Tree bark: The Living Work Beneath the Surface
What grows from the ground with a woody trunk and branches, and has bark that is a covering rather than a sound? A tree is the answer, and tree bark protects its trunk and branches while living tissues beneath the surface carry food and build new growth.
The trunk lifts leaves above competing plants, giving them better access to sunlight. Trees developed this towering form independently in many plant groups, rather than descending from a single group made up only of trees. Their woody tissues provide strength, and their vascular systems move water, nutrients, and other materials through the plant. Below ground, roots anchor the trunk and draw moisture and minerals from soil. Above ground, branches divide into shoots bearing leaves, where photosynthesis turns captured light energy into sugars that support growth. Some trees live for several thousand years, maintaining this traffic between crown, trunk, and roots.
For dicotyledonous trees and conifers, the outermost covering consists largely of dead cork cells. This tree bark forms a thick, waterproof barrier around living tissue, protecting it from weather, disease, animal attack, and fire. Tiny pores called lenticels allow oxygen to pass through. A living layer called the cork cambium continually replaces the covering. As the trunk expands, older outer layers may split into fissures. Different species shed them in different ways: the London plane loses large flakes, while silver birch peels in strips. Protection is therefore continually renewed as the growing trunk increases in circumference beneath it.
At the inner edge of tree bark lies the phloem, a soft, spongy layer of living cells that carries sap containing sugars made in the leaves. Some cells join end to end as tubes, while neighboring cells provide support and strengthening fibers. Immediately inside sits the vascular cambium, a layer only one cell thick. Its dividing cells produce phloem toward the outside and wood, called xylem, toward the inside. New xylem becomes sapwood, carrying water and minerals upward from the roots. Older sapwood gradually becomes heartwood, the dense central core that gives the trunk much of its rigidity.
Tree bark has chemical defenses as well as physical ones. Pines release sticky resin that discourages attackers, rubber trees ooze milky latex, and the quinine bark tree contains bitter substances that make its covering unpalatable. Even so, boring insects can penetrate. Beetles lay eggs in crevices, and their larvae chew tunnels through the tissues. Those openings can admit fungi. Dutch elm disease spreads when beetles carry a fungus between elm trees; the tree responds by blocking the xylem that carries sap upward. Branches lose nourishment, and eventually the entire tree can die. In Britain, the disease killed 25 million elms by the 1990s.
Underground, a seedling's first root grows downward before lateral roots spread horizontally through upper soil layers. In most trees the original taproot eventually withers, leaving those spreading laterals. Fine root hairs touch soil particles directly and absorb dissolved nutrients and water. Roots also need oxygen for respiration, so only a few species, including mangroves and pond cypress, can survive permanently waterlogged soil. Many roots also partner with fungi, exchanging the sugars produced by photosynthesis for minerals such as phosphorus. Fungal threads can link different trees and move nutrients and signals between them. Some trees, including alders, harbor nitrogen-fixing bacteria in root nodules, allowing them to grow in soils where limited nitrogen would otherwise make survival difficult.
Seasonal growth changes the tree's outward appearance. In cold or dry conditions, expansion commonly stops, and buds enclose the growing points in protective scales. Inside are miniature leaves and a small stalk ready for the next growing season. When conditions improve, a shoot pushes out and sheds its scales, leaving scars on the twig. Much of a year's growth can take place within a few weeks. Deciduous trees shed foliage and pass through a leafless dormant period, while evergreens retain green foliage throughout the year. Larches are conifers that follow the deciduous pattern, dropping their needles each autumn.
The long history of trees reaches back around 400 million years. Their forests now shelter other plants and animals, reduce erosion, and store carbon in woody tissues. Dense northern conifer forests face long winters and a short summer growing season, with little light reaching the ground beneath their canopy. In temperate regions with rainfall spread through the seasons, broadleaf and mixed forests include oak, beech, birch, and maple. Drier tropical savannas have more open canopies, with grass and scrub below trees such as acacias and baobabs. In tropical rainforests, broad buttress roots brace large trunks against wind where poor soils keep roots near the surface.