Hardwood forests of the Great Lakes region

Hardwood forest tree species common across the Great Lakes region include such broad leaf deciduous species as sugar maple, basswood, oak, beech, birch and aspen.  The particular type of hardwood forest found in any given location is determined by many factors including the climate zone, soil types and land-use practices. In the cooler, more northern parts of the region we often see these hardwood species mixed with conifer or evergreen tree species such as spruce, fir, and pine.

forest of large mature trees with thick green forest floor vegetation
One of the most concerning aspects of earthworm invasion is the question of long-term effects to hardwood forests. In this beautiful sugar maple dominated forest, there are virtually no seedling, sapling or sub-canopy layers. As the canopy trees age and begin to break up, what will replace this forest?

While earthworms can invade all of these forest types, each forest type has the potential to support different earthworm populations because of differences in the amount and nutritional quality of the litter that each produces. Field and laboratory based feeding trials have clearly demonstrated that when given a choice, earthworms will be very selective in the types of litter they consume. The nightcrawler, L. terrestris, will eat basswood leaves first, and then sugar maple and only when all of those are exhausted, then they will eat the oak leaves. This order of selection closely mirrors the nutritional quality of these leaves. Also, the more litter there is the more earthworms there will be. So, the type of forest and the litter it produces largely control the size and species composition of the earthworm population.

The size and species composition of the earthworm population in a forest also affects the magnitude of impacts we see in the forest. Sugar Maple and basswood dominate forests appear to be the most negatively impacted by exotic earthworm invasion with the biggest changes in soils, forest floor, and understory plants. Oak and beech forests (many of which have a sugar maple understory) support smaller earthworm populations, but still a large enough and diverse enough population to have noticeable impacts. In contrast, mixed aspen-spruce-fir forests of the north appear to support relatively small and less diverse earthworm populations and the level of impact is also less, though not non-existent (just much less well studied!)

What about conifer dominated forests?

Conifer dominated forests perhaps have the least ability to support large earthworm populations because their litter is quite unpalatable to earthworms. However, many conifer dominated forests have hardwood dominated understories such as red and sugar maple, alder, and oak. So, to predict what kind of earthworm populations these forests may support, you need to look at the whole picture and not just the major canopy trees. So far, very little research has been done on the patterns and impacts of exotic earthworm invasion in conifer dominated forests of the Great Lakes region.

How do earthworms affect the different layers in a hardwood forest?

The succession of tree species from seedlings to sapling to sub-canopy and eventually to the canopy is a long and very important part of forest regeneration. If earthworms cause a disturbance in this regeneration process, it could lead to long-term changes in our hardwood forests. Some of these changes may take many decades to manifest, so it’s important to think about the long-term consequences now to see if there are any restoration or management strategies that can help minimize these long-term effects.

dense deciduous forest
Here you can see some of the “structure” of a typical hardwood forest with the canopy, sub-canopy, sapling and seedling layers. Notice how light levels have decreased dramatically by the time you reach the forest floor.
dense deciduous forest
The nearly continuous canopy in hardwood forests plays an important role in controlling light levels in the forest. The light regime of a forest can be very important in determining which plants and trees grow there because some species are more shade tolerant than others.

The Canopy

The canopy is composed of the tops of the largest and most mature trees in the forest. On average, there may be 150 to 250 canopy trees per hectare (2.47 acres). These mature trees usually range in size from 20 to 50 or more centimeters in diameter at breast height (DBH*) depending on site conditions and the tree species. In the Great Lakes region, canopy trees may be 15 to 25 meters tall and from 50 to 150 or more years in age, depending on the overall age of the forest. Old-growth hardwood forests in the Great Lakes region may have trees 200 to 400 years old. In other parts of the country and other forest types average trees size, height and age can be much greater, for example, the old-growth conifer forests of the Pacific Northwest.

*DBH (diameter at breast height) is the standard measurement used to access size clas and tree volume in a forest. The DBH is measured 1.5 meters (approximately 5 feet) from the base of the tree using a “DBH tape” which has been specially calibrated to read “diameter” when the tape is wrapped around the “circumference” of a tree. You can find more about how to conduct forest measurements in the Research Methods section of the website.

Thus far research has not shown the canopy trees to be strongly affected by earthworm invasion. However, anecdotal observations suggest that as earthworms disturb the tree root systems by their feeding and burrowing activities, it may lead to increase large limb loss of the canopy trees. If this is so, it may indicate a long-term effect that has not been fully appreciated. Also, even a small amount of limb loss form the canopy could lead to increased light levels in the forest and this can lead to changes in the understory plant communities.

Close up of base of a large deciduous tree surrounded by green vegetation
In an earthworm-free forest, the root crown of canopy trees are well protected by the thick forest floor and the understory plant community grows right up to the base of the tree.
Close up of base of a large deciduous tree surrounded by dry leaves
After earthworm invasion the protective forest floor and associated plants are gone and the soil has been compacted exposing the tops of the root crown which may affect the trees ability to maintain the full pre-invasion canopy.

The Sub-Canopy

dense deciduous forest with green plants covering ground
Sub-canopy trees are the slightly smaller trees that have grown up to the bottom of the canopy layer, waiting for their chance to advance upwards.

The sub-canopy is composed of smaller and younger trees than those in the canopy. On average, there may be 150 to 250 sub-canopy trees per hectare. They usually range in size from 10 to 20 centimeters in DBH. The tree height may range from 5 to 15 meters (just below the canopy) and trees may be between 10 to 60 years of age. The sub-canopy trees are waiting for an opening in the canopy above that they can take advantage of and then grow to fill the available gap. While the sub-canopy is generally dominated by tree species that will eventually reach the canopy, there are also sub-canopy tree species that rarely ever get to the canopy including Ironwood, Black Cherry, Chokecherry, and American Hornbeam. No research exists on the impacts of earthworm invasion on the sub-canopy layer of forests, but we might assume they would be affected similarly to the canopy trees.

The Saplings Layer

The saplings in a forest are a very important part of the overall regeneration of a forest. Saplings are much smaller trees than those found in the canopy and sub-canopy, but having grown beyond the seedling layer and they have passed a critical phase in their growth. In a well regenerating hardwood forest there may be as many as 300 saplings per hectare. Despite their small size, usually between .5 to 5 meters tall and between 1 and 10 centimeters DBH, saplings can range in age from just a few years old to 30 or more. Sapling growth is the most suppressed when a dense canopy is in place. As a result, the species at this level may stay small for decades as they wait for the trees above to die or be blown down by a windstorm.

dense deciduous forest
Tree saplings often form a dense layer beneath the canopy and sub-canopy of a forest. This layer creates even more shade for the seedling and herbaceous plants below and is the first line of regeneration when gaps open in the canopy or sub-canopy. The sapling layer is also an important structural layer for forest birds, many of which feed and nest in this layer.
base of deciduous sapling surrounded by bare ground
When the root crowns of saplings are so severely exposed during initial earthworm invasion it can lead to increased sapling mortality.

When earthworms invade a forest, the previously thick forest floor is removed and the upper layers of soil are compacted. This can lead increased sapling mortality. This same process of root disturbance and exposure of the root crown occurs in the canopy and sub-canopy trees, but they are not as negatively affected as the smaller and more vulnerable sapling trees. It is unclear what the long-term effects will be on the sapling layer for forests that have been invade by earthworms for many years or decades.


Seedlings are the smallest and youngest of the trees in the forest. In earthworm-free sugar maple dominated forests, there can be 100 or more tree seedlings per square meter (that’s 1 million per hectare) that range in size from a few inches with only two leaves to half a meter tall with multiple small branches. While seedlings are small, they can be much older than one might expect and can range on average from1 to 20 years old depending on the species (some sugar maple seedlings have been aged at 40 years!). Few of the hundreds of seedling produced each year grow to become canopy trees. But without a stable and healthy supply of seedlings the long-term regeneration of hardwood forests would be in jeopardy.  Following earthworm invasion, tree seedling densities have been shown to decline dramatically from over 100/m2 to as little as 0-2/m2.

Green forest floor plants closely spaced
Before earthworm invasion, sugar maple forests have a nearly continuous layer of small seedlings mixed with herbaceous plants covering the forest floor.
Green forest floor plants with space and dry leaves between them
However, after earthworm invasion these tree seedlings may be completely eliminated.