Approximately 25% of all living plant species are trees, made up from 100,000 tree species. But what are the defining characteristics of a tree?
Trees are long-lived perennial, woody plants. Trees parts include the trunk, branches, twigs, leaves and roots. The outer wood is covered by bark. They generally have a single stem with a crown of secondary branches held above a clear trunk, although some have multiple stems. Trees vary greatly in size but an accepted guideline is a minimum height at maturity of 3 metres, with a 30 cm trunk girth. Other woody plants that do not meet these requirements are called shrubs.
Trees are very significant within the ecosystem. They produce oxygen and reduce carbon dioxide through photosynthesis, as well as preventing erosion and moderating ground temperatures. Their branches and leaves provide a valuable ecosystem protected from the weather. A single oak tree can be home to over 300 insect species.
Trees evolved in the Triassic period, when gingkos, cyads and other gymnosperms produced woody tissues. Flowering trees followed later in the Cretaceous period. Most trees today are Angiospems (flowering plants) or conifers and are deciduous or evergreen.
Trees come in a variety of sizes, forms, shape, leaf shape and bark characteristics. They are categorised in their genus or species according to their shape and characteristics of their bark, flowers, fruit and leaves. The can range in size 3m tall to in excess of 100m. The tallest recorded tree is a Sequoia sempervirens (Coast redwood) in Redwood National Park, California which measures 115.56m (379 ft).
Trees can live for decades to several centuries, with some specimens living for several thousand years old. The record for the oldest tree is the Great basin bristlecone pine Pinus longaeva aged 4,844 years.
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