Tend To Tree Growth And Deterioration
August 1, 2012 Trees speak to me. I see how they grip the ground with a strong embrace. They stretch and ache, always upward. They exhale life-giving oxygen. Their leaves or needles fall timely and gracefully to nourish the soil. They travel around the sun with us, in life and death providing shade, protection, and structure. Many take them for granted, but a few of us may choose to study them in appreciation of their service to our human species.
To understand a pesticide or chemical affect on a tree or its wood, it is important to understand the growth and deterioration process. Whether it is a herbicide, insecticide, fungicide or preservative, knowing about the cell structure of trees will help in good decisions making for pest control operations.
Tree growth is what makes wood one of our most valuable natural resources. As a building material, only wood is a sustainable, renewable resource. Using water and nutrients from the air and soil, trees build wood. Photosynthesis is the process. Tree roots move water and nutrients from the soil to the leaves through hollow cells in the xylem or sapwood. Tree leaves absorb carbon dioxide from the air, using chlorophyll and sunlight to make sugary food for the tree. These sugary solutions manufactured by the leaves are moved through phloem in the inner bark to the growth areas of the tree, such as the tips of branches and roots and the trunk. The trunk hosts the cambium, a layer of reproductive cells found between the inner bark and the sapwood.
Water and nutrients up = xylem
Sugary solutions down = phloem
This narrow cambium layer does the work of making wood, creating new sapwood cells toward the inside and new inner bark cells to the outside, responsible for the tree’s growth in diameter. As the tree grows, older phloem cells are pushed out, die, and become part of the outer protective layer of trees called bark. Bark serves as the skin of the tree, protecting tender cells near the cambium from attack by insects, animals, birds, fungi, cold, wind, or fire.
The wood of the tree is the xylem, and includes both the sapwood and heartwood. The heartwood is darker colored dead xylem cells, providing the strength of the tree.
As the circumference of the tree grows, the thin ring of cambium grows equally. Climatic conditions affect the growth of cells; near the tropics, the growth in nearly constant. In temperate areas, the wood cells grow quickly in spring and slow in the fall. This pattern of fast/slow growth of cells gives trees the distinctive annual rings; thin walled cells in spring, thick walled in fall.
The function of these cells is critical to chemical control and preservation. To kill unwanted invasive tree species (preserving space for the desirable trees) a cut-stump or girdle herbicide application works because the phoem cells transport the herbicide down to kill the roots. Herbicides applied just inside the bark provide the best control. An injection of a systemic insecticide, alternatively, needs to reach into the inner living xylem cells to move up throughout the tree. Preservatives for wood will only penetrate effectively if the outer cells are permeable, influenced by the type of cells, pits, ray cells, and presence of extractives.
Stay On Top Of Tree Care ctd.
Trees are givers. As a byproduct of all this photosynthesis, sugar making and wood growth, trees give off oxygen. Even in their death and decomposition, trees give back, contributing nutrients to our soils. Wood can give centuries of service, but may eventually succumb to attack and degradation by fungi, insects and other organisms, providing food and shelter.
Decay of wood is a reversal of the growth process. In growth, the action of the sun on leaves combined with water and carbon dioxide forms sugars. Sugars transported to growing parts of the tree form cell walls, mostly cellulose. With decay, cellulose and starch are returned to sugars by enzymes and then to carbon dioxide and water.
Fungi are one of wood’s attackers. Conks and mushrooms are good examples of the fruiting bodies producing and spreading fungal spores. Wood-rotting fungi generally require temperatures between 50F◦ and 90F◦ with the optimum 70F◦ – 85F◦, moisture content of approximately 30%, oxygen, and the cellulose and lignins of wood as a food source.
Insects are another attacker of wood, using the giving tree as food or shelter. Termites, Ants, Beetles and Borers are some of the insects with a wood appetite. Subterranean termites are the most destructive insect pests of wood. They are social insects living in colonies in the ground, with one of three jobs: reproducer, soldier, or worker. The workers are wingless, sterile and blind, and tear into wood with heavy jaws. Wood particles are ingested and digested into cellulose, a food for other colony members.
Tall and majestic members of the plant kingdom, a tree may be defined as a woody that will grow to 20 feet in height with a definite crown. There are two categories of trees – hardwoods and softwoods. Ironically, these terms have nothing to do with the hardness or softness of the wood, but rather the type of seed covering produced. Softwoods, or gymnosperms include the important group called conifers bearing seeds inside opened cones. Their leaves are actually needles or scales, and most remain on the tree thoughout the winter, hence the name “evergreens.” Hardwoods are angiosperms, producing seeds enclosed in a fruit or nut. Most hardwoods have broad leaves and are typically deciduous, dropping leaves in the fall season.
Angiosperms – enclosed seeds – hardwoods
Gymnosperms – naked seeds – softwoods
In softwoods, over 90% of the wood is made of longitudinal tracheid cells. These long, thin cells are parallel, vertical conduits for movement of water and nutrients in the naked seed tree. Hardwoods have a more variable cell structure. Large, hollow vessels lie within a bed of fiberous tracheids. In hardwoods, these vessels conduct water and nutrients; the tracheids provide mostly strength. These vessels are large enough they can appear as holes on cross sections of wood. Both angiosperms and gymnosperms also have ray cells laying horizontally, transporting waste materials toward the heartwood. Rays are wider in hardwoods and form the distinctive wood grain patterns.
Trees are durable. Some species have natural resistance to decay and insect damage due to the extractives in the heartwoods. Extractives are the waste products transported by ray cells, deposited in the dead heartwood cells. Often toxic to insects and fungi, extractives can act as preservatives. Many tree species with natural durability are becoming scarce and expensive as wood products. For instance, old-growth Bald Cypress, Cedars, Black Locust, Red Mulberry, Redwood and Black Walnut show a great resistance to decay. Younger Bald Cypress, with a greater proportion of sapwood, and Longleaf and Slash Pine show moderate resistance to decay. Other pine species join Sweetgum, Magnolia and Sycamore as slightly decay resistant.
The crown of a tree – the leaves and branches at the top – filter dust, provide shade, sequester carbon, and give habitat to wildlife. The roots of a tree bring water and nutrients from the soil and provide anchor and support. The tree trunk or stem gives shape and strength with a network of cells offering us beautiful wood. We must appreciate our trees.
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