Des arbres au bord de l'étang aux couleurs vives.
Un arbre est une plante ligneuse terrestre comportant un tronc sur lequel s'insèrent des branches ramifiées portant le feuillage dont l'ensemble forme le houppier, appelé aussi couronne.
Les arbres sont des plantes pérennes qui vivent plusieurs années, de plusieurs décennies à plusieurs siècles, et dans de rares cas plusieurs millénaires.
Selon les estimations, la planète compte entre 60 000 et 100 000 espèces d'arbres.
Les formations végétales dominées par des arbres poussant les uns à cñté des autres sont des forêts, mais les arbres peuvent aussi croître de manière plus ou moins isolée hors des forêts (dans la savane notamment).
Ils jouent un rñle majeur dans le fonctionnement écologique terrestre, en raison de leur capacité à stocker le carbone (leur production de matière sèche annuelle correspond à deux tiers de la production mondiale des plantes terrestres), à prendre une part active dans le cycle de l'eau et de manière générale à constituer les écosystèmes complexes que sont les forêts, sources et refuges de biodiversité.
In botany, a tree is a perennial plant with an elongated stem, or trunk, supporting branches and leaves in most species. In some usages, the definition of a tree may be narrower, including only woody plants with secondary growth, plants that are usable as lumber or plants above a specified height. Trees are not a taxonomic group but include a variety of plant species that have independently evolved a woody trunk and branches as a way to tower above other plants to compete for sunlight. Trees tend to be long-lived, some reaching several thousand years old. In wider definitions, the taller palms, tree ferns, bananas, and bamboos are also trees. Trees have been in existence for 370 million years. It is estimated that there are just over 3 trillion mature trees in the world.
A tree typically has many secondary branches supported clear of the ground by the trunk. This trunk typically contains woody tissue for strength, and vascular tissue to carry materials from one part of the tree to another. For most trees it is surrounded by a layer of bark which serves as a protective barrier. Below the ground, the roots branch and spread out widely; they serve to anchor the tree and extract moisture and nutrients from the soil. Above ground, the branches divide into smaller branches and shoots. The shoots typically bear leaves, which capture light energy and convert it into sugars by photosynthesis, providing the food for the tree's growth and development.
Trees usually reproduce using seeds. Flowers and fruit may be present, but some trees, such as conifers, instead have pollen cones and seed cones. Palms, bananas, and bamboos also produce seeds, but tree ferns produce spores instead.
Trees play a significant role in reducing erosion and moderating the climate. They remove carbon dioxide from the atmosphere and store large quantities of carbon in their tissues. Trees and forests provide a habitat for many species of animals and plants. Tropical rainforests are among the most biodiverse habitats in the world. Trees provide shade and shelter, timber for construction, fuel for cooking and heating, and fruit for food as well as having many other uses. In parts of the world, forests are shrinking as trees are cleared to increase the amount of land available for agriculture. Because of their longevity and usefulness, trees have always been revered, with sacred groves in various cultures, and they play a role in many of the world's mythologies.