Tree

 

      

    Most plants classified as trees have a single self-supporting trunk containing woody tissues, and in most species the trunk produces secondary limbs, called branches. tree, woody plant that regularly renews its growth (perennial). Although the majority of Earth’s terrestrial biomass is represented by trees, the fundamental importance of these seemingly ubiquitous plants for the very existence and diversity of life on Earth is perhaps not fully appreciated. Their vast trunks and root systems store carbon dioxide, move water, and produce oxygen that is released into the atmosphere. The biosphere is dependent on the metabolism, death, and recycling of plants, especially trees. There are few organisms as important as trees for maintaining Earth’s ecology. The organic matter of the soil develops primarily from decayed leaves, twigs, branches, roots, and fallen trees, all of which recycle nitrogen, carbon, oxygen, and other important nutrients. To many, the word tree evokes images of such ancient, powerful, and majestic structures as oaks and sequoias, the latter being among the most massive and longest-living organisms in the world. more information on the three botanical groups that include trees, see fern (e.g., the tree ferns), gymnosperm (including conifers), and angiosperm (the flowering plants). This article discusses the historical, popular, and botanical classifications of trees, their evolution, their importance to humans, and their general structure and patterns of growth. For general information on plants, see plant. Modern classifications of plants attempt to assign a plant to a particular taxon and establish relationships with other plants based on genetics, cytology, ecology, behaviour, and probable evolutionary lineages, in addition to gross morphology. The ancient Greeks developed a classification about 300 BCE in which plants were grouped according to their general form—that is, as trees, shrubs, undershrubs, and vines. Popular classifications, however, remain useful tools for studying the common stresses that the environment exerts on all plants and the general patterns of adaptation that are shown no matter how distantly plants are related. This classification was used for almost 1,000 years.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 may be contrasted with a shrub, which might be loosely defined as a woody plant with multiple stems that is, in most cases, less than 3 metres (about 10 feet) tall. It is a relic that has been preserved in cultivation around ancient Buddhist temples in China and planted elsewhere as an ornamental since the mid-18th century; the tree probably no longer exists in a wild state.

ginkgo leaves

ginkgo leaves

The leathery fan-shaped leaves of a ginkgo tree (Ginkgo bilo), a gymnosperm.

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Conifers are the largest group of gymnosperms and include trees and shrubs in 7 extant families and 545 species. 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. This requires physiological and morphological complexity as well as biological optimization. By far the greatest number of tree species are eudicotyledon; they are represented by such familiar groups as birches, elms, hollies, maples, oaks, poplars, ashes, and willows.

cherry tree

cherry tree

Cherry tree in bloom with white blossoms.

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Ecological and evolutionary classification

The tree is not an immutable biological category but rather a human concept based on visual criteria. Some paleobotanists suggest that trees are the most primitive members within these plant families. Thus, a given woody species may be a tree in one set of habitats within its range and a shrub elsewhere. An elegant solution came in the form of tremendous structural adaptations: new tissues and organs permitted localization of the functions of the plant body.

The evolution of discrete plant body parts with separate functions allowed plants to move onto the land and undergo an incredible adaptive radiation. It is furthermore true that, among the flowering plants, trees are found not only among the most primitive members (order Magnoliale) but also among the more specialized, or advanced, members, such as the roses (order Rosales).

Consequently, from both a taxonomic and a phylogenetic perspective, the tree is an artificial category. For example, the spruce and fir may thrive in the tree form at the base of a mountain but assume a shrub form near the mountaintop, the variation due principally to stresses exerted by such environmental conditions as altitude, temperature, and oxygen tension.

As seen in the section above, trees are found among many plant families that also include shrubs and herbs, so that the concept of tree is not a phylogenetic one. Familiar representatives are araucaria, cedars, cypresses, Douglas firs, firs, hemlocks, junipers, larches, pines, podocarpsredwoods, spruces, and yews.

monkey puzzle tree

monkey puzzle tree

Cones and branches of a monkey puzzle tree (Araucaria araucan), native to central and southern Chile and western Argentina.

© Caroline Mcauliffe—EyeE/Getty Images

Dominating Earth’s present flora are the angiosperms, with about 300,000 species, among which are the majority of the world’s trees. These graceful trees, which are natives of humid montane forests in the tropics and subtropics and of warm temperate regions of the Southern Hemisphere, have huge lacy leaves; they are the remnants of a vastly more numerous flora that populated much of Earth during the Carboniferous Period (about 358.9 to 298.9 million years ago).

tree fern

tree fern

Tree fern (Cyathe medullari).

Copyright John Shaw/Bruce Coleman Inc.

Cycads compose the Cycadophyt, a division of gymnospermous plants consisting of 4 families and approximately 140 species. Perhaps a general definition would describe a tree as a perennial woody plant that develops along a single main trunk to a height of at least 4.5 metres (15 feet) at maturity. Angiosperms are sometimes divided on the basis of a group of characteristics into two major groups: the monocotyledons and the eudicotyledon. However, tree forms are found in all the vascular plants, from the club mosses and ferns to the gymnosperms and angiosperms. The most numerous of the monocotyledonous trees are palms; others include agaves, aloes, dracaena screw pines, and yuccas. Stems provided mechanical strength as well as a conductive capacity to transport water and nutrients from the roots to the leaves. Natives of warm regions of the Eastern and Western hemispheres, they also are remnants of a much larger number of species that in past geologic ages dominated Earth’s flora.

cycads

cycads

Modjad cycads (Encephalarto transvenosu) in Pretoria National Botanical Garden, Gauteng province, South Africa.

The ginkgo is the only living representative of the gymnosperm division Ginkgophyt. 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. Leaves evolved as specialized photosynthetic organs. Roots provided anchorage and absorption of sufficient water and nutrients to support the remainder of the plant. However, a species fitting the description of either in one area of the world might not necessarily do so in other regions, since a variety of stresses shape the habit of the mature plant. Further, there is no clear consensus as to whether the tree form is the advanced or primitive condition. Phylogenetic classifications

Trees are represented in each of the major groups of the vascular plants: pteridophytes (seedless vascular plants that include the tree ferns), gymnosperms (cycads, ginkgoes, and conifers), and angiosperms (flowering plants).

Although tree ferns account for only a small percentage of ferns, many are conspicuous members of a forest, attaining heights of 7 to 10 metres (23 to 33 feet); some are 15, 18, or occasionally 24 metres tall (49, 59, or 79 feet).