Vegetal On The World: forests and forestry

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Conifers

Conifers

The conifers, which are woody plants consisting mostly of evergreen trees, make up the phylum Coniferophyta, one of four phyla of gymnosperms that have living representatives. The word “conifer”means cone-bearing. Most conifers bear their reproductive structures in cones.

With 50 genera and 550 of the 700 known gymnosperm species, the phylum Coniferophyta includes the bulk of the gymnosperms. Coniferophyta is also the most widespread and, in terms of numbers of individual trees, the most abundant of the gymnospermphyla.

The abundance and economic and ecological importance of the Coniferophyta are out of all proportion to the number of species, which does not begin to compare to the 235,000 species of angiosperms (phylum Anthophyta).

Conifers grow over almost the entire world. They are especially abundant in northern temperate and boreal regions. They predominate in the taiga, or northern boreal forest, which covers immense stretches of northern North America and northern Eurasia.

They dominate that forest by virtue of large numbers of individuals of only about a dozen species,mainly of spruce, fir, and larch. They show greater diversity in mid latitude, mountain forests of the Northern Hemisphere. Conifers also occur in coolmountain areas of the tropics.

In temperate regions of the Southern Hemisphere, they are widespread but less dominant than in the north. Junipers and pines are the most wide-ranging conifers, occurring across the northern continents and into the tropics. In the Southern Hemisphere, the most widely distributed conifer genus is Podocarpus.

Due to their abundance and wide range, their typically arboreal habit, and their stem structure, conifers are the only gymnosperms that yield timber on a commercial scale. They are the source of all of the world’s softwood timber and nearly half of its total annual lumber supply.

A Long Fossil Record

A Long Fossil Record

The oldest known conifer fossils date to the late Carboniferous period of the Paleozoic era, some 300 million years ago. The earliest known angiosperms are less than half this old.

Conifers diversified greatly during the Paleozoic’s relatively dry, cold Permian period (290-245 million years ago), which followed the Carboniferous. The drought-resistant, needlelike leaves characteristic of many modern conifers may have evolved during that dry period.

The modern families of conifers began to differentiate in the Mesozoic era (245 million to 65 million years ago). Conifers were the dominant vegetation during much of that era.

They gradually gave way to the angiosperms, which achieved worldwide dominance by about ninety million years ago, during the Cretaceous period of the Mesozoic, and remain dominant today. In the early Tertiary period of the Cenozoic era, which dawned about 66 million years ago, a diverse coniferous flora covered large areas of the Northern Hemisphere.

Vegetative Features

Vegetative Features

Conifers have certain notable vegetative characteristics. For example, many, though not all, of them have needlelike leaves, and such leaves occur only in conifers. Some conifers have scalelike leaves, and a few have bladelike ones.

The stems and roots of conifers are also note worthy, resembling the woody eudicots and magnoliids of Anthophyta in producing a dense mass of secondary xylem—that is, wood.

Reproductive Characteristics

In conifers, the ovules, which contain the eggs and eventually ripen into seeds, are not enclosed in ovary tissues within flowers as they are in the angiosperms.

Instead, like other gymnosperms, conifers have naked ovules. These ovules are borne on scales that are arranged spirally along the axes of cones. Due to a complex evolutionary history, the structure of these cones is compound rather than simple.

Reproductive Characteristics

In addition to ovulate, or female, cones, conifers produce male cones, which are smaller, shorter-lived, and generally less woody. The axis of a male cone bears reduced leaves, each with two pollen sacs on the undersurface. The pollen grains formed within the sacs contain the immature sperm. In most conifers, cones of both genders are present on the same tree.

Although gymnosperms other than conifers may produce cones, conifers are distinctive in combining simple pollen cones with compound ovulate cones. The yews (genus Taxus) bear their ovules singly, at the tips of branchlets, rather than in cones. The seeds that develop from these ovules are enclosed in a fleshy covering called an aril.

In the spring, the male cones of conifers release their pollen to the wind. Some of it reaches female cones, which have their cone scales widely separated to receive it.

After pollination, the female cones close their scales. Fertilization usually occurs, within the ovule, about three days to three or four weeks after pollination. In the pines, however, it is delayed for about fifteen months.

In most conifers, the mature seeds, with their contained embryos, are dispersed in the autumn of the year in which fertilization occurs. The most common dispersal agent is the wind, aided in many species by wings on the seeds.

Conifer seeds that lack wings generally depend on dispersal agents such as birds or mammals. In some pines, the cones need the heat of a forest fire to open the scales and release the seeds.

Pine Family

Of the families of living conifers, the Pinaceae, or pine family, is the largest, containing ten to twelve genera, with about two hundred species world-wide. Most of these are resin-bearing trees.

This family is the most widespread and abundant conifer family in the Northern Hemisphere and very important economically. It supplies a great deal of lumber, pulpwood, and paper products. It is also important ecologically, providing essential cover, nesting habitat, and food for many species of wildlife.

Pine Family

The pine family’s range ismainly in Eurasia and NorthAmerica. Genera that occur all across this expanse are Pinus (pines), Picea (spruces), Abies (firs), and Larix (larches).

Douglas firs (Pseudotsuga) and hemlocks (Tsuga) are restricted to North America and Asia. The true cedars (Cedrus) occur only in warm-temperate regions of Eurasia and northern Africa. Three genera of the pine family are restricted to China.

Pinus, the oldest genus in the pine family, arose at least 130 million years ago, in the early Cretaceous period. It grows predominantly in the Northern Hemisphere. The pines number more than ninety species, nearly half the total species of the Pinaceae.

One of the world’s oldest known plants is the bristlecone pine (Pinus longaeva), which grows in Utah, Nevada, and a small area of western California. Some bristlecone pines are about forty-six hundred years old. In contrast to other conifers, pines produce their adult needles in fascicles, or bundles. The number of needles per fascicle depends on the species.

Other Conifer Families

According to a commonly used classification system, there are seven families of conifers in addition to the Pinaceae. A notable one is the Taxodiaceae, which is today represented by widely scattered species that are the remnants of populations that were much more widespread during the Tertiary period.

Bald cypress belongs to the Taxodiaceae, as do two redwood species renowned for their size and longevity. The coast redwood (Sequoia sempervirens) grows along coastal California and Oregon, and the “big tree” (Sequoiadendron giganteum) occurs on the western slope of the Sierra Nevada mountains in California.

The coast redwood is generally the taller of the two species, reaching up to 117 meters in height. The big tree, however, generally exceeds the coast redwood in total mass and can live to be several thousand years old.

Another notable member of the Taxodiacae is the dawn redwood (Metasequoia glyptostroboides). This species was first known to science only from fossils discovered in the early 1940’s. Not long afterward, the tree was found growing in China.

A more recent living-fossil find belongs to the Araucariaceae, a conifer family restricted to the SouthernHemisphere. This tree, the Wollemi pine (Wollemia nobilis), had been thought to have gone extinct fifty million years ago. Then, in 1994, a stand was found growing in Australia.

Wollemi pine (Wollemia nobilis)

The Cupressaceae, or cypress family, includes Chamaecyparis (false cypress), Juniperus (junipers, often called “cedars” in North America), and Thuja (arbor-vitae). These plants are distributed throughout the world. Some are important as timber or ornamentals.

The Taxaceae family, which includes the yews, is widely distributed in the Northern Hemisphere. The Podocarpaceae is largely restricted to the Southern Hemisphere. The Sciadopityaceae and Cephalotaxaceae are confined to east Asia.

environmental issues , forests and forestry

Deforestation

Hinhvv

6:28 AM

Deforestation

Deforestation is the loss of forestlands through encroachment by agriculture, industrial development, nonsustainable commercial forestry, or other human as well as natural activity.

Concerns about deforestation, particularly in tropical regions, have risen as the role that tropical forests play in moderating global climate has become better understood.

Environmental activists decried the apparent accelerating pace of deforestation in the twentieth century because of the potential loss of wildlife and plant habitat and the negative effects on biodiversity.

By the 1990’s research by mainstream scientists had confirmed that deforestation was indeed occurring on a global scale and that it posed a serious threat to global ecology.

Deforestation as a result of expansion of agricultural lands or nonsustainable timber harvesting has occurred in many regions of the world at different periods in history. The Bible, for example, refers to the cedars of Lebanon.

Lebanon, like many of the countries bordering the Mediterranean Sea, was thickly forested several thousand years ago. A growing human population, over harvesting, and the introduction of grazing animals such as sheep and goats decimated the forests, which never recovered.

Countries in Latin America, Asia, and Africa have also lostwoodlands. While some of this deforestation is caused by a demand for tropical hard-woods for lumber or pulp, the leading cause of deforestation in the twentieth century, as it was several hundred years ago, was the expansion of agriculture.

nonsustainable timber harvesting

The growing demand by the industrialized world for agricultural products such as beef has led to millions of acres of forestland being bulldozed or burned to create pastures for cattle.

Researchers in Central America have watched with dismay as large beef-raising operations have expanded into fragile ecosystems in countries such as Costa Rica, Guatemala, and Mexico.

A tragic irony in this expansion of agriculture into tropical rain forests is that the soil underlying the trees is often unsuited for pastureland or raising other crops. Exposed to sunlight, the soil is quickly depleted of nutrients and often hardens.

The once verdant land becomes an arid deserts, prone to erosion, that may never return to forest. As the soil becomes less fertile, hardy weeds begin to choke out the desirable forage plants, and the cattle ranchers move on to clear a fresh tract.

percentage of annual deforestation by country

Slash-and-Burn Agriculture

Beef industry representatives often argue that their ranching practices are simply a form of slash-and-burn agriculture and do no permanent harm. It is true that many indigenous peoples in tropical regions have practiced slash-and-burn agriculture for millennia, with only a minimal impact on the environment. These farmers burn shrubs and trees to clear small plots of land.

Anthropological studies have shown that the small plots these peasant farmers clear can usually be measured in square feet, not hectares as cattle ranches are, and are used for five to ten years. As fertility declines, the farmer clears a plot next to the depleted one.

The farmer’s family or village will gradually rotate through the forest, clearing small plots and using them for a few years, and then shifting to new ground, until they eventually come back to where their ancestors began one hundred or more years before.

Slash-and-Burn Agriculture

As long as the size of the plots cleared by farmers remains small in proportion to the forest overall, slash-and-burn agriculture does not contribute significantly to deforestation.

If the population of farmers grows, however, more land must be cleared with each succeeding generation. In many tropical countries, traditional slash-and-burn agriculture can then be as ecologically devastating as the more mechanized cattle ranching operations.

Logging

Although logging is not the leading cause of deforestation, it is a significant factor. Tropical forests are rarely clear-cut by loggers, as they typically contain hundreds of different species of trees, many of which have no commercial value. Loggers may select trees for harvesting from each stand. Selective harvesting is a standard practice in sustainable forestry.

However, just as loggers engaged in the disreputable practice of high-grading across North America in the nineteenth century, so are loggers high-grading in the early twenty-first century in Malaysia, Indonesia, and other nations with tropical forests.

High-grading is a practice in which loggers cut over a tract to remove the most valuable timber while ignoring the damage being done to the residual stand. The assumption is that, having logged over the tract once, the timber company will not be coming back.

Logging

This practice stopped in North America, not because the timber companies voluntarily recognized the ecological damage they were doing but because they ran out of easily accessible, old-growth timber to cut. Fear of a timber famine caused logging companies to begin forest plantations and to practice sustainable forestry.

While global satellite photos indicate significant deforestation has occurred in tropical areas, enough easily harvested old-growth forest remains in some areas that there is no economic incentive for timber companies to switch to sustainable forestry.

Logging may also contribute to deforestation by making it easier for agriculture to encroach on forestlands. The logging company builds roads for use while harvesting trees. Those roads are then used by farmers and ranchers to move into the logged tracts, where they clear whatever trees the loggers have left.

Environmental Impacts

Despite clear evidence that deforestation is accelerating, the extent of the problem remains debatable. The United Nations Food and Agriculture Organization (FAO), which monitors deforestation worldwide, bases its statistics on measurements taken from satellite images. These data indicate that between 1980 and 1990, at least 159 million hectares (392 acres) of land became deforested.

Environmental Impacts

The data also reveal that, in contrast to the intense focus on Latin America by both activists and scientists, the most dramatic loss of forestlands occurred in Asia. The deforestation rate in Latin America was 7.45 percent, while in Asia 11.42 percent of the forests vanished.

Environmental activists are particularly concerned about forest losses in Indonesia and Malaysia, two countries where timber companies have been accused of abusing or exploiting native peoples in addition to engaging in environmentally damaging harvesting methods.

Researchers outside the United Nations have challenged the FAO’s data. Some scientists claim the numbers are much too high, while others provide convincing evidence that the FAO numbers are too low. Few researchers, however, have tried to claim that deforestation on a global scale is not happening.

In the 1990’s the reforestation of the Northern Hemisphere, while providing an encouraging example that it is possible to reverse deforestation, was not enough to offset the depletion of forestland in tropical areas. The debate among forestry experts centers on whether deforestation has slowed, and, if so, by how much.

Deforestation affects the environment in a multitude of ways. The most obvious effect is a loss of biodiversity. When an ecosystem is radically altered through deforestation, the trees are not the only thing to disappear. Wildlife species decrease in number and in variety.

As forest habitat shrinks through deforestation, many plants and animals become vulnerable to extinction. Many biologists believe that numerous animals and plants native to tropical forests will become extinct from deforestation before humans have a chance to even catalog their existence.

Other effects of deforestation may be less obvious. Deforestation can lead to increased flooding during rainy seasons. Rainwater that once would have been slowed or absorbed by trees instead runs off denuded hillsides, pushing rivers over their banks and causing devastating floods downstream. The role of forests in regulating water has long been recognized by engineers and foresters.

Flood control was, in fact, one of the motivations behind the creation of the federal forest reserves in the United States during the nineteenth century. More recently, disastrous floods in Bangladesh have been blamed on logging tropical hardwoods in the mountains of Nepal and India.

Conversely, trees can also help mitigate against drought. Like all plants, trees release water into the atmosphere through the process of transpiration. As the world’s forests shrink in total acreage, fewer greenhouse gases such as carbon dioxide will be removed from the atmosphere, less oxygen and water will be released into it, and the world will become a hotter, dryer place.

Scientists and policy analysts alike agree that deforestation is a major threat to the environment. The question is whether effective policies can be developed to reverse it or if short-term economic greed will win out over long-term global survival.

ecology , forests and forestry , gymnosperms , methods and techniques , paleobotany

Dendrochronology

Hinhvv

2:08 AM

Dendrochronology

Dendrochronology is the science of examining and comparing growth rings in both living and aged woods to draw inferences about past events and environmental conditions.

In forested regions with seasonal climates, trees produce a growth ring to correspond with each growing season. At the beginning of the growing season, when conditions are optimum, the vascular cambium produces many files of large xylem cells that form wood.

As the conditions become less optimal, the size and number of cells produced decreases until growth stops at the end of the growing season. These seasonal differences in size and number of cells produced are usually visible to the unaided eye.

The layers produced during rapid early growth appear relatively light-colored because the volume of the large cells is primarily intracellular space. These layers are frequently called springwood because in northern temperate regions spring is the beginning of the growing season.

Wood formed later, summerwood, is darker because the cells are smaller and more tightly compacted. The juxtaposition of dark summerwood of one year with the light springwood of the following yearmarks a distinct line between growth increments.

The width of the ring between one line and the next measures the growth increment for a single growing season. If there is a single growing season per year, as in much of the temperate world, then a tree will produce a single annual ring each year.

Tree Rings and Climate

Leonardo da Vinci is credited with counting tree rings in the early 1500’s to determine “the nature of past seasons,” but it was not until the early 1900’s that dendrochronology was established as a science.

Tree Rings and Climate

Andrew Douglass, an astronomer interested in relating sunspot activity to climate patterns on earth, began to record the sequences of wide and narrow rings in the wood of Douglas firs and ponderosa pines in the American Southwest.

Originally, trees were cutdown in order to examine the ring patterns, but in the 1920’s Douglass began to use a Swedish increment borer to remove core samples from living trees.

This instrument works like a hollow drill that is screwed into a tree by hand. When the borer reaches the center of the tree it is unscrewed, and the wood core sample inside is withdrawn with the borer.

The small hole quickly fills with sap, and the tree is unharmed. Borers range in size from 20 centimeters to 100 centimeters or more in length, so with care, samples can be taken from very large, very old living trees.

Counting backward in the rings is counting backward in time. By correlating the size of a ring with the known regional climate of the year the ring was produced, a researcher can calibrate a core sample to indicate the surrounding climate during any year of the tree’s growth. By extending his work to sequoias in California, Douglass was able to map a chronology extending back three thousand years.

Tree Rings and History

Tree Rings and History

In order to extend his chronologies so far back in time, Douglass devised the method of cross-dating. By matching distinctive synchronous ring patterns from living and dead trees of the same species in a region, researchers can extend the pattern further into the past than the lifetime of the younger tree.

Archaeologists quickly realized that this was a tool that could help to assign the age of prehistoric sites by determining the age of wood artefacts and construction timbers.

In this way archaeologists could calculate the age of pre-Columbian southwestern ruins, such as the cliff dwellings at Mesa Verde, Arizona, by cross-dating living trees with dead trees and the latter with timbers from the sites. In 1937 Douglass established the Laboratory of Tree-Ring Research at the University of Arizona, which continues to be a major center of dendrochronological research.

Fine-Tuning

In the mid-1950’s Edmund Schulman confirmed the great age of living bristlecone pines in the Inyo National Forest of the White Mountains of California. In 1957 he discovered the Methuselah Tree, which was more than forty-six hundred years old. The section of forest in which he worked is now known as the Ancient Bristlecone Pine Forest.

During the next thirty years, Charles Ferguson extended the bristlecone chronology of this area back 8,686 years. This sequence formed the basis for calibrating the technique of radiocarbon dating. In the 1960’s, radiocarbon analysis began to be used to determine the age of organic (carbon-based) artefacts from ancient sites.

Fine-Tuning

It has the advantage of being applicable to any item made of organic material but the disadvantage of having a built-in uncertainty of 2 percent or more. Tree-ring chronologies provide an absolute date against which radiocarbon analyses of wood samples from a site can be compared.

At about the same time, Valmore La Marche, a young geologist, began to study root growth of the ancient trees to determine how they could be used to predict the erosional history of a site.

By crossreferencing growth ring asymmetry to degree of exposure and slope profiles, he was able to estimate rates of soil erosion and rock weathering, which in turn could be cross-referenced to the climatic conditions predicted by growth rings in the stem.

La Marche and his colleagues, particularly Harold Fritts, continued to “fine-tune” the reading of growth rings to be able to take into account factors such as soil characteristics, frost patterns, and daily, weekly, and monthly patterns.

The Oldest Tree

The Methuselah Tree, mentioned above, is the oldest known living tree. Schulman also cored a forty-seven-hundred-year-old living specimen in the White Mountains, but he did not name it or identify its location.

While most of the living specimens older than four thousand years are found in the White Mountains, the oldest living tree was discovered in the Wheeler Peak area of what is now Great Basin National Park in eastern Nevada.

This tree, variously known as WPN-114 and the Prometheus Tree, was estimated to be between forty-nine hundred and fifty-one hundred years old when it was cut down in 1964 as part of a research project. The controversy that followed has left many interesting but unanswered questions.

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