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Understanding Forest Ecology

TREES are the first things that come

to mind when someone mentions

a forest. However, if you have taken a

walk in a forest, you know that a forest

holds many forms of life besides trees.

You will also find squirrels, deer,

herbaceous plants, vines, insects, and

other organisms there.

Objective:

� Describe forest ecology and forest ecosystems.

Key Terms:

�

Forest Ecology

Ecology is the science of the interrelationships between living things and their environ-

ment. Forest ecology is the study of the forest ecosystem. Forest ecology deals with the

interrelationships between various trees, plants, and other living organisms that make up the

community. It also deals with the interrelationships between those organisms and the physical

environment in which they exist.

An ecosystem is a system formed by the interaction of a biological community with its

nonliving environment. Forest ecosystems can vary in size from a few hundred acres to thou-

sands of acres. Forest ecosystems are generally classified into types based on their dominant

tree species. Some ecosystems are simple, whereas others can be very complex. An apple

orchard or a pine plantation is a simple ecosystem because it has only one species of plant. A

E-unit: Understanding Forest Ecology

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bole

ecological succession

ecology

ecosystem

forest ecology

hardening-off

hardiness

material cycle

mycorrhizae

overstory

photoperiod

stratification

understory

true pine-hardwood forest, an oak-hickory forest, or a

beech-maple forest is a complex ecosystem because it

has more than one species.

The more complex an ecosystem, the more resis-

tant it is to either change or damage by insects, dis-

eases, ice storms, fire, and other stresses. When an

ecosystem contains many different plant species, the

plants are not all affected if the ecosystem suffers

from one of the damaging factors. In a mixed hard-

wood forest ecosystem, one species of tree may be

killed by a disease, yet other tree species take its place.

A total loss in the timber resource does not occur, and

the ecosystem continues to function. On the other

hand, a simple ecosystem can be destroyed because of

its susceptibility to a single damaging factor.

CHARACTERISTICS OF

FOREST ECOSYSTEMS

Various characteristics and processes are typical of

the forest environment. Characteristics include strati-

fication, zonation, diversity, and stability.

Stratification

Stratification refers to the various layers that can be observed in a forest. The upper can-

opy of trees, trees just below the main canopy, saplings, seedlings, and small herbaceous plants

near the forest floor are typical layers.

Zonation

In a forest, various kinds of trees that have similar needs for moisture and nutrients are

often found in the same zone. The wet area near a stream supports different tree species than a

dry area farther away from the stream. A different type of zone defined by different tree species

may exist between the wet and dry zones.

Diversity

Natural forest ecosystems are very diverse, with many kinds of trees and plants. The various

layers in a forest provide a variety of habitats for both plants and animals. The location and type

of habitat in which a species lives, together with its functional role in the forest ecosystem,

determine the niche of a species.

Stability

Diversity results in a stable environment that is both resistant to change and resilient.


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FIGURE 1. Because this forest ecosystem is

relatively complex, it is resistant to damage from

nature.

NATURAL ECOSYSTEMS

There are many processes and relationships of natural ecosystems. They include energy

flow, material cycling, competition, and succession.

Energy Flow

Energy flow is an important process. Green plants, also known as producers, combine

energy in the form of sunlight with water and minerals from the soil and with carbon dioxide

from the air. Energy captured by green plants as sunlight is considered a flow because as it is

used, it is constantly converted to heat energy that goes back to outer space, not to be used

again in the ecosystem. The nonliving materials that green plants use to survive and grow are

termed the abiotic part of the ecosystem. Animals, or consumers in the ecosystem, survive on

the green plants for food.


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Canopy

Forest Floor

Understory

Herb Layer

ShrubLayer

FIGURE 2. Various layers in a typical forest.

Material Cycling

The material cycle is a circular process in which

materials are used over and over again. Plants, which use

materials from the environment, are eaten by animals.

Eventually, all plants and animals die, and the wastes cre-

ated decompose. Decomposition is very important.

Decomposers are the fungi and bacteria found primarily

in the upper soil layer. They break down plant and ani-

mal matter, which is recycled by plants and other life

forms.

Competition

Competition for light, space, water, nutrients, and

other resources is constant in ecosystems. Both plants

and animals compete for the resources they need for sur-

vival.

Succession

Many forest ecosystems are constantly changing

through a very slow process called ecological succession.

Ecological succession is a fairly predictable and orderly change in the composition or

structure of a community of organisms. For example, a tree species declines and dies and is

replaced by other kinds of trees. Some of the tree species were not previously present in the

forest, and over time the ecosystem changes. A mature forest is usually considered the climax,

or terminal stage, which is very stable and diverse.


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FIGURE 3. These soybean plants are

competing with weeds for nutrients.

AnnualPlants

PerennialPlants andGrasses

Shrubs Softwood Trees —Pines

Hardwood Trees

TimeFIGURE 4. The stages of succession.

TREES AND THE ENVIRONMENT

The total tree environment involves a complex interaction between a variety of physical and

biological factors. An environmental change is rarely the result of a single factor. Knowledge of

the nature and interrelationships of the major environmental factors is helpful in understand-

ing how plants grow and how they respond to environmental change.

Physical Factors

The physical environmental factors include climate, soil, and fire activity.

Climate directly affects both the daily growth processes and the seasonal development of

plants. Chemical reactions tend to speed up as temperature increases and slow down when the

temperature approaches the maximum at which plants can survive.

The sun is the source of the visible light portion of solar radiation and is important as a

source of energy for photosynthesis. The photochemical reaction is one triggered by the sun.

Sunlight is important in regulating the processes of growth, leaf fall, flowering, fruiting, and

dormancy.

The quality, intensity, and duration of light affect the photosynthetic process. The light

received by the understory, or trees growing beneath the canopies of the tallest trees, is quite

different in color and intensity from the light received by the overstory, or upper-canopy

trees. Photoperiod, or day length, influences diameter growth of trees and the time of flush-

ing or new growth in the spring and the beginning of dormancy and defoliation in the fall.

Trees and other plants change physiologically to better tolerate and/or resist the extremes of

environmental conditions. This change is known as hardening-off.

Air is the principal source of carbon dioxide required for photosynthesis and of oxygen used

for respiration.

Of the environmental factors

that influence the growth of trees,

moisture is the most limiting ele-

ment. There is a wide range of

annual precipitation in the for-

ested areas of the United States.

Silvicultural methods retain more

of the moisture available to the

tree crop. Increasing available

water generally increases tree

growth more than any other

practice.

Soil directly affects a tree by

providing anchorage and by serv-

ing as a growth medium. Soil

serves as a reservoir for moisture

and provides the essential ele-


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FIGURE 5. Because trees help to hold soil in place, sediment is kept to a

minimum in nearby streams. (Courtesy, Natural Resources Conservation

Service, USDA)

ments for tree growth. Such soil characteristics as organic-matter content, texture, structure,

chemical composition, depth, and position influence the supply of available moisture and

nutrients to the tree.

Fire is a natural part of a forest ecosystem. Periodic fires are even necessary for some trees to

reproduce. Fires generally burn accumulated debris on the forest floor. The process returns

nutrients to the soil. Many tree species have thick bark that protects them from fire.

Biological Factors

Certain biological factors affect tree growth. They

include:

� The genetic make-up of trees

� The interaction between trees and other plants

� The interaction between trees and small animalsthat use the forest for food and cover

� The interaction between trees and fungi andmicroorganisms in the soil

Reproduction is a vital process of every self-perpetu-

ating tree species. However, the reproductive habits vary

widely between trees. Seed formation depends on the

amount of reserve carbohydrates and nitrogenous mate-

rials in a tree. The amount of seed produced varies by

species and certain climatic factors. Seed germination is

influenced by moisture, temperature, light, and other

factors.

A timber tree normally directs a maximum of growth

energy into the development and maintenance of one

main stem, or bole. Deviations

from this typical tree form may

cause a tree to take on the form of

a bush or shrub. Many tree spe-

cies assume the form of a tall,

pipe-like bole in moist, fertile

soils but become shrub-like on

dry, windy sites. Variations in tree

form also occur when certain

trees are shaded by others in a

forest stand. A silvical factor

related to form is a tree’s toler-

ance, or its ability to withstand

unfavorable conditions. Light is a


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FIGURE 6. The plant life on this forest floor is a

biological factor.

FIGURE 7. These wood poles were made from timber trees.

major factor commonly associated with the tolerance of a tree, and tree species are often

grouped or classified based on their tolerance to shade.

Root growth is related to the inherited characteristics of a tree. Root development varies

considerably from species to species. Root systems are adaptive and tend to modify themselves

to suit the environmental conditions in which they grow. The presence of certain soil fungi is

essential for the successful growth of many tree species. Mycorrhizae are soil fungi that

enable trees to more fully utilize water, minerals, and nitrogen in the soil.

The hardiness of a tree is its ability to withstand cold. Trees also have adaptations to toler-

ate heat, droughts, insect infestations, disease attacks, and other elements limiting survival and

growth. The level of adaptation differs between species and between individual trees within a

species.

Summary:

� Forest ecology is the study of the forest ecosystem. An ecosystem is a systemformed by the interaction of a biological community with its nonliving environ-ment.

Characteristics and processes typical of the forest environment include stratifica-tion, zonation, diversity, and stability. Stratification refers to the various layers thatcan be observed in a forest. Zonation refers to various kinds of trees in a portion ofa forest that have similar needs. Natural forest ecosystems have many kinds of treesand plants.

Processes and relationships of natural ecosystems include energy flow, materialcycling, competition, and ecological succession. The material cycle is a circular pro-cess in which materials are used over and over. Many forest ecosystems are con-stantly changing through ecological succession.

The total tree environment involves a complex interaction between a variety ofphysical and biological factors. The physical environmental factors include climate,soil, and fire activity. The biological factors include genetic considerations and theinteraction between trees and other plants, between trees and small animals that usethe forest for food and cover, and between trees and fungi and microorganisms inthe soil.

Checking Your Knowledge:

� 1. What is forest ecology?

2. What are some characteristics and processes typical of the forest environment?

3. What role do energy flow, material cycling, competition, and succession havein a natural forest ecosystem?


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4. What is ecological succession?

5. What physical and biological factors play a role in the tree environment?

Expanding Your Knowledge:

� Conduct a field study of a forested area near you to determine the interrelationshipsof forest organisms. Prepare a brief report.

Web Links:

� Forest Ecology

http://en.wikipedia.org/wiki/Forest_ecology

Logging for the 21st Century: Forest Ecology and Regeneration

http://www.extension.umn.edu/distribution/naturalresources/DD6517.html

Temperate Forests

http://library.thinkquest.org/17456/temperateall.html

Forest and Rangeland Ecosystem Science Center

http://fresc.usgs.gov/products/factsheets.aspx

Ecological Succession

http://www.psu.edu/dept/nkbiology/naturetrail/succession.htm


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http://en.wikipedia.org/wiki/Forest_ecology

http://www.extension.umn.edu/distribution/naturalresources/DD6517.html

http://library.thinkquest.org/17456/temperateall.html

http://fresc.usgs.gov/products/factsheets.aspx

http://www.psu.edu/dept/nkbiology/naturetrail/succession.htm

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