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A tree survives through the intricate relationships of leaves to roots, branches to bark, water to air, and people to plants. Trees live longer, grow taller and have greater mass than any other living organism. To sustain the health and longevity of these beautiful, complex plants in an urban landscape, humans need a basic understanding of how trees function.
Trees need sunlight to power photosynthesis, the fuel-making system for green plants. To get first crack at the sun, trees keep their "solar panels" (leaves) above the competition by growing ever taller and wider. Leaves use energy from the sun, carbon dioxide from the air, and water from the soil to manufacture carbohydrates, the basic fuel for plants, humans and animals.
Trees use their fuel supply for cell maintenance, growth, defense and reproduction, storing any reserves for future use. The amount of fuel allocated to each process varies by season, and by tree age, health and species. Because a tree's fuel supply is finite, circumstances that cause a tree to direct more of its energy toward defense, for example, result in less fuel for growth and other processes.
Trees grow in height and diameter. Trees grow taller and branches grow longer due to cell division at the branch tips. Trees gain girth from cell divisions in the cambium, a single layer of cells between the bark and the wood. Each year, as the cambium cells divide, new layers of wood and inner bark are added between the previous year's growth. The cambium produces wood (xylem) cells to the inside, and bark (phloem) cells to the outside.
Tree root systems consist of large woody roots and smaller, short-lived absorbing roots. The woody roots anchor the tree in the ground and store food reserves. Absorbing roots take in water and elements from the soil. Roots grow much closer to the soil surface than is commonly believed.
Although some may be found in the subsoil, most roots, especially the absorbing roots, proliferate near the soil surface where water and oxygen are most available. This is especially true in urban soils, which are typically shallow, compacted, infertile and droughty. Landscaping and site development practices commonly cut, crush or suffocate roots. Root injury is a leading cause of premature death of urban trees.
Trees are composed primarily of carbon, hydrogen and oxygen. Thirteen additional elements are required for healthy plant growth, but most are needed in minute amounts. Soils usually contain adequate levels of trace elements. Major elements, particularly nitrogen, can be lacking in urban soils where topsoil has been removed, and leaves and twigs raked away.
Fertilizer can supplement these essential elements. Fertilizer is not tree "food," however, because it does not supply energy. Unless a tree is unhealthy because essential elements are lacking, it is unlikely that fertilizer will restore a tree to health. Excessive fertilization can damage tree roots, increase drought stress and insect feeding, worsen certain diseases, and contaminate runoff to streams and lakes.
The water and mineral elements necessary for growth are supplied to cells by the xylem, which transports these materials from the roots to the leaves. Water and mineral elements from the soil enter the tree through tiny "hairs" on the surface of the absorbing roots. This water moves upward in the tree because the air is generally drier than the tree tissues.
Like a sponge, air soaks up water vapor from the leaves and other plant parts in a process known as transpiration. Since water molecules bond very tightly, a continuous water column is pulled up the entire height of the tree. The water lost to the atmosphere is replaced by soil water taken up through the roots.
Most of the carbohydrates produced by photosynthesis in the leaves are transported in the phloem. In young trees, the majority of carbohydrates are sent to the roots in the latter part of the growing season before dormancy begins. As buds open in spring, the process is reversed, and carbohydrates are transported to the rapidly growing shoots. As shoots, flowers and leaves finish growing, food is directed to the cambium and roots for further growth. As trees grow larger, shoots growing near the crown draw on food produced and stored nearer the treetop.
This essential transport system of sap and surrounding tissues lies close to the surface, between the bark and wood. It is vulnerable to injury, especially in young or thin-barked trees. Lawn mowers and string trimmers, tree wrapping and guying materials, and certain insects and diseases are among the potential causes of injury to the tree's vascular system.
Trees cannot heal their wounds. They have no means of repairing or replacing damaged cells. Instead, trees form physical and chemical barriers to seal off injured areas. This system of defense prevents the spread of decay and other pathogens within the tree.
Generally, the healthier the tree, the greater the fuel reserves, the stronger the defenses, the greater the resistance to decay. When a tree is unhealthy or when injury is chronic, repeated or sufficiently severe, fuel reserves are depleted. The tree responds with poor growth, sparse leaves, premature leaf drop, general dieback or by sacrificing specific limbs. Incipient decay often spreads further into the tree.
Sometimes wound-response symptoms are not apparent for several years. By the time symptoms are recognized, the tree may be in a state of irreversible decline. Opportunistic insects, drought or other injury often deliver the final blow.