A tree is a perennial woody plant with a single trunk that supports secondary branches clear of the ground. The definition also includes a minimum height although this varies amongst authors - 3-6 metres for an adult plant is typical.
Woody perennials that are smaller than this and/or having multiple stems are technically shrubs. Note that what would normally be termed a tree, may have multiple stems (e.g. as a response to extensive injury).
Trees are a diverse classification that include a number of botanical groups such as angiosperms (flowering plants) and gymnosperms (incl. conifers and cycads). The world's earliest forests (Carboniferous era) consisted of tree ferns, horsetails and lycophytes.
Although smaller species typically have shorter lives of about 10-15 years, many varieties can live a lot longer (e.g. 1000 years for English Oak). Individual specimens of English Yew, Great Basin Bristlecone Pine, and Sequoia have been recorded with ages between 2500 and 4500 years old.
Trees are grown for a wide range of reasons, including:
- Visual landscaping (tree shape and/or blossom)
- Fruit or nuts
- Wind breaks
- Livestock control (i.e. as a hedge)
- Wood for construction or fuel
- To attract animals, especially mammals and birds
- Sap production (e.g. Maple or Rubber)