Venation (arrangement of the veins)

There are two subtypes of venation, namely, craspedodromous, where

the major veins stretch up to the margin of the leaf, and camptodromous,

when major veins extend close to the margin, but bend before they intersect

with the margin.

Feather-veined, reticulate — the veins arise pinnately from a single

mid-vein and subdivide into veinlets. These, in turn, form a complicated

network. This type of venation is typical for (but by no means limited to)

dicotyledons.

Pinnate-netted, penniribbed, penninerved, penniveined; the leaf has

usually one main vein (called the mid-vein), with veinlets, smaller veins

branching off laterally, usually somewhat parallel to each other; e. g.

Malus (apples).

Three main veins branch at the base of the lamina and run essentially

parallel subsequently, as in Ceanothus. A similar pattern (with 3—7 veins)

is especially conspicuous in Melastomataceae.

Palmate-netted, palmate-veined, fan-veined; several main veins diverge

from near the leaf base where the petiole attaches, and radiate toward

the edge of the leaf; e. g. most Acer (maples).

Parallel-veined, parallel-ribbed, parallel-nerved, penniparallel — veins

run parallel for the length of the leaf, from the base to the apex. Commissural

veins (small veins) connect the major parallel veins. Typical for most

monocotyledons, such as grasses.

Dichotomous — There are no dominant bundles, with the veins forking

regularly by pairs; found in Ginkgo and some pteridophytes.

Note that although it is the more complex pattern, branching veins appear

to be plesiomorphic and in some form were present in ancient seed

plants as long as 250 million years ago. A pseudo-reticulate venation that

is actually a highly modified penniparallel one is an autapomorphy of

some Melanthiaceae which are monocots, e. g. Paris quadrifolia (Truelover's

Knot).

Margins (edge)

The leaf margin is characteristic for a genus and aids in determining

the species:

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entire: even; with a smooth margin; without toothing;

ciliate: fringed with hairs;

crenate: wavy-toothed; dentate with rounded teeth, such as Fagus

(beech);

dentate: toothed, such as Castanea (chestnut);

coarse-toothed: with large teeth;

glandular toothed: with teeth that bear glands;

denticulate: finely toothed;

doubly toothed: each tooth bearing smaller teeth, such as Ulmus

(elm);

lobate: indented, with the indentations not reaching to the center, such

as many Quercus (oaks);

palmately lobed: indented with the indentations reaching to the center,

such as Humulus (hop);

serrate: saw-toothed with asymmetrical teeth pointing forward, such

as Urtica (nettle);

serrulate: finely serrate;

sinuate: with deep, wave-like indentations; coarsely crenate, such as

many Rumex (docks);

spiny: with stiff, sharp points, such as some Ilex (hollies) and Cirsium

(thistles).

THREATS

By now, it is a well-known fact that forests everywhere are facing a

range of threats. The forests that remain today cover a fraction of the area

that was forested even a few hundred years ago. And the speed of destruction

is only increasing. Everything from the direct and obvious effects of

over-cutting to the more subtle effects of climate change are threatening to

destroy the last of the remaining natural forests.

Threatened forests are more than just threatened trees. When the trees

disappear, so does everything that depends on them, from fungi and microorganisms

to tigers and bald eagles. Each species has its own particular requirements

for habitat, and therefore preserving only certain forest ecosystems,

which in many cases aren't prime sources of timber, does not protect

the habitat of all forest species.

When a forest is cleared and new trees are planted, the diversity of

animals is not brought back. A plantation of one or a few kinds of trees

does not support as many life forms as a natural forest. These plantations

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will not produce as much high-quality timber. The trees in natural forests

have been growing for hundreds and even thousands of years. Planted trees

are cut as soon as eighty years after they are planted.

Planting trees and logging, then planting and logging again in continuing

cycles can degrade soil and water. Water often washes soil away on

cleared slopes, making the area unsuitable for new trees, and destroying

riparian zones below. Plantations are also much more susceptible to pests

and diseases. Pest controls and fertilizers are often used on plantations,

which may have other unfavorable effects on soil and water.

Much of the human-caused forest destruction stems from overpopulation.

In many places, there are too many people trying to make a living

from too few forest resources. Trees are cleared not only for wood and

other products, but so that the land they once grew on can be turned into

pastures for cattle and agricultural fields to feed growing numbers of people.

As cities expand, forests are cleared to make room for housing developments,

shopping malls, golf courses, and other structures that require

large amounts of land.

Other threats to the health of forests are more indirect. In certain areas,

including much of Europe and the eastern United States and Canada, forests

are declining because of air pollution. This pollution is from the fossil

fuels burned in vehicles — cars, trucks, buses — and from industry. Carbon

dioxide (CO2), a gas which contributes to "global warming", is a well

known emission from these sources, but vehicles and industry also produce

sulphur dioxide and nitrous oxide gases, which create acid rain. Acid

deposits can kill trees directly by leaching nutrients from them, resulting in

the death of leaves and needles. The most damage, however, is caused

when acid gets into the soil and releases poisonous heavy metals, which

are naturally present but usually inaccessible. At the same time as the acid

rain releases these poisons, it also dissolves and washes away vital nutrients

in the soil.

People have been destroying forests for hundreds of years, but the rate

of destruction has been increasing so rapidly that some forests won't last

much longer. Some areas, such as the American Southeast, that were cut a

century ago are now recovering gradually. However, much of the choice

timber is now gone and these "new" Southeast forests, which are only beginning

to re-grow, are once again being cut. More remote areas are also

being logged heavily, often to supply the growing market for wood-chip

products. Coastal temperate forests in Chile, which are home to over

700 species of plants are being cut for this reason. Remote forests in Russia

are also being cut, mostly for raw logs. About one fifth of the world's

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forests are found in Russia. In some cases, these logs are not allowed to be

shipped to North America because of fears that species of insect pests

might also be unknowingly imported.

For thousands of years, humans have played an ever increasing role in

global deforestation. Throughout history, one empire after another stripped

forests to build their ships and dwellings, and for fuel. Once devastated,

those forests didn't recover for a thousand years or more, and some never

did like parts of the Mediterranean, the Middle East, and Great Britain.

Global deforestation has accelerated dramatically in recent decades.

The tropical forests of South America and Southeast Asia are being cut

and burned at an alarming rate for large and small agricultural uses, from

huge palm oil plantations to slash and burn subsistence farming. Fires set

for these purposes often rage out of control.

Any reduction in the forest is a problem for its ecosystem. Deforestation

occurs when the forests are turned into farms to feed people or converted

into cash crops or used for raising livestock. Also, logging trees for

commercial use or for firewood leads to the destruction of the forests and

to their use as land for agriculture.

Deforestation isn't just about a loss of trees. It also has a large impact

on the environment. Many living creatures depend on the trees, so when

the trees disappear, the animals die off as well (decreased biodiversity).

Potentially valuable medicines and materials are lost, as well as clean water

and air. Indigenous peoples suffer, and eventually national economies

suffer too. The futures of people and forests are still interconnected.

Trees also store water and then release it into the atmosphere (this

process is called transpiration). This water cycle is an important part of the

ecosystem because many plants and animals depend on the water that the

trees help to store. When the trees are cut down, there is nothing to hold

the water, leading to a drier climate. Also, the loss of trees causes erosion

because there are no roots holding the soil together. The dirt then washes

away into lakes and rivers, degrading aquatic habitats.

Deforestation leads to the increase of carbon dioxide in the air because

living trees store CO2 in their fibers, but when they are cut down, the carbon

is released back into the atmosphere. CO2 is a major greenhouse gas,

so cutting down trees contributes to the danger of climate change.

No communities of plants and animals are stable. Many factors are

constantly disrupting these ecosystems — weather, predation, food supply,

and, above all, humans. Conditions are favorable to different species at different

times. Ecosystems are constantly changing, and after every change,

it is impossible to re-create the ecosystem that existed before. That is one

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of the problems humans face when trying to "manage" forests. As hard as

they might try, people can't mold the forest into something that has been,

or keep the forest at the state it is now.

Disturbances to forest ecosystems happen all the time; some create

short-term havoc, others take several years to begin showing their effects.

Some disturbance is necessary to create the diversity of habitats. For example,

small, frequent fires in certain areas make it easier for pines to survive

where oaks would otherwise grow. When fires happen regularly, species

adapt, and some trees even depend on fire to release their seeds.

Weather can change forests in less dramatic ways as well. Temporary

cold spells and droughts can seriously affect certain species, though don't

usually change the entire composition of the forest. Other disturbances,

such as pests, may also be confined to one or two types of trees, creating

small gaps in the forest. Diseases can also be responsible for far more serious

changes in forests. The effect of any given disturbance depends on its

frequency, predictability, and magnitude.

Fire is one of the natural disturbances that forests have evolved to deal

with over time. Periodic fires caused by lightning have shaped and altered

forests for millions of years. For example, the heat generated in fires actually

enables Lodgepole pinecones to release their seeds. In this century,

humans have made serious efforts at fire suppression.

The relationships between forests, fire, insects, disease, and climate

are only recently beginning to be understood. The huge fires in Yellowstone

National Park in 1988 caused many people to wonder whether a

more natural approach to fire might be better than complete suppression.

There is no doubt that fire destroys trees, which humans consider a waste.

But the hidden benefits of natural fires might be greater than were previously

thought. Forest fires move in varying and often unpredictable ways.

Ground fires creep through the duff, and fires may smolder below the surface

for long periods of time. Surface fires are another form of forest fire,

and move along at up to 1.3 meters in height. Crown fires occur higher in

the trees (in the upper branches known as the "crown" of a tree), and can

be of several different varieties. These include "dependent crown fires",

which use convection to pre-heat the crown, and most dangerous of all, independent

crown fires which leap from tree to tree.

Fires affect forests in various ways. In intense fires, nitrogen, calcium,

phosphorus, and potassium can be vaporized; and soil microorganisms can

be killed up to 3 inches under the soil, but recover quickly. Mycorrhizal

fungi growing on tree roots can also be killed by fire. Recovery of trees

takes longer than other organisms. A year after the Yellowstone fires of

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1988, grass and inch-tall lodgepole pine seedlings were spotted emerging

from the soil. Certain plants have adaptations which help them recover

more quickly after a fire. Chaparral shrubs benefit from their large root

systems, as their taproot can grow to 3.5 feet in length after only three

months. Other plants sprout from roots instead of seeds, giving them an

advantage.

Animals can be severely affected by fires. Their habitats are most often

destroyed, including their food source which means that even if they

escape the flames, they have to find new territory. Many animals do escape

from fires, and move to another area. Other animals which live in the surrounding

forest come to feed on the new seedlings and young plants growing

in a recently burned area.