Shades of Green: Earth's Forests -- Plants

Plants provide shelter for everything from tiny insects to large mammals. The flowers, seeds, nuts, and leaves produced by plants are the food sources that allow animals to live in the forest. Plants also provide humans with many important medicines. For instance, the yew tree, which was once considered "waste wood", actually contains a substance called taxol, which helps to fight certain types of cancer.

Right and left: trees and bushes in a temperate forest. Photos by Maya Walters

Perhaps most importantly, oxygen needed by all animals is produced by the leaves of plants. Leaves are green because a chemical called chlorophyll is found within every one of their cells, and this chemical uses energy from sunlight and substances including carbon dioxide to produce oxygen.

Trees are, of course, the most obvious plants in the forest, but there are also numerous shrubs, vines, herbs, grasses, and a variety of other plants to be found among the trees. In tropical forests, many of these plants are actually found on the trees. These smaller plants which grow high above the ground on branches or trunks are known as epiphytes, and thrive in the moist tropical rainforest.

Almost ten percent of all higher plants are epiphytes, and in tropical areas, miniature forests of these plants grow on the larger, horizontal tree branches. Some epiphytes even grow on other epiphytes, creating a multi-layered habitat.

Grasses are the first plants to move into an area when the forest has been cleared. Photo by Maya Walters.

When a forest is destroyed, the first plants that move in and repopulate the area are grasses. Most of these plants are annuals, living for only one year, and though they are hardy and opportunistic, they are easily crowded out by larger shrubs. Most also require full sunlight, and therefore can't live in the dense shade of a thick forest. Grasses have thick root systems which can make up 90% of the plant's total weight.

Slightly larger plants, including bushes and shrubs, are perennial, which means they live for many years instead of only one like the annuals. These plants usually have thicker, woody branches, with flowers forming in clusters on the outer branches so they are visible and accessible to pollinating insects. Leaves grow at an angle on most shrubs so they get the best possible exposure to sunlight. Most of the smaller shrubs need full sunlight and are not adapted for dense forests.

Vines growing in a forest on Maui, Hawaii. Photo by Maya Walters.

Trees have an elaborate and complex structure. The tree grows using the compounds created in their leaves, which take in carbon dioxide and water, and combine them using energy from sunlight. The water is brought to the leaves through the trunk, an impressively tall and slender cylinder that is both sturdy but flexible enough to support the tree during high winds. The trunk is supported by the roots, a vast branching network that can extend farther underground than the height of the trunk. The tree is protected by bark, which also helps to keep wood-boring insects out.
find out more...

Almost all vines live in forests simply because they require trees for support. They are most common in tropical forests, and the trees here are often covered with these living ropes. Vines do not belong to any particular family of plants; many unrelated species have evolved to grow in this manner. Vines grow quickly because they use no energy for thick, supportive stalks; instead they hold themselves up by entwining stems around, or clinging onto, the branches of trees. Certain vines can also grow horizontally along the ground, and are therefore not limited to forest habitats.

Thorns on the stems of a wild rose bush. Photo by Maya Walters.

The soft green leaves of most plants are an ideal food source for many vegetarian creatures. Most plants suffer at least some damage from leaf-eating animals, and occasionally extreme pest outbreaks can occur. To help prevent complete defoliation, plants have evolved defenses to protect themselves from hungry insects. Some plants discourage herbivores by growing sharp-edged, spiny leaves. The thistle is one example of a plant that will remain untouched in a field after a herd of grazing animals have passed through.

Though relatively low in energy, leaves are an important food source for many animals. Photo by Maya Walters.

But the majority of foliage damage is the responsibility of insects, which are small enough to walk around thorns and not be bothered by sharp-edged leaves. It is for this reason that plants have evolved chemical defenses. We avoid poison ivy because of the plant's chemical defense--it contains oils which are responsible for the skin irritation it causes. Stinging nettles "sting" because, when disturbed, the plant's tiny hairs give off an irritating acid.

Flowers can be found on all kinds of plants, even trees which have such inconspicuous blooms they often go un-noticed. While some plants depend on wind to move pollen from one flower to another, insects are much more reliable pollinators. Many insects depend on flowers for their nectar, just as the flowers depend on insects for pollination.
find out more...

Huge variations in leaf shape between different plants are obvious, and there are also huge variations in patterns of leaf arrangement. Trees that grow in open meadows and clearings have much more light available and their small leaves seem to be arranged almost at random. These leaves are usually notched or lobed and not spaced very thickly on the branches. This means that each layer of branches doesn't cast much of a shadow, allowing more light to reach lower branches of the tree. In deep shade, however, trees have leaves arranged in a more definite pattern, with larger leaves growing towards the ground where almost all of the sunlight has been filtered out by higher branches.

A close-up of the texture on a backlit hazlenut tree leaf. Photo by Maya Walters.

Different types of forests have very different numbers and types of plants. Tropical rainforests have the most diverse plant communities of any habitat. Several hundred species of trees can exist in a small area of tropical rainforest, compared to one species of tree which can dominate the same amount of area in a boreal forest. The groundcover in rainforests is sparse because the thick tree canopy blocks most of the light, while in a temperate coniferous woodland, the trees are spaced far apart with large open spaces where grasses grow.

Seeds, nuts, and fruit are a major food source for mammals and birds. Migrating birds follow the ripening of seeds. Rodents such as squirrels store large collections of tree seed cones every fall. Still, plenty of seeds survive to sprout, often growing from a tiny dot into giant trees. Fruits, with their seeds stored inside, are eaten by another range of animals, and often the seeds inside are too large to be consumed and are left to sprout.
find out more...

sub-topics
[bark, wood, roots, & leaves] [flowers & pollen] [seeds, nuts, & fruit]

view the regular version of the plants article for faster load time

return to the list of condensed articles