Shades of Green: Earth's Forests -- Bark, Wood, Roots, & Leaves

Depending on the species of tree, bark has different characteristics and functions. It provides trees with essential structural support, conducts nutrients from the leaves down to the roots, and offers protection from wood-boring insects and twig-gnawing mammals. In some cases, however, the bark itself becomes a food source, attracting animals such as porcupines which feed on the bark during the winter.

All trees have bark of some form and color. Some is smooth and shiny or papery, some is rough, thick and ridged. The variety of bark coloration is as diverse as its textures--tree trunks come in every shade from light silvery-white to deep orange-brown.

Above and top: Just a few examples of the variety of bark textures and colors. Photos by Maya Walters

Bark is sensitive and can be susceptible to both "sunburn" and "frostbite". If a tree that has grown for many years in deep shade is suddenly exposed to hot sun (when all the trees around it are cut down, for example), the bark may become red and flake off, injuring the tree. In northern regions, a frost too late in the spring or too early in the fall can kill new bark tissue, as well as leaves and new branches.

Why is there such variation in the color and texture of bark? Certain characteristics have obvious advantages in certain habitats. The thick, rough bark of pine trees withstands fire far better than the bark of oak trees, and therefore more pines survive the frequent fires in temperate coniferous forests. In other cases, the advantage of one type of bark over another is less well understood. Smooth bark might take less energy to produce, or it might help to discourage the growth of moss and lichen. Rough bark might be more effective in keeping wood-boring insects out. No one knows quite why the types of bark have evolved the way they have. One possibility is that the appearance of bark is not adaptive, and while all trees must have bark of one form or another, its exact color and texture are not particularly important.

Left: The bark on a Ponderosa pine trunk. Photo by Maya Walters.

Right: Cross-sections of two coniferous and one broad-leaved tree trunk. Photos by Maya Walters.

The central column of wood in a tree is called the "heartwood". All the cells in this layer are dead and clogged with resin, and are therefore unable to move nutrients through the tree. The heartwood's only job is to provide structural support. The outer layer of wood is called the "sapwood". This is the living layer of the tree, where all the nutrients are channeled. This explains how an old hollow tree can remain alive, for it is only the central heartwood that has rotted away. The diameter of either layer compared to the total size of the tree trunk varies depending on the species.

There is a major difference between the wood of most coniferous and broadleaved trees. Broadleaved trees have "pipes" (called vessels) running through the wood to carry nutrients between the roots and the leaves. Coniferous trees don't have these vessels. Instead, nutrients are moved up the trunk through small chains of special cells within the wood, and down the trunk from the leaves to the roots through cells in the bark. The vessels in broadleaved trees are many times larger than the chains of cells in coniferous trees (which can be as small as .2 millimeters), and therefore nutrients can flow much faster: sap can move at rates of 20 meters an hour in some oak trees, compared to only half a meter during the same amount of time in coniferous trees.

Above: The roots of a rainforest tree spread across the ground. Left: Some trees send out aerial roots from partway up their trunks. Photos by Maya Walters.

Tree roots are the essential framework for preventing soil erosion. Without roots to hold the soil in place, forest slopes simply wash away. Tree roots can extend great distances, and in some cases, roots from separate trees (of the same species) can "graft" themselves together. Roots from many trees can grow together into a single enormous network that supports many individual trees. Thus, when nutrients from a healthy tree are brought down into the roots, they may be "stolen" by a different individual tree that gets very little sunlight. Stumps connected to this network of roots can remain alive by receiving nutrients from the other trees.

A mature coniferous tree has several million needle-shaped leaves.

Above: New leaves begin to grow on deciduous trees. Below: Large-leaved plants in the tropical rainforest need little direct sunlight. Photos by Maya Walters.

The leaves are where "food" is created for the tree. Leaves are green because they contain a chemical called chlorophyll. This chemical allows the plant to manufacture sugars from carbon dioxide and water in a process called photosynthesis. Most leaves have a relatively tough, waxy, water-proof coating which protect them from hungry insects. It is only through the coating's tiny pores, called "stomata", that the required carbon dioxide can enter the leaf.

During photosynthesis, plants capture red and blue light wavelengths, and use their energy to combine the component atoms of water with carbon dioxide. The plant uses the resulting sugars for its own growth. Oxygen is simply a by-product of the reaction. All green plants use the chemical chlorophyll for photosynthesis. In fact, all plants are green because of chlorophyll -- it absorbs the red and blue light wavelengths, and only reflects green.

Above: The branches and leaves of taller trees form the forest canopy, home to a range of completely arboreal animals. Below right: Far beneath the canopy, fern leaves grow on the forest floor. Photos by Maya Walters.

The stomata* in the leaves, however, also let water vapor escape from the leaves. While this does work to cool leaves in the hot sun, plants in dry areas have fewer stomata or keep them closed for most of the day. This slows this rate of water loss (called transpiration) and prevents dehydration. In most trees, the fact that much more liquid is being moved upwards from the roots to the leaves than in the opposite direction is obvious when you consider that cells within the entire outer trunk carry nutrients upwards, but only the thin layer of bark carries them downwards.

When a plant appears to wilt during a dry season, it is because there isn't enough water in each of its cells to keep them rigid, and they have "deflated".

How does the water get up to the leaves to begin with? Of course it is brought up from the tree roots through long chains of cells in the tree trunk, but what is the force that makes it rise? This question has puzzled people for a long time. The most popular explanation now is that since water tends to "stick" to itself, any water that is evaporated at the top of the tree (through the leaves) is immediately replaced by more water being sucked up through the roots, keeping the tree's "plumbing" full of liquid at all times.

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