What Is a Tree?

Trees are almost everywhere. They are planted along streets and around playgrounds. They create pleasant, cool, shaded areas around homes in the summer. Trees provide food, shelter, and homes for birds and other wildlife. City parks and gardens are valued for their beautiful trees, and forests are enjoyed by hikers, naturalists, campers, and bird-watchers.

Is It a Tree or a Shrub?

Trees — and shrubs — are large plants with hard woody trunks or stems. Most have leafy branches or groups of leaves. Size is generally the best way to tell the difference. Shrubs usually grow less than 15 or 20 feet tall and often spread out close to the ground. A mature (fully grown) tree can grow tall and straight to a height well over 20 feet.

Most trees have a single main trunk that branches out into many smaller branches and twigs. But most shrubs have several woody stems rising right from the ground and then dividing into thinner branches. Your school, house, apartment, or nearby grocery store parking lot probably has several shrubs planted around it.

Here are some examples of shrubs:

* flowering lilac

* blueberry bushes

* azaleas

* honeysuckle bushes

* flowering forsythia

* others: hawthorn trees, witch hazels, and sumacs can often grow more than 15 feet tall, so they are described in field guides as small trees or large shrubs.

A single plant family can include both trees and shrubs — and even small woodland wildflowers!

It can be confusing to tell a tree from a shrub. Some trees don't grow tall because they are stunted by insect damage or poor growing conditions, such as drought. And many trees are pruned and trimmed so that they remain small. Even though they could become much taller, they are kept to the size of a shrub (less than 20 feet tall). Here are a few examples:

* Apple trees can grow more than 20 feet tall, but in orchards they are usually pruned and trimmed to keep them smaller to make it easy to harvest ripe apples from them.

* Rhododendrons can grow well over 20 feet high in the wild, but when planted in front of houses or public buildings they are kept trimmed to stay small.

* Hemlock trees growing naturally in the wild can grow 60 to 70 feet tall — with some huge specimens reaching more than 100 feet! But they are sometimes planted in rows near a building and trimmed and pruned to form a low hedge.

* A balsam fir can grow to be a large tree — usually 50 to 60 feet tall. But on a tree farm where Christmas trees are raised and grown, they are trimmed so they can fit inside a house for the holidays. Balsam firs in poor growing conditions may only grow a few feet tall.

Parts of a Tree

On the ground at the base of a tree, you can often see the top surface of large roots spreading away from the trunk. If you could dig up a large tree, you would find that the big roots divide into smaller roots, then into tiny rootlets, and finally into tinier roots called root hairs. You can sometimes see the entire root system of a tree when it has been blown down by a severe storm and the roots have been ripped out from the ground.

The trunk of most trees rises upward to spread out into large branches, then smaller branches, and then smaller and shorter twigs. The twigs support buds and leaves.

The leafy top of a mature, fully grown tree is called the crown. The tops of many trees, including maples, birches, oaks, and ash trees, are rounded. An American elm has branches and a crown that grow in a fan or fountain shape. A blue spruce, often planted near homes and buildings, has a triangular shape with a pointy top. Balsam fir trees have a triangular shape too (also called a pyramid shape).

Parts of a Leaf

The flat part of a leaf is called the blade. Some leaves have a very wide blade, like those of the American sycamore, which can be eight inches across. Other leaves are quite narrow. The leaf of a black willow tree is only about one-half inch wide. Pine leaves are called needles because they are long and very thin.

Leaves are attached to twigs by a stem called a petiole (PET-ee-ole). The petiole on the leaves of elms, chestnuts, birches, and oaks is fairly short. But the petiole on the leaf of a bigtooth aspen or on most maples is long. Look at the petiole on the leaf of any tree or shrub, and observe how long it is. Is the petiole shorter than the length of the leaf blade, or is it longer than the leaf? Bigtooth aspen, cottonwoods, and trembling aspen all have long, flat petioles, which cause the leaves to wiggle in a breeze.

Leaf Shapes and Designs

Leaves have many different shapes. Each species (type) of tree has its own leaf shape. There are many different species of maples and birch trees, and the leaves of each species have a distinct shape and size.

Leaves can be long and narrow, like those of willows. They can be oval, like those of cherry and plum trees. Some trees have leaves that come to a long point. The leaves of a white oak have rounded, fingerlike lobes. The leaves of sassafras trees can have three different shapes: a single oval, an oval with a "thumb" (a shape like a mitten), and a three-part leaf. All three shapes can be found on one sassafras tree!

The edge of a leaf also can be important in identification. Some leaves have a mostly smooth edge, like those of a southern magnolia. Others, like red maples and American chestnuts, have a toothed or serrated edge.

You may also notice that on some trees (and other plants) the leaves grow from the twigs opposite from each other. On other trees, the leaves are alternating (or staggered) along the branch. Once you start to look at the shape, design, and placement of different leaves, you will develop an eye for noticing details. This will help you to identify common trees, and to keep a lookout for anything different or unusual.

LOOK FOR

THE UNDERSIDE OF A LEAF

You need good light indoors for this activity, or a bright day outside.

MATERIALS

* A few different leaves, picked from trees or shrubs

* Magnifying glass

1. Pick a few leaves from different trees or shrubs. (It doesn't matter if you can't identify them.)

2. Compare the upper side of the leaf with the underside. The underside is probably lighter in color.

3. Now look at the underside more closely with a magnifying glass. You will find that the veins are easier to see.

4. Look for a different texture than on the upper side. The underside may be smoother, or it may be rough, fuzzy, or hairy.

On the underside of hemlock leaves (which are short, flat "needles"), you can see white or silvery lines on each side of the central vein. These white lines are...