They differ from shrubs, which have a similar growth form, by usually growing larger and having a single main stem; made more confusing by the fact that trees may be reduced in size under harsher environmental conditions such as on mountains and subarctic areas.
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.
A new layer of wood is added in each growing season, thickening the stem, existing branches and roots.
A commonly applied narrower definition is that a tree has a woody trunk formed by secondary growth, meaning that the trunk thickens each year by growing outwards, in addition to the primary upwards growth from the growing tip.
The cork cambium gives rise to thickened cork cells to protect the surface of the plant and reduce water loss.
Both the production of wood and the production of cork are forms of secondary growth.
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. 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.
Most conifers are evergreens but larches (Larix and Pseudolarix) are deciduous, dropping their needles each autumn, and some species of cypress (Glyptostrobus, Metasequoia and Taxodium) shed small leafy shoots annually in a process known as cladoptosis.
In many tall palms, the terminal bud on the main stem is the only one to develop, so they have unbranched trunks with large spirally arranged leaves.
If insufficient water is available the leaves will die.