fire adapted species such as Oaks to survive.  It was only the wetter areas, typically the bottomland forests along rivers, which were too wet for fire where the Maple-Basswood climax would have been found historically.

This mature Oak forest at the Indian Creek Nature Center near Cedar Rapids, Iowa, is managed by having controlled burns every few years.  These surface fires kill trees not adapted to fire such as Maples, Basswoods, or Hickories.  Oaks however, survive due to their thick bark.  

How to look into a forests past and/or future
1. What types of trees are present in the canopy?  Are they pioneering, mid-successional, or climax species? Use the list of trees in the descriptions above to find out.  You can also use this list of trees by shade tolerance to help you find out.
2. What types of trees grow in the understory?  If they are Oaks, Maples, or Basswood they will eventually replace the current forest type.
3. Look for evidence of disturbance.  Do you find old stumps as evidence of logging?  Dead trees as evidence of flooding or disease?  These things should result in a return to a pioneering forest.  Or do you find charred woody materials as evidence of burning?  This should result in an Oak forest or savanna and the absence of other species.
4. Where are you in the landscape?  Uplands or bottomlands?  Is the soil sandy and therefore dry or have higher clay content and therefore wetter?  Drier soils will more likely climax with Oaks, especially if fire is present.  Wetter soils will more likely climax with a Maple-Basswood forest.

So along with simply enjoying the magnificent trees you hike through, with this information you can hopefully do a little detective work.  Often this process can be difficult but it sure can add a lot to you knowledge of the forest making you hike a little more interesting.