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Shrubs in the Understory.

by James P. Engel, © 2012

Walk into any woods and you might be forgiven for overlooking them.  They blend right into the canvas of leaves, twigs and stems that make up the forest background.  Look a little closer at the leaves, the stems and bark.  Try to match them to the large canopy trees that make up the forest overstory.  Nothing seems to match, maybe because they aren’t trees at all but the group of plants that are called shrubs.

In woodlot management, shrub species tend to just get in the way of other management priorities like producing timber.  They have no economic value. They never get large enough to cut for firewood. The stems are usually forked and crooked. The forester recommends cutting them out to reduce competition with the more valuable tree species. So what good are they? What role do shrubs play in the forest ecosystem?

People have a way of trying to simplify everything, so we can get our heads around it.  In nature complexity is the name of the game.  Change the smallest component and it can have unexpected and unforeseen effects on many other organisms around it.  Change happens even if we aren’t aware of them.  

The forest shrub layer is often defined as the physical space between ground level and 15 to 20 ft in height.  The forests understory consists of several types of plants that as a whole make up the understory.  The first group and often the most numerous in number are the seedlings and saplings of the dominant canopy trees.  The next group is the understory trees, species that are adapted to grow in the shade of the canopy trees.  These tend to be small to mid-sized species like Hop hornbeam, American hornbeam, Serviceberry and Flowering dogwood . Shrubs make up the third group and grow from ground level up to about 10 to 15 ft in height.   The last group includes all of the herbaceous plants often referred to collectively as herbs.  The herbaceous layer includes all non-woody plants including annuals, perennials, grasses and sedges.

Shrubs are physiologically different from trees. Shrubs are generally described as multi-stemmed plants with numerous stems originating from the root crown. Trees generally have a single dominant stem growing from the root system.  In trees the main stem exhibits strong apical dominance while the stems of shrubs are all codominant with no single stem expressing dominance. Shrub stems are all about the same size and can regenerate new stems by suckering from the root system.

The most common shrubs found in the forest understory are Spice bush, Maple leaf viburnum, Beaked hazelnut and Witch hazel.  Other shrubs and shrub like plants such as Prickly ash, Round-leaf dogwood, Bladdernut and Deer berry are locally abundant in some woodlands but not in others.  Occasionally other species that require more sunlight will be present in second growth woodland but these shrubs will not persist over time and eventually be shaded out.

Shrubs provide many ecological benefits in the forest understory that are not similarly provided by small trees. Shrubs provide more leaf surface area closer to the ground where the majority of forest birds forage. Their dense irregular branching habit provides more cover and structural complexity near the ground such as forked branches and multiple stems that birds prefer for nesting.

Most forests contain only one or two dominant shrub species and some contain no shrubs at all. It is unclear why this is. There are large variations in the composition of the understory between one forest and another. Could it be that those one or two species are so finely adapted to the site that they are the only species that can reproduce and survive over time or is it that the species present today are the only ones that survived or were able to recolonized after some past human activity eliminated all of the forest understory. 

Many factors can bear on the differences in understory composition; the amount of sunlight, soil composition, pH, drainage patterns and topography.  The shrubs that live in the understory need to be able to survive and reproduce with the amount of sunlight available to them.  Sunlight or the lack of it, shade, may be one of the more significant selective factors influencing species survival.  The amount of sunlight will determine what species can survive and how well they grow and persist.  The difference may also be due to the past history of the forest. What human activity might have impacted the forest in the past, what species were eliminated and which ones survived?

Most of our current woodland has regenerated on former abandoned agricultural fields or woodland that was used to pasture animals. If your woodlot is a second growth forest that regenerated after agricultural abandonment it might have only a few species remaining compared to a forest that was only used for firewood or timber harvest. A woodlot that was used for pasturing animals may have lost all of its understory species.  Pasturing animals in woodlots was a common practice of small subsistence farms from colonial times through the early 1900’s and is still. In use today although on a more limited basis.  Small farms utilized every piece of available land for what ever returns it could provide. The fall was a particularly important time of year when oak, hickory and chestnut trees were producing a mast crop of nuts. Cows, sheep and pigs would be turned loose into the forest to fatten up on the bounty of the forest.

No matter whether you currently have shrubs in your woodland or not you should consider the ecological importance of shrubs to the over all health and biological diversity of your woodland. Increasing forest bird populations by improving understory habitat can have the added benefit of suppressing insect pest populations in your woodlot.  More browse for deer on the forest floor might help a few more tree seedlings avoid being eaten.  More foliage in the understory can help protect the soil surface from erosion by pounding rain and add more leaf litter to the soil surface.  Those are just a few more obvious benefits.  The complexity of forests ecology tells me that there will be many more but no less significant benefits to the health of the forest.

this page updated Feb 10, 2012