Objectives:

• to demonstrate that trees grow from their buds after a period of dormancy

• to observe growth form of branches and roots of different trees and identify the advantages of different growth forms

• to interpret graphs of the growth rates of trees.

Lesson Information:

	Grade Level	11
	Subject	Biology 20
	Curriculum Correlation	Agricultural Botany of Saskatchewan
	CEL's	Independent Learning, Numeracy, Communication.
	Duration	up to 2 weeks
	Group Size	class of students working independently or in small groups
	Setting	outdoor and indoor
	Vocabulary	terminal buds, lateral buds, terminal bud scar, cambium, crown, dominant crown, suppressed trees, shade-tolerant, conical.

Activities:

• The Growing Parts of Trees and Factors That Affect Growth

• Factors That Affect the Growth of Trees

Materials:

Paper, pencils, drawing and model building materials, copies of the background information on Biosphere II.

Advance Preparation

1. In the spring, before buds have flushed, collect twigs as part of a field trip activity. Cuttings can be kept frozen in plastic bags until needed, then slowly thawed. Stress that permission should be asked before cutting twigs from trees or shrubs on private or public property. Cut twigs with a sharp knife blade or snipping shears. Remove twigs that won't noticeably affect the general appearance of the tree, and don't take all the twigs from the same spot or side.

2. Make copies Activity #1 (experiment sheet) and Activity # 3 as required.

Background Information:

How does a small, hard seed become a 30 m tall tree? In the competitive world of the forest, the vast majority of seeds do not in fact ever produce mature trees. Only a few that land on suitable ground, get enough exposure to sunlight and water, and are not eaten or damaged by animals and diseases eventually grow up to become adult trees in the forest.

Look at the trees on your local streets and in parks. Every tree has a different size and shape, depending on the rate and pattern in which new cells are added to it each year. You can think of tree growth in two ways. First, the trunks grow outwards and upwards from their tips, carrying their leaves towards the sun. This growth produces extra height and spread. Second, the trunk and branches grow thicker and sturdier, allowing them to support the added weight of the larger branches.

Despite their variations in appearance, all trees have essentially the same basic structure. They have a central column - the trunk - supporting a framework of branches. The branches in turn bear an outside covering layer of leaves. Anchoring the tree in the ground is a network of roots, which spreads and grows thicker in proportion to the growth of the tree above the ground. In a mature tree, most of the cells of the trunk, roots, and branches are dead or inactive. All growth of new tissue takes place at only a few points on the tree, by the division of specialized cells. These actively growing areas are located at the tips of branches and roots and in a thin layer just inside the bark.

If you look at a branch on a deciduous tree during the winter when the leaves are gone, you can see terminal buds at the very ends of the twigs. These are the points from which the next spring's growth will take place. Twigs also have side or lateral buds, from which side branches develop. Buds are formed in the summer and fall and remain dormant over winter. Dormancy is broken when suitable growing conditions return, usually in the spring.

If you look further back along a twig, you will come to a ring of thickened bark. This scar tissue marks the place where the terminal bud grew the previous year and is called the terminal bud scar. The distance between the scar and the new terminal bud is therefore the amount of growth that took place in one year. Knowing this, you can find the age of several branches of different sizes on your tree.

As the main trunk of the tree grows taller, it also grows thicker. Its structure can be thought of as an extremely elongated cone shape, with the base of the trunk, being the oldest part, slightly thicker than the rest of the trunk. You can visualize this growth pattern by inverting and stacking a number of paper cups on one another. Each cup column represents a year's growth. The narrow bottom of each cup (pointing upwards in the column) represents the terminal bud. Each year's growth fits tightly around the previous year's and increases the height of the tree. While the growth in height comes from the terminal buds, the growth in girth comes from a thin layer of cells called the cambium, found between the inner bark and the wood of the trunk and branches. In the paper cup model, the cambium is represented by the outside surface of the cups.

The growth rate and the ultimate height and shape of a tree are governed partly by the growing conditions of its environment, but also by its genes. In optimal conditions of sunlight and moisture, different species reach different maximum heights. The tallest species of trees are the coastal redwoods, which grow on the west coast of California and Oregon and reach heights up to 112 m. A tree can continue to grow for as long as it lives, so its ultimate height depends on its growth rate and its longevity. Height can however be adversely affected by environmental factors such as long periods of cold summers and insect attacks.

The shape of a tree depends on many factors, such as the amount of space it has to grow in, the amount of sunshine and moisture it receives, the relative growth rate of its terminal and lateral branches, and any damage it receives from such factors as lightning, fires, prevailing winds, snow, animals, and disease. The part of the tree above the trunk, called the crown, is built up bit by bit and year by year in order to maximize the leaves' exposure to the sun's energy. The direction of growth of the branches is towards the light. Trees that can reach above their neighbours get the most sunshine and are said to have dominant crowns. Those that are shaded by other trees may be slower growing and are called suppressed trees. In some species, such as poplar, continued shading over several years will eventually lead to their death. Other species, however, called shade tolerant, can survive long periods in low levels of light. Species such as white spruce establish themselves under a canopy and eventually become dominant as pioneer species die off.

While a great height and large crown give a tree more exposure to sunlight, they make the branches more vulnerable to damage from severe winds or heavy snow. To balance the spread of its branches, a tree tends to grow deeper roots to anchor it firmly against toppling by storms. Jack pines have large deep tap roots which enable them to root themselves well in shallow sandy soils and access water. Spruce trees have a shallow root network which leaves the trees susceptible to wind blow when the tree is exposed. Such exposure may come from removal of adjacent trees during logging operations or when a road is being built or, naturally when a fire leaves a strip of trees along a lake as it goes through an area.

In general, the crowns of trees growing in Saskatchewan take on two basic forms: conical and round. In conical forms - such as the familiar spruce tree - the terminal bud grows much faster than the lateral branches. The resulting conical form helps the tree shed snow and ice, and its compact shape resists wind pressure. For this reason, the conical form is most common in trees growing farther north. In the milder climatic areas associated with mixed wood forest and southern parkland, the predominant form is that of the spreading, more rounded crown, typical of aspens and elms.

Besides its branches, roots, and leaves, a mature tree grows one other important structure -- the flower (or cone, in the case of evergreens). These are the reproductive structures from which the seeds are produced. Fast-growing species may mature and produce seeds after as little as five to 10 years' growth, while other species take 30 to 40 years to mature. A certain size is needed before flowering takes place, and suppressed trees may not flower even at an age of 50 to 100 years or more. With the growth of flowers, fruit, and seeds, the life cycle of the tree comes full circle. The seeds are dispersed by wind, water, or animals, and the few that find a suitable spot will germinate and begin their journeys of growth toward the sun.

Notes:

1. The purpose of the Activity # 1 is to observe and measure twig growth and to observe the effects of a sucrose solution on the buds. The experiment demonstrates not only how new growth occurs from buds, but also the phenomenon of dormancy. This is a spring activity, as twigs require a cold period before dormancy can be broken.

2. Activity #2 and the two graphing exercises for Activity #3 are intended to help students appreciate some of the factors that affect the growth of trees.

Resources:

l. How a Tree Grows, a poster available from the Saskatchewan Forestry Association.

Related Internet Links:

• Saskatchewan Education Evergreen Curriculum

• Canadian Forests