Figure 1. A cross section of a young pine tree.
©Shutterstock: Tonello Photography
What do trees need so they can grow? Some of their needs are the same things people need. For example, trees need air and plenty of water and food. But while people and other animals eat their food, trees get food in a different way: they produce it in their leaves from carbon dioxide and water using energy from the sun. And just as people and animals need certain vitamins for growth, trees need nutrients, such as nitrogen and phosphorus, which they get from the soil.
As a tree grows, it produces new layers of wood around the trunk and branches, just under the bark. If you cut a tree’s trunk, you can see these layers (see Figure 1). They look like a set of circles that go from smaller to larger from the center of the trunk. These circles are called tree rings or growth rings.
Figure 2. This close-up of a tree’s growth rings shows the lighter (early wood) and darker (late wood) parts of each ring.
©Shutterstock: Enrico Bortoluzzi
If trees don’t get enough water, sunlight, or nutrients, they may grow slowly or die. A tree’s growth rings show this graphically. In general, wide rings mean that the tree received plenty of water, sunshine, and nutrients, while narrow rings indicate that the tree grew slowly because these resources were limited.
A tree’s growth rings have two parts: a wide, light part (early wood) and a narrow, dark part (late wood). The early wood grows in the wet spring, which tends to be a time of fast tree growth (see Figure 2). As the seasons change to drier summer and then to fall and winter, growth slows and the late wood forms.
Tree rings provide clues about the life of the tree. They can tell us the tree’s growing conditions over the years and whether it has been through fire, drought, or attacks from insects or diseases.
Tree rings provide clues about the life of the tree. In this example, you can see the scars of past events.
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Studying tree rings can also tell us about an area’s climate over time. The shape and width of the tree’s growth rings often differ from year to year because of varying annual growth conditions. During a moist growing season, a tree may produce a wide ring. During a drought or cold winter, a tree will produce a narrow ring. Climate scientists use information from tree rings to infer climate conditions in years past.
Discussion Questions
- What do trees need to grow? (Answers may include air, sunlight, water, space to grow, and nutrients.)
- What causes a tree’s growth rings?
- What can you learn about a tree from its growth rings?
