Tree

I

Introduction

Tree, perennial plant having an upright woody main stem, and usually the tallest of plants at maturity. A tree differs from a shrub in that it usually produces a single, well-defined main stem, or trunk, and from a herbaceous plant in that the stem is composed almost entirely of woody tissue. Trees of some smaller species sometimes develop with more than one stem, like a shrub, but most larger species of tree never produce shrubby forms. Some species, when they reach maturity, are only a few metres tall, with trunks as slender as 15 cm (6 in) in circumference; the largest species may reach heights of more than 112 m (367 ft), with trunks that have a diameter of more than 6 m (20 ft).

Trees are popularly grouped into two broad categories: evergreen and deciduous trees. (These categories do not correspond strictly to the scientific classifications described below.) Evergreens are those that bear a full canopy of foliage throughout each year, only shedding a small proportion of the older leaves and replacing them with new leaves. Two evergreen leaf types are common: (1) needle leaf, typified by the tough, narrow or scale-like resinous leaves of most conifers; and (2) broadleaf, most common in angiosperms of tropical areas, but found in temperate areas as well. Deciduous trees are broadleaved and lose all their foliage each year, usually at the approach of the harshest season, which may be the coldest, darkest, or driest season. Most deciduous trees are broadleaved but some needle-leaved species, such as larches, are also deciduous.

II

Classification

Most trees are seed-bearing plants: either gymnosperms, which are mostly cone-bearing plants commonly called softwoods; or angiosperms, which are flowering plants, the trees of which are commonly called hardwoods. Trees may be either dicots or monocots, depending on their seed structure. Of the 60,000 to 70,000 species of trees and tree-like plants, all are dicotyledonous (dicots) except a few hundred monocotyledonous (monocot) species and fewer than 1,000 gymnospermous species.

All five living orders of gymnosperms consist primarily of tree species; the most important gymnosperm orders are the Pinales (pines) and Taxales (yews), comprising the conifers. Among the angiosperms, no true tree species are monocots. The only monocotyledonous family containing a preponderance of tree-like species is the palm family, Arecaceae, the genera of which are native to tropical and subtropical regions throughout the world. Despite their large size, these plants, together with the so-called tree-ferns, are not truly woody as are other trees.

III

Evolution

Trees have existed since the Devonian period (408.5 million to 362.5 million years ago) of the Palaeozoic era. The oldest trees known to palaeobotanists are those of the genus Cordaites, which originated in the early Devonian period and became extinct by the end of the Palaeozoic era (245 million years ago). The oldest known surviving order of trees, the broadleaved, gymnospermous Ginkgoales, is now represented by a single species, the maidenhair tree, Ginkgo biloba (see Ginkgo). Coniferous trees have existed since the middle of the Carboniferous period (about 285 million years ago). Angiospermous trees first appeared in the early Cretaceous period (145.6 to 97 million years ago) of the Mesozoic era, and by the beginning of the Pliocene epoch (5.2 million years ago) of the Cenozoic era virtually all tree genera now in existence were growing. The majority of fossil tree leaves found in Pliocene rocks are indistinguishable from leaves of present-day trees. See also Palaeontology.

IV

Climate and Soil Requirements

Trees grow wherever adequate groundwater is available for part of the year. Trees do not grow profusely in deserts or where the groundwater table is sufficient only for grassland vegetation; in such areas trees grow successfully only under careful cultivation, in desert oases, or along the banks of rivers and streams. Moreover, in areas bordering a grassland or desert, trees are frequently stunted and gnarled in growth. In high mountains or at the edge of the northern coniferous forests, such scattered, stunted, twisted trees are called krummholz. Under optimum conditions, however, trees can densely cover large areas called forests.

The climatic and soil requirements of trees are somewhat different for each species. A tree species may grow over a wide geographical range, of which only a small proportion permits optimum growth of the plant. The most common tree species in a given area is called the dominant species. In the boreal forests of northern Eurasia and North America coniferous trees are dominant, while further south, forests are more commonly dominated by various broadleaf species such as oak, beech, or lime.