Introduction

The principles of succession covered in the previous section provide a background on the processes at work in the general landscape. Similar but distinct principles govern the growth, health, and maturation of trees. Because woodlands dominate many natural areas, we will now focus on trees. It is appropriate to cover a few basic principles of forestry and silviculture, which is the development and care of forests.The activities in this section will help you identify ways to improve your natural area. They will also help you to pursue the interests you noted in your stewardship journal after reading “What Do I Have and What Do I Want: Creating Your Stewardship Journal” in Module 1 here.

Principle 1

Principle 1: Tree size is not directly related to age.
Most people assume that all small trees are young. But some kinds of trees, such as redbuds and dogwoods, just do not get very big. Other kinds, such as oaks and maples, often grow quite large. But not every individual oak and maple tree will grow large or at the same rate. A tree’s final size depends on its genes, on site quality, where it grows, and on the quantity and quality of light (Table 2). If the site is too wet or too dry, too windy, too sunny or too shady, the tree will remain small compared to others of the same species growing in better circumstances.

If a tree does not shoot above the surrounding trees within the first few years of its life, it is likely to remain in the understory. For a shade-intolerant tree, this less-than-ideal situation can produce a smaller, weaker, misshapen tree that dies before it produces seed.

You may have heard that if you remove all the big trees from a site, the little ones will grow up to replace them. That is not necessarily true. After a number of years of being small and suppressed in the understory, a tree often loses its ability to grow big.

Table 2 Preferred Site Requirements for Selected Eastern Forest Trees
Species	Moisture Preference	Nutrient Preference	Tolerance to Shade
Ash, white	M*	H **	L ***
Aspen	M	M	L
Bald Cypress	W	M	M
Basswood	M	H	L
Beech, American	M	M	H
Birch, black	M	L	M
Birch, yellow	W	H	M
Blackgum	M	M	H
Boxelder	W	M	M
Cedar, Atlantic white	W	H	M
Cedar, eastern red	D	L	L
Cherry, black	M	M	L
Cherry, fire	M	L	L
Cottonwood, eastern	W	M	L
Cucumbertree	M	H	M
Dogwood, flowering	M	M	H
Hackberry	D	L	M
Hemlock, eastern	M	M	H
Hickories	D	L	M
Holly, American	M	M	H
Larch	W	L	L
Locust, black	D	L	L
Maple, red	M	M	M
Maple, silver	W	H	M
Maple, sugar	M	H	H
Oak, black	D	M	M
Oak, chestnut	D	L	M
Oak, pin	M	M	L
Oak, northern red	M	M	M
Oak, scarlet	D	L	L
Oak, southern red	D	L	M
Oak, white	M	M	M
Pawpaw	M	H	H
Pine, loblolly & shortleaf	M	L	L
Pine, red	D	L	L
Pine, Virginia	D	L	L
Pine, white	M	M	M
Poplar, yellow	M	H	L
Redbud, eastern	M	M	L
Sourwood	D	L	H
Sweetgum	W	M	L
Sycamore	W	M	M
Willow, black	W	H	L
Walnut, black	M	H	L
Yellow buckeye	M	H	H
* Key: D=dry to moist, M=moist, W=moist to wet Key: H=excellent soils; M=average soils; L=poor soils * Key: H=high; M=medium; L=low Adapted from: Mollie Beattie, Charles Thompson, and Lynn Levine. Working with Your Woodland: A Landowner’s Guide.© 1993 by University Press of New England. Reprinted with permission; Forest Landowner Fact Sheets, http://dendro.cnre.vt.edu/LandownerFactsheets/index.htm; and USDA Forest Service Ag Hndbk. 654, Silvics of North America, http://www.na.fs.fed.us/spfo/pubs/silvics_manual/table_of_contents.htm

Principle 2

Principle 2: Different tree species require different conditions to thrive.
Some factors in a site’s suitability (Table 2) for a tree species include:

soil depth
soil composition (rocky, sandy, or clay-based )
site precipitation and wind
Slope direction (south facing slopes are hotter and drier, north facing cooler and moister)
slope position (top, middle, or bottom of the slope)
species’ range

(Species requirements vary by location within their range of distribution. For example, at the middle of a species’ range, it may grow in a wider range of site conditions than at the outer edges of its range.)

Table 2 Preferred Site Requirements for Selected Eastern Forest Trees
Species	Moisture Preference	Nutrient Preference	Tolerance to Shade
Ash, white	M*	H **	L ***
Aspen	M	M	L
Bald Cypress	W	M	M
Basswood	M	H	L
Beech, American	M	M	H
Birch, black	M	L	M
Birch, yellow	W	H	M
Blackgum	M	M	H
Boxelder	W	M	M
Cedar, Atlantic white	W	H	M
Cedar, eastern red	D	L	L
Cherry, black	M	M	L
Cherry, fire	M	L	L
Cottonwood, eastern	W	M	L
Cucumbertree	M	H	M
Dogwood, flowering	M	M	H
Hackberry	D	L	M
Hemlock, eastern	M	M	H
Hickories	D	L	M
Holly, American	M	M	H
Larch	W	L	L
Locust, black	D	L	L
Maple, red	M	M	M
Maple, silver	W	H	M
Maple, sugar	M	H	H
Oak, black	D	M	M
Oak, chestnut	D	L	M
Oak, pin	M	M	L
Oak, northern red	M	M	M
Oak, scarlet	D	L	L
Oak, southern red	D	L	M
Oak, white	M	M	M
Pawpaw	M	H	H
Pine, loblolly & shortleaf	M	L	L
Pine, red	D	L	L
Pine, Virginia	D	L	L
Pine, white	M	M	M
Poplar, yellow	M	H	L
Redbud, eastern	M	M	L
Sourwood	D	L	H
Sweetgum	W	M	L
Sycamore	W	M	M
Willow, black	W	H	L
Walnut, black	M	H	L
Yellow buckeye	M	H	H
* Key: D=dry to moist, M=moist, W=moist to wet Key: H=excellent soils; M=average soils; L=poor soils * Key: H=high; M=medium; L=low Adapted from: Mollie Beattie, Charles Thompson, and Lynn Levine. Working with Your Woodland: A Landowner’s Guide.© 1993 by University Press of New England. Reprinted with permission; Forest Landowner Fact Sheets, http://dendro.cnre.vt.edu/LandownerFactsheets/index.htm; and USDA Forest Service Ag Hndbk. 654, Silvics of North America, http://www.na.fs.fed.us/spfo/pubs/silvics_manual/table_of_contents.htm

In sum, these site characteristics help to define site quality. Some species grow well on a variety of site characteristic mixes while others are more sensitive and require certain site characteristics. For example, oaks are adapted to drier sites, whereas yellow poplars require fertile, deeper soils near streams and along slope bottoms. White pine does poorly in wetlands while red maple thrives in both wet and dry soils. Site quality is an important consideration in choosing which trees and shrubs to nurture or plant.

Principle 3

Principle 3: Different species grow at different rates depending on site quality and competition.

On a given site, the mix of tree species reflects site quality as well as individual tree and species competition for light, nutrients, and water. Some trees will benefit, while others will have slow growth rates and lose vigor. Thinning, meaning cutting select trees, reduces the competition, reallocates resources, and produces more vigorous trees.

The trees that first appear after a timber harvest, natural disaster, or old-field succession tend to grow very quickly. In doing so, they retain their place in the sun and prepare the site for later successional stages by modifying the site. For example, they shade the ground and use soil moisture, pulling down the water level in the soil. These early successional species are shade-intolerant. Typically, many more trees (thousands) occur per acre in successional stages 2 and 3 than the 20 - 120 trees per acre that can be expected in stage 4. So what happens to all those young trees? The weaker ones and the ones with less desirable positions relative to light, water, and nutrients die, freeing additional resources for those that remain.


The center tree has space to grow on three of its four sides.	Looking up into a crowded canopy.	Looking up into an uncrowded canopy.

Trees grow most rapidly and produce the strongest branches if their canopies get light on all sides. Foresters typically recommend spacing new hardwood seedlings 10 feet apart on all sides, knowing that as the trees grow their crowns will eventually grow closer together. In any given situation, some trees will grow taller than their neighbors. When trees begin to get crowded and their growth rate declines, thinning to create more space around selected trees can accelerate the natural process of forest development. Choose to keep trees that have already shown their competitive edge because they are bigger, have thicker trunks, and have a better shape. Give them more space by cutting the trees around them (see "Thinning" in Module 4 here).

Gardens serve as an example of thinning in action. More seeds are planted than can thrive in an area. Later, the weaker plants are pulled out to obtain proper spacing and at the same time weeds are removed. This reallocates resources to the remaining plants.

In a stand of trees with trunk diameters greater than 6 inches DBH, proper thinning creates space on 2–3 sides of the tree crown we are favoring. The bigger the tree, the more space it should have around it. After thinning, the growth rate of the remaining trees increases and their vigor improves. Consider using trees cut in this process for firewood or to create brush piles for wildlife.

Activity 5, "Assess Competition Among Trees," will help you determine if thinning is a good idea in your habitat areas. You may pause to complete the Activity here or continue reading.

Principle 4

Principle 4: Healthy forests are three-dimensional.

A hole in the canopy lets sunlight reach the forest floor.

Openings in the forest canopy develop naturally as trees die from crowding, attack by insects and disease, or windstorms, ice, or other weather events. Gaps in the canopy allow sunlight to reach the forest floor, and the mix of light conditions stimulates the growth of new and existing plants. The result is the development of plants of different heights, known as vertical stratification. This gives the visual impression of a layered, three-dimensional forest with tall trees, a mid-story of trees, and an understory with trees, shrubs, and herbs, that extends over horizontally. Openings that are as wide as the trees are tall usually result in more species diversity as well as more individual plants. This occurs because the mix of direct sunlight and shade benefits both shade-tolerant and shade-intolerant species.

A diverse forest that consists of a variety of species adapted to the site is like a diverse investment portfolio. It is more balanced and better able to handle natural cycles such as insect and disease outbreaks. Plant diversity helps ensure you will retain cover in the event of loss – you are protecting your investment to meet your longer-term needs.

Not all forests exhibit health in all three dimensions. Sometimes something will diminish or remove an important component. For example, overabundant deer can over-browse understory plants, including native wildflowers, important shrubs, and desirable tree seedlings. Over time this removes a component in the forest’s dimensional structure resulting in a forest of large trees that lacks the desirable plant diversity for wildlife. Such a forest is incapable of sustaining itself into the future. This is not always obvious as plants deer do not browse begin to dominate and give the impression of health. An example might be an understory of ferns, exotic shrubs, or small trees such as striped maple.


A forest with vertical stratification.	A forest with little vertical stratification.

Principle 5

Principle 5: Trees reproduce from seeds or stumps.

Young deciduous trees will often sprout from a stump.

Deciduous trees can regrow from stumps or root sprouts, depending on the size, age, and species of the tree. You can cut some deciduous tree species to within a few inches of the ground and one or more new shoots will sprout from the stump if they receive enough light. Large (> 2 feet in stump diameter) mature trees may not sprout prolifically. Sprouts will grow faster than seedlings because they take advantage of the existing root system. They typically become dominant of co-dominant trees in the next forest. Deciduous species are more difficult to establish than conifers by planting seedlings because deciduous trees are more susceptible to weed competition and wildlife damage. If you do plant deciduous trees, they will require more care and protection. For more information about sprouting potential of various trees, see Appendix A here.

Extensive red oak regeneration.

Coniferous trees or conifers generally do not sprout from stumps, so if you want to establish them, it is often best to plant seedlings or rely on natural seeding, which may require special conditions. See “Planting Trees and Shrubs” (in Module 4 here) and consult the “Planting trees” section of the Resource List here for more information.

The future of your natural area depends on having plant reproduction. Some species, such as hickory, oak, red maple, hemlock, and beech, establish themselves relatively easily under an existing tree canopy. Sooner or later the mature trees will decline and die opening the canopy for smaller trees to grow. If deer or competitive plants are too high in numbers, few saplings will be ready to occupy the “free” space. If this is the case, new forest establishment will be delayed or only fast-growing tree species or species deer do not browse will survive. Many invasive and/or exotic species are able to establish themselves in these conditions. See “Planting Trees and Shrubs” (in Module 4 here) and consult the “Planting trees” section of the Resource List here for more information. For more on deer management, go to “Planting Trees and Shrubs” (in Module 4 here).

By completing Activity 6, "Assess Tree Reproduction," you will determine if there are young trees in your forested natural area. You can pause your reading to complete Activity 6 here, or continue reading. The link will also appear at the end of the reading.

Principle 6

Principle 6: Trees die and dead trees are valuable for wildlife and soil.

Dead standing trees, called snags, provide important wildlife habitat.

Trees do grow old and die. Just as different animal species have different average lifespans, so to different tree species. For example, an eastern red cedar tree that thrives in early successional habitat will usually have a shorter life span than a late successional tree such as hemlock or American beech. Many times, early successional species are overtopped as the late successional species grow taller. Appendix A (here) shows the relative longevity of some tree species common in the eastern United States.

Trees are usually killed by a specific event such as an insect attack, an ice storm, a strong wind that topples them, or a series of events that sets them on a decline path. Even relatively small Injuries can leave trees open to diseases, insects, or rot and decay. As trees age, they become more susceptible to injury and disease. Knowing this, it is important to protect roots near the soil surface, bark, and branches from unnecessary damage.

When a tree dies, it may remain standing for years. If there are dead trees in your natural area that do not pose a safety threat to people or buildings, it is best to let two or three per acre stand. Dead trees, both standing and fallen, provide nesting habitat for cavity-dwelling birds and mammals. Insects feed on the deadwood, attracting insect-eating birds and animals. Other plants may grow right out of a dead tree. The wood slowly decomposes, helped by insects, microbes, and fungi, which then are eaten by other organisms. In this way nutrients are carried out of the tree and slowly recycle back into the forest soil, where they will eventually nourish another tree. It is tempting to turn all dead trees into firewood, but it is important to leave some to add complexity and value to your wooded area. Studies have suggested woodland can benefit from two to five cords per acre of standing dead trees, downed trees, and woody debris. A healthy woodlot is not necessarily clean and tidy.

In Activity 7, "Survey for Broken and Dead Trees," you will survey your own natural areas for broken and dead trees. You may pause your reading to complete Activity 7 here, or you may continue and return to it later.

Principle 7

Table 3
Common Invasive/Exotic Plants in the
Eastern United States Region

Tree-of-heaven (ailanthus)	Kudzu
Japanese barberry	Japanese honeysuckle
English ivy	Oriental bittersweet
Golden bamboo	Autumn olive
Multiﬂora rose	European buckthorn
Japanese stiltgrass	Garlic mustard
Bush honeysuckle	Japanese Privet
Japanese knotweed	Mile-a-minute
Winged euonymus (burning bush)	Butterfly bush

Principle 7: Invasive species will always be there.

No matter how you manage your land, invasive and exotic species will be there. We live in a world of global commerce and virtually every part of the globe is now affected by non-native or exotic species. Some of these exotic species are invasive, which means they tend to spread very rapidly and will out-compete native species. Common exotic invasive plant species you might encounter are bush honeysuckle, Japanese honeysuckle, multiflora rose, privet, autumn olive, Russian olive, ailanthus, wine berry and garlic mustard. Invasive and exotic plants, animals, and insects such as gypsy moth, EAB, and HWA often displace native species and upset ecosystem balance. Table 3 lists a few common and particularly troublesome invasive/exotic plants. Photos of these plants are included at the end of this reading. Check the Resource List here or the World Wide Web for more information. The additional case studies on the “Beyond the Workshop” page will familiarize you with other invasive/exotic plants in our area.

It is virtually impossible to get rid of all invasive and exotic plants, but if you make no effort to control them, they will overrun your natural area.

Grapevines can strangle or pull down trees or branches, but their value as wildlife food may be more important if a landowner is managing the area to attract wildlife.

Herbicides, properly used according to label, are generally essential to really control invasive species. See “Herbicides to Control of Exotic/Invasive Plants” (in Module 4 here) and the Resource List (here) for information on control measures. You may successfully control small patchy populations of invasive/exotic plants by pulling or cutting. Be especially careful to remove the plant’s seeds and fruits from the site. Know that the sooner you find and begin to control invasive/exotic plants, the more likely you are to be successful. The first time you see one, address the problem then! Even one plant can become a management challenge in just a few years.

Nonnative gypsy moth caterpillar.

To reduce future problems with invasive and exotic plants, it is best to plant only native species. Most nurseries sell exotic and invasive species as well as natives, so do some research and/or ask a cooperative extension agent before you buy. Avoid the species listed in Table 3. Most of the species sold by state-run nurseries (see “Planting Trees and Shrubs,” in Module 4 here) are natives.

In Activity 8, "Assess Invasive and Exotic Plants in Your Natural Area," you will identify plants that are candidates for removal. Activity 8 can be found in the section after the quiz.

Common Invasive Plants
(Click for a larger image of each species)


Bush honeysuckle.	Autumn olive.	Oriental bittersweet.	Multiflora rose.	Garlic mustard.

Golden bamboo.	Japanese honeysuckle.	English ivy.	Kudzu.	Mile-a-minute.

Ailanthus.	Japanese stiltgrass.	Butterfly bush.	European buckthorn.	Japanese barberry.

	Japanese knotweed.	Japanese privet.	Winged euonymus (burning bush).