Terrific Trees
Trees are all around us. These complex plants have adapted to stand tall and strong, and they are essential to the world around them. There are over 70,000 species of trees on Earth; each species has amazing adaptations and unique stories hidden within.
Parts of Trees
A tree is a tall plant with a single trunk and branches that support its leaves. Trees are easily recognized by their height, as they are usually the tallest plants. Like all plants, they have roots which anchor them in the ground and absorb water, a stem which is called the trunk, leaves which absorb sunlight and exchange air, and seeds which help to produce new trees. Some of them even have flowers and fruits.
Trees are different from other plants because of their hard stems and branches. Rather than the fleshy green stems of many flowering plants, tree trunks are armored with bark to help strengthen and protect the tree. As the tree ages, the trunk and branches thicken. This is called secondary growth and the reason why trees can grow so tall and stretch their branches so wide.
Level up! As you can see on the tree cookie diagram below, the trunk consists of six main parts. All plants have a phloem and xylem within their stem to move water and sugar up and down the plant. The xylem moves water from the roots to the leaves, and phloem moves sugar from the leaves to the roots to be stored. The core of the trunk is called the pith. It is the oldest part of the trunk and stores lots of nutrients for the tree. Heartwood is the hard, dead tissue surrounding the pith. Its role is to provide structural support. The lighter wood after heartwood is called sapwood; this is where the xylem is located. Next is the layer on the outside rim of the xylem called the cambium. The cambium is a layer of tissue that creates the xylem and phloem. It is located right in between them and adds new growth to each of these layers over time. The last two layers are the phloem and then the outer bark which protects the trees from the environment.
Deciduous vs Coniferous
The two main categories of trees are deciduous and coniferous. Deciduous trees’ most defining characteristic is that they drop their leaves in the fall. These trees have broader, flatter leaves and their crown (branches and leaves) forms as a rounder shape. Oak, maple, birch, and ash are some deciduous examples that are responsible for the beautiful fall foliage. These trees drop their leaves to protect themselves from breaking their branches in the winter. If their broad leaves remained all winter the snow would collect on top, weighing done their branches and causing them to snap. The winter is a time of dormancy for these trees as they store their energy for spring and summer growth.
Coniferous trees are also known as evergreens. They keep their leaves throughout fall and winter because their needle-like or scaly leaves have less chance of collecting snow. The reason these trees were named coniferous is because they all have cones. Whether the cone is as large as the pinecone from the Coulter Pine or as small as the cone of a black spruce, they are all a part of the coniferous family. These trees continue to grow throughout the year even in the cold weather, and many of them grow in a triangular shape like Christmas trees. Species of coniferous trees include firs, cedars, and hemlocks.
Aging
Just like all other living organisms, trees grow. Each year, trees add another layer of growth within the trunk. This acts as a record so that we can see how old a tree is. Look at the “Reading the Rings” handout in the Resources section below. The growth of the tree slows in late summer and fall. This is when trees create the darker rings in the trunk which are called summerwood. In between each dark ring is a light-colored wood. This is called the springwood; it grows during the spring. Every dark ring on the tree cookie accounts for a year. So, starting from the center, you can count each dark ring to determine the tree’s age.
The study of aging trees is called dendrochronology. Scientists can read the age rings within the trunk and determine information about the trees, the past climate, and other past environmental impacts. For example, the further apart the dark rings are, the more the tree grew that year. This may mean that there was a lot of rain that year. If the rings are close together, the tree did not grow much, meaning there could have been a drought that year. Other markings that scientists look for are fire scars and signs of harm from insects.
Activity – Paper Plate Tree Cookie
Materials: paper plate, colored pencils
You will be making your own tree cookie to describe your life like trees do. Tree cookies, like the one in the diagram above, are a cross-section of the tree trunk that scientists use to show tree growth and layers. Use the paper plate to draw a ring for every year of your life. Draw your rings closer or further apart depending on whether you think you had a year of growth or not. Your growth can either be based on actual growth in height or big events or special moments you had during the year. Add text or drawings between the rings to note anything of importance that happened that year. For example, a soccer ball could be drawn to represent playing with your team that year. Have everyone in your family, classmates, or others draw their own tree cookies and talk about why your rings are close, far apart, and the other notes and drawings that describe what happened each year.
Nature Exploration
Use the leaf identification handout to learn about different parts of a leaf and how to identify trees. When identifying a tree, look at the leaf edges and shape, the color and texture of the bark, and the height of the tree. These are all important indicators that show differences in tree species. See if you can identify all the trees in an outdoor space as either deciduous or coniferous.
Level up! Use a dichotomous tree guide to identify tree species. Dichotomous keys help us identify plants by asking two questions at a time about the plant’s characteristics. Once you answer a question, it guides you to the next pair of questions until you reach the correct plant. You can use the Arbor Day Foundation’s website to help you on your exploration: https://tree.oplin.org/.
To learn more about the aging of trees, you can watch videos about dendrochronologists and the tools that they use. Some tools that these scientists use include measuring tables, microscopes, increment borers, and computer software. Look for logs and take a moment to observe their trunks. Count the rings to determine the age and see if you can find any other markings to uncover the tree’s past.
Books and Resources
Little Sap by Jan Hughes
Backpack Explorer: Discovering Trees by Nature-Watch
Peterson First Guide to Trees of North America by George A. Petrides
Reading Between the Rings
Leaf Identification