Consequently, from both a taxonomic and a phylogenetic perspective, the tree is an artificial category. On an ecological basis, however, the tree can be recognized as a natural construct, as it represents an adaptive strategy by many different taxa to exploit and dominate the habitat above the ground.

In the early stages of the development of terrestrial life, land plants were rootless and leafless. Since they had their origins in aqueous environments, they did not require the specialized conducting and supporting tissues afforded by roots and stems, nor did they require localized regions of carbohydrate synthesis, since each cell was involved in metabolism, water and nutrient absorption, and respiration. Habitats farther from the water as well as aerial habitats represented available uninhabited environments.

One key to exploiting terrestrial habitats is increasing complexity of the plant’s form to allow specialization of function. This requires physiological and morphological complexity as well as biological optimization. If all the tissues of massive tree trunks were alive, for example, the physiological cost of maintaining these structures in the living state would be enormous and probably unattainable. An elegant solution came in the form of tremendous structural adaptations: new tissues and organs permitted localization of the functions of the plant body