Woody perennial plant. Most trees have a single self-supporting trunk containing woody tissues, and in most species the trunk produces secondary limbs called branches. Trees provide many valuable products, especially wood, one of the world's chief building materials, and wood pulp, used in papermaking. Wood is also a major fuel source. Trees supply edible fruits and nuts. In addition, trees take in carbon dioxide and release oxygen during photosynthesis. Their root systems help retain water and soil, preventing floods and erosion. Trees and forests provide habitats for a wide variety of animals, and they beautify both natural and altered landscapes. Growth rings in the trunk indicate the age of most trees. The tallest trees are the Pacific coast redwoods; the oldest are the bristlecone pines, some of which are over 4,000 years old. See alsoconifer; deciduous tree; evergreen; forest; shrub; softwood.