Forest ecosystems of Canada

FAQs

• Why are leaves green?
• Why do leaves change colour in autumn?
• What is the role of a tree's roots, trunk and crown?
• How can the age of a tree be determined?
• Why do trees have bark?
• How does a tree grow?
  
  
  

Why are leaves green?

Leaves are green because they contain chlorophyll, a pigment that plays an important role in photosynthesis.

The green is due to the fact that chlorophyll absorbs all colours (wavelengths) for photosynthesis, except green, which it reflects outward.

 

Why do leaves change colour in autumn?

At the end of the growing season, as winter approaches, trees stop producing sap in order to protect themselves. Without sap, photosynthesis can no longer take place and the chlorophyll breaks down and eventually disappears. In the absence of chlorophyll, the other pigments present in the leaves become visible. These pigments—anthocyanins (pink, red and purple) and carotenoids (yellow and orange)—are also present in common fruits and vegetables such as corn, carrots, turnips, tomatoes and red peppers. Have you ever stopped to think about how our forests, when arrayed in their fall colours, are like a basket of fruits or vegetables?

As for conifers, their foliage is more cold resistant because they are smaller and covered by a protective layer of wax. They therefore do not lose their chlorophyll and they remain green throughout the winter, with the exception of tamarack (larch).

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What is the role of a tree's roots, trunk and crown?

From bottom to top, a tree is composed of a root system, a support structure (trunk and branches) and foliage. What are the roles of each of these parts?

Roots

The root system of a plant or tree serves as its foundation. Roots play roles that are both mechanical and biological:

Roots:

• anchor the tree to the ground;
• absorb water and nutrients from the soil;
• conduct these nutrients and water (raw sap) to the trunk;
• store nutrient reserves.

The root system consists of three parts:

• structural roots. The large roots that serve primarily to anchor a tree to the ground;
• lateral roots. A network of medium to very fine roots that spread out in the soil and absorb nutrients and water. The very fine roots live only for a few months. The production and maintenance of fine roots require a lot of energy from the tree;
• root hairs. Fine, hair-like roots that ensure water and nutrient uptake. They are located on very fine roots. In many tree species, root hairs are replaced by fungi that live in symbiosis with the tree and increase the roots' area of absorption.

Roots do not grow very deep in the cool and humid soils of our northern forests. They stay near the surface where the soil is warmest and the nutrients most abundant. Thus, the largest part of a tree's root system is usually found less than one metre from the surface. However, roots spread out horizontally over a distance that may reach several times the height of the tree!

Support structure

The trunk and branches form the support structure of a tree. They support the crown, carry raw sap from the roots to the leaves and take elaborated sap from the leaves back to the roots. The trunk, like the roots, also stores energy reserves.

The trunk is the main stem of the tree; it supports all of the tree's weight. The trunk does not grow in height; it only increases in diameter (see How does a tree grow?). The trunk gets longer because the branches at the top get thicker and become the trunk.

The main branches, which are attached directly to the trunk, play the same role as the trunk; they support the weight of the lateral branches and leaves. Secondary branches and their smaller branches are responsible for the growth of the crown in height and width.

Leaves

Most leaves have two parts: the petiole and the blade. The petiole connects the blade to the branch. It carries sap, supports the leaf and directs it towards the light. Leaves attached directly to the branch without a petiole are called sessile leaves.

The wide, green flat part of the leaf is the blade. Different species of trees are characterized by different types of blades. For instance, oaks have large, deeply notched leaves, while firs have flat needles.

Deciduous trees and larches (tamaracks) drop all their foliage every year. In conifers, needles can stay on the tree for as long as 10 years, but their life span varies by species.

Leaves are often thin and wide to absorb as much light as possible. Networks of veins carry water and nutrients. Conifer needles are small, but their arrangement on the branches enables them to absorb light efficiently.

The leaves on a tree:

• collect solar energy;
• transform raw sap into elaborated sap (photosynthesis and respiration);
• facilitate gas exchanges (intake of oxygen and release of carbon dioxide).

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How can the age of a tree be determined?

Growth rings

Determining the age of a tree is fairly easy. It involves simply counting the growth rings that can be seen on the surface of a log or on an increment core from the trunk of a tree. Each ring represents the growth produced during one growing season but contains two parts that can be distinguished on the basis of colour: the early wood, less dense, is whitish in colour; and the late wood, more dense, is darker in colour and forms during the summer. Counting the growth rings tells us the number of growing seasons the tree has been through, and hence the age of the tree.

Can growth rings tell us more about a tree?

Growth rings are not used solely to determine a tree’s age. They can also be examined to discover important events that have marked the tree’s life. In a way, they are like a history book.

For example, changes in the thickness of the rings usually point to events that have affected the tree’s growth, such as a period of drought, a forest fire, a mechanical injury or an insect infestation. Anything that has a strong enough impact on the tree’s growth—even the need to compete with nearby trees—will show up, or be revealed, in its annual growth rings.

Tree rings are therefore studied for a variety of reasons: to determine a stand’s response to a particular silvicultural treatment; to trace the fire history in a given region; to date the passage of an insect epidemic or a major ice storm, and so on. They can also be used to delineate the climatic variations that have occurred in the past.

Here is an example of what can be learned by examining a tree’s growth rings. The tree in question is a jack pine.

By counting the annual rings from the right hand side toward the centre, it was determined that the tree’s first year of growth occurred in 1926. When the tree was cut down, in 2002, it was 76 years old.

In 1951, a fire swept through the forest in which this jack pine was growing and destroyed some of its cambium (living tissue just under the bark) on the left side of the trunk. The tree was 25 years old then. Diameter growth stopped in this part of the trunk, but continued on the right hand side. This explains the rather unusual shape of this trunk.

1. Tree size in 1951
2. Section destroyed

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Why do trees have bark?

The answer to this seemingly simple question is quite fascinating. Trees have bark for the same reason that we have skin, that is, for protection. Bark serves mainly as a protective cover for the cambium, the thin layer of living cells located beneath it. Cambium cannot be seen with the naked eye. It plays a vital role, though, since it is the living part of the tree. Indeed, this layer of cells enables the tree to grow and to produce wood, roots, leaves and so on.

Bark protects the tree from mechanical injury, the cold, the sun’s rays and above all pathogens. When a tree’s bark becomes damaged, this provides a point of entry for a wide variety of pathogens (diseases, fungi, etc). Similarly, when we cut ourselves, the tissues involved become exposed to the air and are at risk of becoming infected.

Another key function of the bark is to allow the tree to breathe. Just as our skin aids in our respiration, a tree’s respiration occurs partly through the bark, which allows gaseous exchanges to take place between the living cells of the cambium and the outside environment.

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How does a tree grow?

Like all living beings, trees grow because they are continually producing new cells, at least during the growing season. These new, or living, cells, arise from the active tissue layer called the cambium, which is situated just beneath the bark (the cambium cannot be seen with the naked eye). Trees grow in height through the elongation of the tips of their branches, but diameter growth is centred in the trunk. Becauseof this, if a branch is cut off near the trunk, the resulting scar will always be at the same height. Similarly, if you make a mark on the trunk of a tree, it will remain at the same level.

There are some easy-to-recognize signs that a tree is growing taller: the new part of the branch generally has a different colour and it is also softer and less fibrous, because its constituent tissues are young. This new part of the branch is called the annual shoot. The main branch is called the terminal shoot or leader. At the tip of every branch, there is an apical bud. In the spring, the apical bud induces height growth, which continues until the end of summer. The part of the branch that corresponds to the previous year’s growth does not elongate, but it does grow in diameter.

Diameter growth

Trees grow in diameter as a result of the cells produced by the cambium layer. The cells that the cambium produces in an outward direction (towards the bark) form what is called the phloem tissue. Phloem is made up of tiny tubes that transport the sap downwards within the tree. The cambium layer also produces cells towards the inside, forming the xylem tissue (the tree’s wood). Xylem, too, is made up of tiny tubes, but these tubes carry the sap upwards in the tree. Every year, the cambium produces new cells which form a ring around the trunk. New cells are always produced on top of those making up the previous year’s growth. As a result, old phloem is pushed outwards and forms the inner part of the bark.

Last Modified: 2007-10-09