Hence it is essential for the tree to supply the water and nutrients to the leaves from the ground through the stem. A tree ring is simply a layer of wood produced during one tree’s growing season. A cross section of a tree often shows a distinct pattern of concentric tree rings. Each tree ring marks a line between the dark late wood that grew at the end of the previous year and the relatively pale early wood that grew at the start of this year. One annual ring is composed of a ring of early wood and a ring of late wood.
The growth occurs in the cambium (the thin, continuous sheath of cells between bark and wood).
In spring, the cambium begins dividing. This creates new tissue and increases the diameter of the tree at two places: 1. Outside the cambium. The outer cells become part of the phloem. The phloem carries food produced in the leaves to the branches, trunk, and roots. Some of the phloem dies each year and becomes part of the outer bark. 2. Inside the cambium. The inner cells become part of the xylem. These cells contribute most of a tree’s growth in diameter.
The xylem carries water and nutrients from the roots to the leaves. These cells show the most annual variation: * When a tree grows quickly, the xylem cells are large with thin walls. This early wood or springwood is the lighter-colored part of a tree ring. * In late summer, growth slows; the walls of the xylem cells are thicker. This late wood or summerwood is the darker-colored part of a tree ring. What Causes Variations in Tree Rings? When conditions encourage growth, a tree adds extra tissue and produces a thick ring. In a discouraging year, growth is slowed and the tree produces a thin ring.
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Much of the variation in tree rings is due to variations year-to-year in: * Higher springtime temperature. If spring starts early, the growing season is likely to be longer than usual, causing a tree to have a wider ring. * Lower springtime temperature. A late spring is likely to shorten the growing season, causing a tree to have a narrower tree ring. * Abundant rainfall increases growth, producing a wider ring. * Drought decreases growth, producing a narrower ring. * Species of tree do differ in their response to weather changes.
One might respond strongly to changes in overall rainfall, another might be more sensitive to the amount of rain during the late summer, and another to a temperature change that alters the length of the growing season. * Crowding from neighboring trees. This causes a series of narrow rings. Crowding is suspected when the series of narrow rings is more than three, because droughts are usually only one to three years. * If the rings are narrow on one side of a tree with wide rings on the other, the tree was crowded on the side of the tree where the rings are narrow. A series of many narrow rings followed immediately by wide rings probably means that an encroaching neighbor died, releasing the crowded tree into a growth spurt. * Fire scars suggest past forest fires. The number of annual rings between fire scars shows the period between fires. * Scars due to insect plagues indicate insect infestations What Makes A Ring? Because trees live longer than most plants, their stems (trunks) grow differently. Trees develop woody stems that contain two types of tissues: Primary tissues- The first tissues formed by a plant.
Primary xylem and primary phloem Secondary tissues- Tissues generated by the growth of the cambium. Examples: secondary xylem and secondary phloem Secondary tissues are added each year. These additions cause something known as secondary thickening in the stems. This thickening is what makes a woody stem as woody stem. It’s part of what makes a tree a tree. You can see evidence of stem (trunk) growth by looking at a cross-section of an older tree. Each ring shows one year’s worth of growth. But, what causes the rings? A young stem has bundles of vascular tissues (tissues that carry fluid).
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The bundles are made up of xylem, phloem, and cambium layers. As the tree ages, the cambium layers between the bundles come together. As the tree gets older, a vascular cylinder is formed. About a year after the vascular cylinder is formed, a tree’s first ring will appear. Remember, the cambium is a meristem – a place of growth. Each growing season, new xylem cells are produced. These cells eventually clog and no longer transport sap. They become heartwood. These are the cells that make up the rings you see. Year after year, new layers of xylem tissue are added and year after year a new ring is produced.