Mary L. Duryea and Marlene M. Malavasi School of Forest Resources and Conservation, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida
What is tree growth?
Trees have six organs: leaves, stems and roots (vegetative structures) and flowers, fruits, and seeds (reproductive structures). Tree growth is the increase in size and numbers of the vegetative structures. Trees use the sun, carbon dioxide, water, and minerals to produce sugars. Sugars are the building blocks of tree growth. As a result, tree growth is as much a response to the environment as it is to the trees genetic make-up.
Where does growth occur?
Growth occurs in meristems. A meristem is a tissue containing cells that have the capacity to divide to make new cells. In general, during growth cells divide, cells elongate, and cells differentiate into structures such as roots and shoots. Meristems can also produce new meristems called primordia. For example, an apical meristem in a bud produces new meristems called leaf primordial. Each leaf primordium will grow into a new leaf.
Above Ground Growth
Shoots elongate or grow in height at the tips of the branches. Apical meristems are located in the terminal buds at the tips of the branches. Cells at the apical meristem divide, elongate and differentiate in distinctly visible steps: (1) The bud at the tip of the branch opens, (2) Leaves emerge and enlarge, and (3) The area between the leaves expands (i.e., the stem grows). Lateral (side) buds grow in the same way but often these are dormant and do not grow until they are released after such activities as pruning.
On the surface of the apical meristem in the bud, a new meristem is formed. This new meristem is called a leaf primordium where cells divide and grow into a leaf. Soon after leaves develop, a new bud primordium (meristem) is formed at the base of each leaf stem. Once formed, this auxiliary bud has the capacity to become a branch, but may lie dormant for many years.
Between the wood and bark is a thin layer of dividing meristematic cells called the vascular cambium. The cambium divides producing new wood towards the inside and bark on the outside. These new cells increase the diameter of the trunk and branches. The new wood cells, called xylem, carry water and minerals up from the roots to the leaves. The old wood in the middle is the heartwood. Heartwood, while dead, supports the weight of the tree. The inner bark cells, called phloem, carry sugars and other materials to the growth and storage locations of the tree.
New layers of wood are added each year between the bark and the previous year’s wood. These are called growth or annual rings and may be used to age a tree. Annual rings vary in size and thickness according to the season that they are formed. Cells that are produced in the spring are larger with thinner cell walls. These are the light-colored rings, and the wood is called “early” or “spring” wood. Cells produced in the summer are smaller, and this “late” or “summer” wood has a higher density and darker color.
All woody trees have an outer bark that constantly renews itself and protects the tree from pest attacks and environmental impacts such as fire and mechanical injury. Some trees have thick bark that is resistant to injury. Others are easily injured because they have thin bark. As the tree grows in thickness, the outer part has to give, forming ridges and cracks in the bark. Eventually the outer bark sloughs off.
Growth below ground
Roots can grow in length diameter, and number. At the tip of a root is the root cap. This cap protects the root and must be constantly replaced as a root pushes through the soil. Behind the root cap is a meristem which produces new cells for the root cap and for root elongation. These new cells elongate, divide and differentiate into root parts as it pushes through the soil.
Root diameter growth is similar to growth in the stem with the vascular cambium producing wood (xylem) and bark (phloem). A couple differences between root and shoot diameter growth are: (1) cambial growth is much more irregular in roots resulting in roots that are oval or irregular in cross section, and (2) there is greater variation in diameter with age and with horizontal roots compared to vertical roots.
Roots can also grow new lateral roots that form and branch off the main root. Some cells located in a layer inside the root produce a new root primordium. This new meristem divides and elongates pushing the root out through the parent root.
During the process of transpiration, water evaporates from the foliage through openings in leaves called stomata pulling adjoining water molecules with it. This pulling action helps draw water and nutrients up the trunk and into the leaves. In addition, some trees may be capable of exerting a pumping action to push water up the tree.
Photosynthesis, carried out in foliage, twigs and other green plant parts, produces sugars (and other components) that are used by the tree to carry out its many functions. Sugars, growth regulators, and proteins are moved about the plant in the phloem. Once sugar arrives at a location, it is used as energy for normal processes or it is stored as starch for later use. Trees need stored starch to carry out normal functions, especially to break dormancy in temperate trees.
Tree species have a wide range of lifespan. For example, while peach trees may live only 30 years, oaks may live 200 years, cypress 1,600 years, and bristle cone pine up to 5,000 years in an undisturbed site in the forest. The average tree in urban settings, however, has only one tenth the lifespan of a tree at a rural site.
When a tree ages: (1) Growth slows down; (2) Trees are more susceptible to diseases and insects; (3) The tops of the trees are more likely to die back; (4) Wounds compartmentalize more slowly; (5) There are fewer numbers of leaves relative to the size of the tree; and (6) There is an increased number of dead branches. All of these characteristics can also be seen in urban trees as they begin to die and/or show signs of poor health.
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