2017-11-01
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The trunk of a tree is important for two reasons: first, it acts as a support rod, giving the tree its shape and strength; second, it acts as the central “plumbing system” in a tree, forming a network of tubes that carries water up and down to the branches, trunk and roots. The easiest way to see how a tree works is to look at a cross section of a trunk. Here are the five main layers you would see, and what each layer does. The outer layer of the trunk (and branches) is called the outer bark or just the bark. Tree bark can be smooth, scaly, rubbery, flaky, craggy, or bumpy. Its texture, thickness and flexibility depend on the type of tree. Although bark looks different from tree to tree, it serves the same purpose – to protect the tree from injury and disease. Some trees have very thick bark that helps prevent damage from fires. Others have bad-tasting chemicals in their bark that discourage hungry insects. And some bark is covered with spines or thorns that keep browsing mammals away. The bark of large Douglas firs and sequoias may be more than two feet (6 m) thick. The layer next to the outer bark is called the inner bark, or phloem. The phloem is a thin layer that acts as a food supply line from the leaves to the rest of the tree. Sap (water containing dissolved sugars and nutrients) travels down from the leaves through channels in the phloem to the branches, trunk and roots, supplying all the living parts of the tree with food. At certain times during the year, the phloem also transports stored sugars up from the roots to the rest of the tree. If you were to cut a band around the trunk, through the bark and phloem, the tree would probably due. That’s because the phloem would be severed and food could no longer flow to the lower trunk and roots.) Next to the phloem is a very thin layer called the cambium. (It is often only one or two cells thick, and you need a microscope to see it well.) The cambium is one of the growing layers of the tree, making new cells during the growing season that become part of the phloem, part of the xylem, or more cambium. The cambium is what makes the truck, branches, and roots grow thicker. The layer next to the cambium is called the sapwood or new xylem. The sapwood is made up of the youngest layers of wood. (Each year the cambium adds new layers of woody tissue.) The sapwood is a network of thick-walled cells that forms a pipeline, carrying water and minerals up the tree from the roots to the leaves and other parts of the tree. The sapwood also stores nutrients and transports them across the tree, from one part to another. Most of the trunk in an old tree is dead wood called heartwood, or just plain wood. The heartwood is old xylem that no longer transports water and minerals up a tree. (After a few years the sapwood in most trees gets filled in with resin-like material and slowly changes into heartwood. The new xylem is the only part of the wood that works as a transport system.) The heartwood is often much darker in color than the sapwood. The heartwood gives the tree support. But sometimes it rots away, leaving a hollow, living tree. Hollow trees often topple over or split apart in storms because they are very weak after the heartwood has decayed. A trees’ roots are long, underground branches that spread out to help anchor the tree and to absorb water and nutrients from the soil. Some trees have long taproots that reach straight down for 15 feet (4.5 m) or more. Other trees have more shallow root systems that lie closer to the surface of the ground. Large taproots and lateral roots branch into smaller and smaller roots. An average tree has millions of these small rootlets, each covered with thousands of fine root hairs. The root hairs make it easier to soak up water and dissolved minerals from the soil. (Most of the rootlets lie very close to the surface of the ground where most of the water and nutrients are located.)
From skinny pine needles to broad palm leaves, all tree leaves serve the same purpose – to make food for the tree. Leaves use carbon dioxide from the air, water from the roots, and sun’s energy (in the form of sunlight), to make sugar (glucose). This food-making chemical reaction is called photosynthesis. Photosynthesis can take place only in the presence of chlorophyll – the green pigment that is found in all green plants. Chlorophyll absorbs the sunlight needed for photosynthesis. During photosynthesis the leaves release oxygen that becomes part of the air that we and other animals breathe.
(4527 signs)
The buds
If you look at a branch on a deciduous tree during the winter when the leaves are gone, you can see terminal buds at the very ends of the twigs. These are the points from which the next spring’s growth will take place. Buds are protected from freezing by a covering of moisture-conserving bud scales. Twigs also have side or lateral buds, from which side branches develop. Buds are formed in the summer and fall and remain dormant over winter. Dormancy is broken when suitable growing conditions return, usually in the spring.
If you look further back along a twig, you will come to a ring of thickened bark. This scar tissue marks the place where the terminal bud grew the previous year and is called the terminal bud scar. The distance between the scar and the new terminal bud is therefore the amount of growth that took place in one year. Knowing this, you can find the age of several branches of different sizes on a tree.
Buds are actually areas of growth; where the stem or branch lengthens or leaves develop. In vascular plants, most cells that have differentiated – into phloem or xylem for example – lose their ability to divide. Each cell of either xylem or phloem tissue is formed undifferentiated tissue that continues to divide by mitosis as long as the plant lives. This tissue, where ever it might be located in the plant, is referred to as meristem.
Meristematic tissue can be found at the tip of a stem. As a stem lengthens, the majority of the meristematic tissue remains at the tip and some remains behind. Branches and leaves develop from this remaining tissue. A bud actually forms before the end of the growth season and contains the differentiated cells (xylem, phloem, mesophyll and so on) of a leaf. These cells are quite small and in the new season, the leaves expand mostly from the enlargement of these small cells.
(1844 signs)
Photosynthesis
The leaves are the food factories of a tree. They contain chlorophyll, which facilitates photosynthesis and gives leaves their green color. Through a process called photosynthesis, leaves use the sun’s energy to convert carbon dioxide from the atmosphere and water from the soil into sugar and oxygen. The sugar, which is the tree’s food, is either used or stored in the branches, trunk and roots. The oxygen is released into the atmosphere.
The process of Photosynthesis (the process by which plants make sugar from sunlight, water, and carbon dioxide):
– sunlight shines through the top of the leaf and reaches the next layer of cells. The light energy is trapped by the chlorophyll in the chloroplasts. In the chloroplasts, a process that uses water changes the light energy into a kind of chemical energy. This chemical energy is stored in the chloroplasts;
– the chloroplasts use the chemical energy to make food. Air enters the leaf through the stomata and moves into tiny spaces around the food-making cells in the leaf. Carbon dioxide from the air passes through the cell walls and membranes of the cells. Carbon dioxide enters the chloroplasts where the previously stored chemical energy converts the carbon dioxide into sugar;
– tubes in the plant carry sugar from the leaf cells to other parts of the plant, such as roots, stems, and fruits. Cells in these parts of the tree store some of the sugar.
(1408 signs)
The root system
The root system is important to the eucalyptusprimarily because its rapid growth and size demands large quantities of water. It needs a strong taproot to secure it and lateral roots to support its size. The taproot needs to sink down into the soil at least six feet for good anchorage. Lateral roots will spread out to one hundred feet which can be a problem when planted near buildings and other types of facilities. The roots and rootlets can disrupt ditches, crack cisterns, clog water pipes and damage septic tanks. Eucalyptus roots are aggressive.
Aspen has an extensive root system, and ramets have been recorded growing up to 40 metres from a parent tree. Because of their access to nutrients through the parent tree’s root system, aspen ramets can grow very quickly – up to a metre per year for the first few years. As the ramets grow, they remain joined through their roots, and all the interconnected trees are called a clone. They are all the same individual organism and are therefore all single-sexed, either male or female. Each clone exhibits synchronous behaviour, with, for example, all the component trees coming into leaf at the same time in the spring. A clone can also sometimes be identified by the specific colour its leaves change to in the autumn. Aspen roots also have the unusual ability of staying alive underground for many years after the death of the parent tree. This leads to the appearance of aspen ramets in areas where there are no mature trees.
(1500 signs)
Hardwood Forests
One major type of hardwood forest is the steamy, tropical rain forest that grows in the tropics and subtropics. These forests are typically composed of big, old evergreen hardwood trees that grow in several layers, or groups according to height. The tallest layer may reach heights of 150 feet (45 meters) or more. Other layers, composed of shade-tolerant trees, reach to about 100 feet (30 meters). Smaller trees and vines cover the ground in the shade in a dense tropical rain forest. These forests are often called jungles, though true jungles are dense thickets of brush and vines that may not have any tall trees.
One of the largest and most important tropical rain forests is in South America. It extends from the mouth of the Amazon River, in Brazil, to the mouth of the Orinoco River, in Venezuela, and covers millions of acres. Another great rain forest girdles the waist of Africa, and much of Southeast Asia is covered with rain forests. In the United States, only the southern tip of Florida and a bit of the coast of Texas have tropical forests.
Beautiful woods such as rosewood, mahogany, ebony, and teak are commercially important products of these forests. They are prized for fine furniture, cabinetry, and artistic uses.
Hardwood forests other than tropical ones cover much of the United States, most of Northern Europe, areas of northern Russia, eastern China, and the east coast of Australia. These forests include broad-leaved trees adapted to higher elevations with more moderate climates.
The largest forest area in the United States is the Central Hardwood Forest. It covers about 130 million acres (52.6 million hectares), from the edge of the central plains, across the lower edge of the Great Lakes states, to the coasts of New Jersey and New York. It straddles the Appalachian Mountains as far south as northern Georgia and spreads westward and covers large parts of eastern Texas. Because this area is blessed with rich soil and a moderate climate, it has been cleared in many places for farming.
Much of the wood for America’s furniture industry comes from this forest. Hardwoods such as black walnut, red oak, white oak, yellow poplar (also called tulip tree), sweet gum, and sycamore are among the most valuable trees. A few softwoods are also found, including the shortleaf and white pines.
(2318 signs)
Softwood Forests
The Northern Forest covers much of the Great Lakes states and the New England states in the United States. It also extends along the ridges of the Appalachian Mountains as far south as northern Georgia. This forest also covers much of Canada and the interior of Alaska and stretches across northern Asia, Russia, and the Scandinavian countries.
The Northern Forest is composed of softwood trees such as white pine, hemlock, and red spruce. Several hardwood trees, such as oak, maple, birch, and basswood, are also present. Trees in this forest tend to be smaller and more slow-growing than those in warmer areas with longer growing seasons.
The softwood forests of the southeastern United States are sources of lumber, plywood, and paper pulp. The more important southern, or yellow, pines are the loblolly, longleaf, shortleaf, and slash. The bald cypress grows in southern swamps.
Hardwoods such as the red oak, white oak, hickory, gum, ash, pecan, and live oak are found along southern rivers. They are a significant part of the forest economy in the South.
Another important forest region in the United States is the softwood forest of the Rocky Mountains. There are almost 90 million acres (36.4 million hectares) of lumber-producing commercial forests. Ponderosa pine, Idaho white pine, Douglas fir, Englemann spruce, larch, lodgepole pine, and western red cedar are among the important softwoods.
The rainy climate of the Pacific Northwest contributes to the growth of one of the United States’ most productive softwood forest areas. It extends in a narrow band from the southern part of Alaska to central California. Inland, on the western slopes of the Coast Range, it extends somewhat farther south. These forests contain California redwood, western hemlock, western red cedar, sugar pine, lodgepole pine, and white fir. Most of America’s old-growth forests, stands which have not been harvested by humans or nature for hundreds of years, are in this region.
(1995 signs)
Mangrove Forests
Mangroves are woody, specialized types of trees of the tropics that can live on the edge, where rainforests meet oceans. Found on sheltered coastlines and river deltas, they grow in brackish wetlands between land and sea where other plants can’t grow. They protect the coastline and prevent erosion by collecting sediment from the rivers and streams and slowing down the flow of water. There are about 39.3 million acres of mangrove forests in the warm coastlines of tropical oceans all over the world.
Mangrove trees look as if they grow on stilts. The stilts are their specialized aerial roots which hold the trunk and leaves above the water line. Mangrove forests are affected by the rising and falling of the ocean’s tides. The aerial roots and tap roots can filter out the salt in the brackish water they grow in. Support roots grow directly into the mud to anchor the tree. Other roots snake up and down with the upward loops rising above the salt water level. Salt crystals taken up by the roots are stored in the leaves. The mangrove rids itself of the salt by shedding its leaves after a while.
One of the most biologically diverse forests, mangrove forests are known as the “rainforests by the sea”. The forests are the breeding grounds for fish, shrimp, prawns, crabs, shellfish and snails. Mud skippers are found in mangrove forests. The mudskipper has made adaptations so it can skim across the mud during low tide when the ground is uncovered in many places, traveling from puddle to puddle. Mangrove forests are also nesting sites for many shore birds and home to crab eating monkeys, fishing cats, lizards, sea turtles, and many more animals. For many species of fruit bats mangrove blossoms and fruit make up a large part of their diet.
Mangroves have a very specialised adaptations that enable them to live in salty waters. Breathing roots allow them to survive in anaerobic sediments. Buttresses and above-ground roots enable them to grow in unstable mud flats. Their foliage removes excess salt from the sap, and they conserve water to cope with periods of high salinity. Their seeds are buoyant to allow them to disperse and establish themselves in new areas.
Today mangrove forests are one of the most threatened habitats in the world. Mangrove roots are very susceptible to pollutants like crude oil clogging their lenticels, and continual flooding from artificial dikes and sea walls. Mangroves act as sinks which concentrate pollutants like sewage, toxic minerals and pesticides and herbicides. Over time the stress of the pollutants and reduced light kill large areas of mangroves forests. Mangrove wood also makes a superior kind of charcoal and many trees are being cut down to sustain local charcoal industries. Mangrove forests are also being filled in for developments and as a form of mosquito control.
Public awareness and education about the mangrove forests may help protect them. It is necessary to involve local communities in managing and protecting their mangroves. Designated conservation areas may also save some of the forests. However, national governments have not been able to enforce laws and regulate development and industry to save the wetlands. Adequate provisions for conservation and restoration have not been included in development of the mangrove wetlands.
(3369 signs)
Improving the Forest
Over many years nature improves forests by removing inferior trees, an example of natural selection through the survival of the fittest. But many natural events and human activities affect this process. Fires, floods, wind and ice storms, and harvesting practices all affect the growth of the forest. In general it is fair to say that nature’s events generally improve the forests over the years and the activities of human beings in the past have harmed them.
Recently, through forest management, human beings have begun to do less harm, perhaps even help forests. In controlling the composition of the stand by planting species they prefer, human beings can improve the quality of the trees they plant. Much has been learned about the development of genetically improved strains of corn, wheat, and other food crops. This improvement can be accomplished quickly because these crops produce a new generation each year. Progress is slower with trees because they often take many years to produce seed.
When forest managers think of improving trees, they want trees with long, straight trunks that can be sawed into lumber or peeled for plywood more economically. They want trees with small crowns, or tops, so more trees can get sunlight and grow on each acre. They want trees that are insect and disease resistant so their chances of maturing are better. And they want trees that will grow to commercial size as rapidly as possible.
Forest scientists have accomplished all these goals with pines and with some hardwood species such as sycamore, cottonwood, and yellow poplar. They have even learned to clone a superior tree, a process that produces more trees exactly like it. This is done in the laboratory by forcing tiny cuttings from the tree to develop roots and leaves.
The most commonly used method of tree improvement involves first locating superior specimens of the species. Branches are collected from their crowns, often by shooting them off with guns.
The branches are grafted, or joined, onto rootstocks in special nurseries where they grow until they are old enough to bear seed. A great advantage of this method is that tree branches that come from mature trees bear seed much sooner than do trees that are grown from seed. The flowers on the selected branches are carefully pollinated with pollen from other superior trees. Thousands of combinations of pollen trees and branch trees are tested. The resulting seed is then planted. Many of the trees that grow from the seed have all the bad characteristics of their parents. But some have all the good characteristics. Forest scientists try to produce and plant as many seeds having the good combinations as they can.
An example of what can be achieved is that improved species of pines produce much more usable wood in the same time as ordinary wild trees produce relatively little. Such improvements mean that more wood can be grown on less land in a shorter time. As the world population grows and requires more land for home sites and food production, this ability to maximize wood production on each acre will be vital to future generations.
