How Trees Grow
To survive and grow, trees need adequate amounts of carbon dioxide, water, sunlight, and nutrients. Factors that influence the availability or use of these elements include tree characteristics, site characteristics, and climate.
Parts of a Tree
Trees are composed mainly of carbon, but to ensure healthy growth, trees also need oxygen, hydrogen, nitrogen, potassium, calcium, magnesium, phosphorus, sulfur, and trace amounts of several other elements. In general, carbon dioxide and sunlight are absorbed by the leaves, water is absorbed by the roots, and nutrients are absorbed mostly by the roots but also by the leaves. Leaves are the food factories in a tree. They contain a green substance called chlorophyll that enables the sun?fs energy to convert carbon dioxide and water into sugar. Trees use sugar as a basic ingredient in all plant parts.
Tree growth occurs from buds on the ends of branches, from the tips of tiny roots, and in the cambium?\a thin layer of cells beneath the bark that covers the whole tree. (Figure 3-1) Cambium cells produce both inner bark and sapwood. Inner bark or phloem is spongy tissue that transports food from the leaves to other parts of the tree. Phloem cells live for a relatively short time, then die and become part of the outer bark, which insulates the tree from weather extremes and serves as a barrier against insects and disease. Sapwood or xylem is the pipeline that transports water from the roots up to the leaves. A new layer (ring) of sapwood grows each year. Light-colored bands of sapwood are called springwood?\thin-walled cells that grow early in the year. Dark-colored bands of sapwood are summerwood?\thick-walled cells produced in the summer when tree growth has slowed. Sapwood cells live for several years, then lose their vitality, die, turn a darker color, and become known as heartwood. Heartwood provides substantial physical support for the tree.