Designing a forest from the ground up

Components of an Eastern Deciduous Forest

The best designer of the eastern deciduous forest in which we live is nature. Given enough time and lack of human disturbance, almost any spot in Northern Virginia with some soil, will turn into this ecosystem. Our climate and environment encourage the type of growth that brings with it tall trees and dense woods. If you looked at all the layers of a healthy forest, what would go into designing a forest from the ground up?

Soil

Without proper soil, you don’t get much growth. There are plants that can handle very poor, thin soils, but that will not get you a full forest very quickly. Because our forest tends to be dominated by species that prefer acidic soils, such as oaks, beech, and tuliptree, you would start with an acidic soil. Almost just as important is the soil composition. Gardeners will attest to the prevalence of clay in their soil, and the difficulty of digging in this clay, but we do also have sand and silt in our soil. A good mix of the three, with a focus on the water-retaining clay, gets us a good soil to start with.

If we end up with an acidic, well-composed, uncompacted soil, that’s step 1 of our forest. Our useable soil typically goes about 1-2 feet down, so that’s a target to hold. Where space is restricted (planters, tree pits), we often ask for more to compensate, but in an actual forest, this is plenty. Bringing in soil from other areas can be costly, so a great method of providing this layer of our forest is to mix organic material with clean existing soil on site, to provide the basis of our forest.

 

Groundcover and herbaceous plants

A forest is not just made out of trees. Ephemerals take advantage of low canopy cover in the earlier part of the year to catch energy from the sun, and then disappear. Grasses and other groundcovers hold the soil, and provide food for earthbound wildlife.

When building from the ground up, a complete forest also has:

– Ephemerals, such as spring beauty (Claytonia virginica), Virginia bluebell (Mertensia virginica), and Dutchman’s breeches (Dicentra cucullaria)
– Grasses and grass-like plants, such as eastern bottlebrush grass (Elymus hystrix) and Pennsylvania sedge (Carex pensylvanica)
– Ferns, such as Christmas fern (Polystichum acrostichoides), and wood ferns (Dryopteris spp.)
– Other Herbaceous plants, such as Jack-in-the-pulpit (Arisaema triphyllum), and mayapple (Podophyllum peltatum)
– Vines, such as Virginia creeper (Parthenocissus quinquefolia) and grape (Vitis spp.)

This layer can be difficult to emulate perfectly, as some ephemerals require unusual soil compositions, and species such as orchids and trilliums often don’t survive transplanting. However, there are many grasses, ferns, and vines are fairly tolerant of transplanting,  which can be used to prepare the site for other seeds to come in naturally. Keeping the soil in place and alive is the best way to have this forest provide the habitat for the rarer plants to come in.

Shrubs

The next level up is the shrub layer. There’s no real physiological difference between most shrubs and trees, but the general definition relates to mature size. Most shrubs don’t reach above 20 feet, when fully grown. This shrub layer provides much of the shelter and cover for forest animals. In forests with an unhealthy population of deer, this layer is often suppressed, along with the lower plants. Healthy native shrub growth is a great indicator of a healthy forest, and usually indicates the other aspects of the forest are in place, in a fairly balanced way. Unfortunately, many invasive shrubs, such as Honeysuckle (Lonicera spp.) and Autumn olive (Elaeagnus umbellata) have out-competed our native shrubs, providing little value to our ecosystem, except for some shelter and berries with little nutritional value.

Common shrubs in our forest include spicebush (Lindera benzoin), witch hazel (Hamamelis virginiana), Arrowwood (Viburnum dentatum), and blueberry (Vaccinium spp.).

Creating the shrub layer can be a great way to improve the forest, but deer pressure can make it a frustrating process. Deer exclosures and individual deer fencing can be a great way to address this issue.

Understory and midstory Canopy

The forest is an ever-changing fight for access to light. The groundcovers and shrubs have typically adapted to lower light conditions, but there are always new trees coming through, trying to get to the top. All of our tree species can exist in this middle level of canopy at one point in their lives, but it typically takes the opening of the canopy to let through the really intolerant species. For example, Tuliptree (Liriodendron tulipifera), will live as a sapling, building up energy, sometimes for up to several decades, to wait for an opening in the canopy. All of the accumulated energy in its roots will then allow for exponential growth into the canopy, to prevent others from taking this access to light.

Understory trees that stay under 30-40 feet are present in a healthy forest, and rise above the shrub layer. These are most commonly dogwoods (Cornus spp), redbuds (Cercis), serviceberry (Amelanchier spp.), hornbeam (Carpinus caroliniana), Pawpaw (Asimina triloba), American holly (Ilex opaca), and Sassafras (Sassafras albidum).

The mid-story is typically composed of all the trees looking to reach the next level, and can be composed of any of our hundreds of native species. This layer can be emulated through transplanting of landscape-sized trees, but care has to be taken with placement. If a plant has no access to light, it will not thrive, and not likely survive.

Overstory Canopy

Ultimately, what is essential to the forest is the overstory canopy. Trees larger than 40 fe

et, providing high cover for animals, intercepting the majority of the rainwater and sunlight. The trees in this layer are typically older, and have accumulated massive reserves of energy through their access to light and lack of competition. Some overstory trees even provide their own habitat for other plants, like vines and epiphytes, plants growing in small soil pockets along branches.

In our region, our overstory is dominated largely by oaks, beech, tuliptree, and hickory. Some unique ecosystems with very high moisture contents also have dominant red maples, willow, and sweetgum canopy.

Creating this layer of the forest takes extreme patience. Transplanting large canopy trees is financially burdensome, and establishment is difficult. The best approach is to plant for the future, with all of the aforementioned layers in place, and, after establishing the plants, letting nature take its course.

The Complete Forest

Arlington is working to incorporate all the elements of the forest into our ecosystem, understanding the stresses of urban influence. While letting nature do its job is an essential component, invasive species, deer pressure,  pollutants, and non-native insects can cause human-made havoc on our environment, and we must stay vigilant in identifying and addressing these problems in time.

Of course soil and plants are not the only aspect of forests that make it a healthy ecosystem. Water, in the form of groundwater, streams, lakes, and wetlands, provides the lifeblood for the forest, animals spread and germinate seed, and fungi make nutrients available to our plants, and break down dead material. The forest does not work without these aspects, and if you are looking for a permanently self-sustaining forest, measuring the presence of key organisms and soil fungus health can be a great indicator of success.

The best way to learn about these layers is to explore all of our region’s forests, and to see all the different growth habits of every habitat. Learn to identify habitat types, and why certain plants are in different situations. This way you can learn to best incorporate these elements into your urban forest.

For more information on forest structure in Virginia, check out this publication by the Virginia State Extension and Virginia Tech: https://pubs.ext.vt.edu/465/465-315/465-315_pdf.pdf