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Lesson 2: Understanding How Trees Grow

TEACHER:

SCHOOL:

GRADE LEVEL: 9-12

TASKS/COMPETENCIES ANR8006.130 Plant forest seedlings.

ANR8006.138 Manage a small woodlot.

ANR8008.074 Plant forest seedlings.

ANR8008.082 Manage a small woodlot.

ANR8040.083 Explain basic tree structure and growth.

ANR8046.132 Explain basic tree structure and growth.

OBJECTIVES AND GOALS

• The student will name the three main parts of atree.

• The student will explain the process ofphotosynthesis.

• The student will define the term transpiration.

• The student will explain how a tree forms annualrings.

• The student will state the difference betweenterminal buds and lateral buds.

• The student will explain why leaves change colorin the fall.

SOL CORRELATIONS

BiologyBIO.1 (plan and conduct investigations)BIO.3 (biochemical principles essential for life)BIO.4 (relationship between cell structure and

function)BIO.5 (life functions of monerans, protists, fungi,

plants, and animals)

EQUIPMENT, SUPPLIES, ANDMATERIALS NEEDED

• Cross-section of tree trunk• Pictorial diagram of a tree• Bulletin board materials• Instructor-generated test on tree growth

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ACTIVITIES

PreparationLesson ApproachFew people understand that a tree is not only a living plant, but that trees are necessary for our very existence. Fewpeople know the different parts of a tree and how these parts function to make an efficient, living organism. Questionsmay be used to determine student’s knowledge of tree growth: • What are the parts of a tree?• How does a tree increase in height?• How does a tree increase in diameter?• Why do leaves change color in the fall?

General SituationStudents should discuss these questions/answers:• Why are trees important to us other than for the products they furnish? (They hold the soil, increase its fertility,

moderate temperature, and create beauty.)• Why is it important to understand how a tree grows? (A crop cannot be properly managed and grown unless its

growth requirements are understood.)

ApplicationTake students on a field trip to an area where timber is being cut. Have students participate in the following activities:• Determine the age of trees cut by making annual ring counts on the stumps.• Determine average height growth of a tree by counting rings on the stump and then the number of rings at each cut

up the tree trunk. The difference in ring counts between any two points, divided into the distance between thesepoints, will give the average height growth per year.Example: Number of rings at stump = 25

(minus) Number of rings at 5 feet = 22 3

5 ÷ 3 = 1.66 feet average growth per year

• Use an increment borer to determine age and growth per year. Bore tree at 4 1/2 feet from the ground (diameterbreast high), count the rings, and add 4 to determine the age of the tree. (This is necessary since it probably took thetree four years to reach this height.)

• Count 10 rings in from the bark edge and measure the width of the wood making up these 10 rings. This is the radialgrowth. To determine the diameter growth over the last ten-year period, double this figure.Example:

10 rings measure 1 1/2 inches2 x 1 1/2 = 3 inches diameter growth in 10 years

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Presentation: What is a Tree?

A tree is a woody plant having one well defined, erect perennial stem and a more or less definitely formed crown,attaining a diameter of at least 8 inches and usually attaining a height of at least 12 feet. Follow the chart below duringthe explanation of the parts of a tree and their functions.

Heartwood (inactive) gives strength.

Sapwood (xylem) carries water and nutrients up the stem.

Cambium is where the diameter growth occurs.

Inner back (phloem) carries manufactured food from leaves tobranches, trunk, and roots.

Outer bark provides protection.

Natural mulch of leaves protects surface against drying anderosion.

Decomposing layer (humus) is inhabited by beneficial insects,bacteria, and fungi.

Enriched soil layer is source of much of the tree’s food; it isthe home of earthworms which leave channels for water andair.

Subsoil contains soil particles and parent material. Deep rootsfind their home here and absorb water and anchor the tree.

Passages through the soil are constructed by insects.

Three main parts of a tree

RootsThe roots of a tree have three main functions:• They anchor the tree.• They absorb water and dissolved nutrients.• They protect against erosion. TrunkThe trunk supports the crown and produces useful wood. The trunk is made up of the following:• The outer bark protects the tree from injuries and heat or cold, just as skin protects the human body.• The inner bark (phloem) carries manufactured foods made in the leaves down to the branches, trunk, and roots.• The cambium consists of a layer of cells where growth in diameter occurs. It forms annual rings of new wood inside

and new bark outside.• The sapwood (xylem) carries water and nutrients from roots to leaves.• The heartwood was once sapwood, but is inactive. It gives mechanical strength to the tree.• The annual rings are made up of springwood and summerwood.

CrownThe crown is the leaf area of the tree, including the branches, which support the leaves. Size and shape are affected byenvironment and genetics.

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• Leaves are the most important chemical factories in the world. Their manufacturing process is called photosynthesis.Without this process there would be no food for people or animals, no wood for shelter, no humus for the soil, nocoal for fuel. Every living thing depends on this process.− Photosynthesis (from photo, meaning light, and synthesis, meaning putting together)

• Inside each leaf, millions of green-colored, microscopic “synthetic chemists” (chloroplasts) manufacturesugar. They trap radiant energy from sunlight for power. Their raw materials are carbon dioxide from theair and water from the soil. Oxygen, a byproduct, is released. This fundamental energy-storing, sugar-making process is called photosynthesis. The chemical formula is chlorophyll + CO2 + H2O + Sunlight =C6H12O6 + O2 (oxygen).

• What happens to this leaf-made sugar in a tree? With the aid of “chemical specialists” (enzymes), everyliving cell, from root tips to crown top, goes to work on the sugar. New products result. Each enzyme doesa certain job, working with split-second timing and in harmony with the others. In general, enzymes breakdown sugar and recombine it with nitrogen and minerals to form other substances.

• Enzymes change some sugar to other foods, such as starches, fats, oils, and proteins, which help formfruits, nuts, and seeds.

• Enzymes also convert some sugar to cell-wall substances, such as cellulose, wood, and bark, and makesome of the sugar into other substances which find special uses in industry. Some of these are rosin andturpentine from southern pines; syrup from maples; chewing gum from chicle trees and spruces; and tanninfrom hemlocks, oaks, and chesnuts.

− Transpiration• Transpiration is the release of water-vapor from living plants. Most of it occurs through the pores

(stomates) on the underside of the leaves. Air also passes in and out. Factors which affect transpiration arerelative humidity, temperature, sunlight, wind, available water, and atmospheric pressure.

• Branches anchor and conduct fluids to the tree’s leaves and flowers.

• Flowers function as the reproductive organs of the tree.

How does the tree grow?

Diameter growthThe diameter of a tree increases as the cambium layer forms new wood cells on the inside and bark cells on the outside.Alternate growth of spring and summerwood forms annual rings. The cambium is a single layer of cells that retains thecapacity to divide throughout the life of the tree.• Cambium gives origin to both wood and bark.• When the cambium becomes active in spring, it begins to split off rows of wood cells to the inside and rows of bark

cells to the outside.• Wood formed in spring consists of light-colored, thin-walled cells; later in season the cells formed are smaller,

darker, and have heavier walls (summerwood).• When growth conditions are favorable and food and water abundant, the rings are wide. Under unfavorable

conditions, such as drought, the rings are narrow.• By counting the annual rings, one can determine the age of a tree.

Sapwood

Heartwood

Inner Bark

Outer Bark

Cambium

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Height growthElongation from terminal buds produces height growth (the same is true of twigs and branches).• Terminal buds complete growth early in the season, usually by the end of June.• In many trees, all cell division for the next year’s growth is completed in the bud by this time.• The next season’s growth takes place mostly by elongation of the bud cells formed in the previous year.• Twigs and branches are formed from lateral buds.

Why do leaves change color?A green leaf is green because of pigments known as chlorophyll. This green color masks out the colors of the otherpigments. During the growing season, the plant replenishes the chlorophyll as it is used. As autumn approaches,chlorophyll is replaced at slower rate. This is when the green masking dwindles and the other pigments begin to show.

AnthocyaninsThis group of pigments causes the red and purple colors.

Caritenoids• This group of pigments causes the yellow, brown, and orange colors.• These pigments exist in many living things, giving characteristic color to carrots, corn, canaries, bananas, and egg

yolks.

Factors affecting leaf colorLeaf color can be affected by light (overcast or sunny periods), temperature, moisture, nutrients, and other factors. Thisis a very complicated process, but basically the fall color change is the result of the nonreplenishing of green chlorophyll,permitting the other dominating pigments to take over.

ADDITIONAL ACTIVITIES

• Display a cross-section of a tree trunk and have students identify the outer bark, inner bark, cambium, sapwood, andheartwood; point out the annual rings and have students determine the age of the tree when it was cut.

• Have students bring samples of terminal buds and lateral buds to use during discussion of height growth of trees.• Have students collect leaves and make a bulletin board display illustrating why leaves change color in the fall.• Arrange a field trip to a timer-cutting site and have students determine the age and growth rate of selected trees.

SUGGESTIONS FOR STUDENT EVALUATION

• Give each student a pictorial diagram of a tree to point out the various tree parts and explain their functions.• Have students complete a short test on tree growth or complete the activities suggested under the Application

section above.

SUGGESTED RESOURCES

• http://www.uga.edu/wsfr• http://interactive.usask.ca/skinteractive/modules/forestry/index.html

Lateral Buds

Terminal

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TEACHER NOTES