Tree

Britannica defines a tree as a woody plant that regularly renews its growth, with most species having a single self-supporting trunk that produces branches. One common definition sets a tree apart from a shrub by describing it as a perennial woody plant that grows along a single main trunk to a height of at least 4.5 meters (15 feet) at maturity. In taxonomy the tree is an artificial category: it is a growth form that many different plant families adopted independently rather than a single branch of the evolutionary tree.

The trunk carries the crown and holds the tree's shape and sturdiness, while the cambium and the vascular system sit just under the bark and run water and nutrients between the leaves and the roots. Roots absorb water and nutrients from the soil, store them for the tree and anchor it in place. Fifty percent of a tree's roots lie in the top 6 inches of soil, with fine feeding roots branching off the larger structural roots.

Leaves make food for the tree: they contain chlorophyll, and during photosynthesis they absorb energy from light to make sugar and oxygen. The overall reaction uses six carbon dioxide molecules and six water molecules, converted by light energy captured by chlorophyll into a sugar molecule and six oxygen molecules, with the oxygen released as a by-product. Photosynthesis is critical for the existence of the vast majority of life on Earth, and almost all the oxygen in the atmosphere comes from the process.

As chlorophyll breaks down in autumn it reveals other leaf chemicals, including yellow flavonols, orange carotenoids and red to purple anthocyanins, whose mixture varies between species. As the tree becomes dormant, abscisic acid triggers a seal at the base of each leaf, reducing water flow and trapping chemicals in the leaf before it drops to the ground.

Trees are commonly sorted into popular groupings that reflect their foliage and reproductive habits. Softwoods are the conifers, while hardwoods are the broadleaf trees; alternatively, trees may be divided into evergreens and deciduous species. Conifers are gymnosperms, usually bearing needle-shaped evergreen leaves with seeds attached to the scales of a woody cone. They are most abundant in cool temperate and boreal regions, where they serve as important timber trees and ornamentals, and they reach their greatest diversity in warmer areas including tropical mountains.

The north coast of Devon and Somerset preserves what is recorded as Earth's earliest fossil forest, with trees dating to around 390 million years ago. The Somerset forest is four to five million years older than the previous record, the fossil forest at Cairo, New York. The trees there, of the form known as Calamophyton, resembled palms but were unrelated to modern trees; their trunks were hollow-centred and ringed by woody supporting strands, and their branches carried hundreds of twig-like structures.

In the evolution of land plants, leaves became specialized photosynthetic organs, stems supplied mechanical strength and conduction, and roots provided anchorage and absorption.

The Great Basin bristlecone pine is the oldest non-clonal tree species, surviving for thousands of years just below the tree line. Growing very slowly in cold, dry high-elevation sites, these pines sometimes fail to add a growth ring in a year, and their dense wood resists insects, fungi, rot and erosion.

Under the crown of a spruce on Fulu Mountain, researchers found four generations of spruce remains, ranging from 375 to 9,550 years old and genetically matching the trees above them.

After the death of Prometheus, the oldest known living tree was a 4,847-year-old bristlecone pine in the White Mountains of California, until 2012 when another bristlecone there proved to be 5,065 years old.

The largest clonal organism is Pando, a quaking aspen grove in the Wasatch Mountains of Utah widely considered the world's largest single organism: more than 47,000 genetically identical stems covering 106 acres (43 hectares) and weighing an estimated 13 million pounds (5.9 million kilograms). Its age is estimated at around 16,000 years, although there is no method of firmly fixing it.

Bristlecone needles stay on the limb for over 40 years, an adaptation that keeps the tree from losing all its foliage in years when it does not grow at all, and the pines carry five needles per fascicle.

Dendrochronology, also called tree-ring dating, is the scientific discipline that dates and interprets past events, particularly past climates, from the analysis of tree rings. Samples are taken with an increment borer, a small metal tube driven into the tree to extract a core reaching from bark to centre, and the rings are then counted and measured in the laboratory. By correlating ring sequences from trees with overlapping ages, dendrochronologists extend climate knowledge back thousands of years; the bristlecone pines of California are especially suitable.

Their vast trunks and root systems store carbon dioxide, move water and release oxygen, while decayed leaves, twigs, branches, roots and fallen trees build the organic matter of the soil.Forested watersheds supply quality drinking water to over 180 million Americans, and New York City invested $1.5 billion in 1997 to protect its watershed instead of building a $6 billion filtration plant.

Most of Earth's terrestrial biomass is represented by trees, and the biosphere depends on their metabolism, death and recycling.

Agricultural expansion remains the main driver of deforestation, with large-scale commercial agriculture accounting for 40 percent of tropical deforestation between 2000 and 2010 according to the FAO.

Nearly 58 percent of all tree species are single-country endemics, and of the 20,334 tree species assessed for the IUCN Red List, 8,056 were globally threatened.

The Arbor Day Foundation reports that global forests remove about one-third of fossil fuel emissions annually.

A single large tree can supply a day's worth of oxygen for up to four people, and the Amazon rainforest alone produces more than 20 percent of the world's oxygen.

According to the FAO, forests cover 31 percent of the global land area, about 4.06 billion hectares, about half of it relatively intact and more than one-third primary forest.

About 10 million hectares of forest were lost each year between 2015 and 2020, and since 1990 an estimated 420 million hectares have been lost through conversion to other land uses, although the rate of deforestation has decreased.

The coast redwood begins as a seed no larger than a tomato seed. Such trees can reach ages of 2,000 years and regularly reach 600 years, and their tannin-rich wood and thick bark make disease virtually unknown and insect damage insignificant. During summer dry periods fog precipitates onto the redwood greenery and drips to the forest floor, and fog accounts for about 40 percent of the trees' moisture intake. The coast redwood has no taproot: its roots go down only 10 to 13 feet (3 to 4 metres) deep before spreading outward 60 to 80 feet (20 to 27 metres), intertwining with those of neighbouring redwoods. Windthrow is the main natural cause of death for mature redwoods while 12-inch-thick bark and a lack of flammable pitch allow mature trees to resist fire, with tannins in the wood keeping insects and diseases at bay.

The four redwood parks have been managed cooperatively since May 1994 under a signed agreement. The parks protect 38,982 acres of ancient coast redwood forest, about 35 percent of all old-growth redwood forest remaining in the world.

Redwoods are closely related to the giant sequoia, and both are members of the cypress family; the sequoia is not as tall but is longer-lived, over 3,000 years, and more massive. On 25 September 1890 President Benjamin Harrison signed legislation establishing Sequoia National Park, the second national park in America, and a week later four square miles of parkland became General Grant National Park.

The tallest verified individual coast redwood, named Hyperion, was measured in 2006 at 379.1 feet (115.6 metres).

By trunk volume, the largest tree is the General Sherman Tree, containing about 52,500 cubic feet (1,486.6 cubic metres) and weighing nearly 1,400 tons. The General Sherman Tree stands 274.9 feet (83.8 meters) above its base, with a circumference at ground level of 102.6 feet (31.1 meters).

A coast redwood may grow to 367 feet (112 meters) tall and 22 feet (7 meters) wide at its base. Redwood National Park was established on 2 October 1968 and expanded on 27 March 1978.

Federal forest management in the United States dates back to 1876, when Congress created the office of Special Agent in the Department of Agriculture. In 1905 President Theodore Roosevelt transferred the western forest reserves to the Department of Agriculture's new U.S. Forest Service, led by Gifford Pinchot as its first Chief. The observance of Arbor Day traces to 4 January 1872, when Morton first proposed a tree planting holiday of that name at a meeting of the State Board of Agriculture.

The first Arbor Day was celebrated on 10 April 1872, when an estimated million trees were planted in Nebraska; in 1885 Arbor Day became a legal state holiday in Nebraska with April 22 as its permanent date.

It is estimated that more than 50,000 plant species are used to make medicines, many of them trees, and nearly every part of a tree can be used.

• Summer of 1964
  Coring the Ancient Bristlecones
  The geographer Donald R. Currey was studying the moraines of Wheeler Peak when the United States Forest Service granted him permission to core ancient bristlecone pines.^{24The Prometheus Story · Great Basin National Park, U.S. National Park Service ↗}
• 1964
  The Felling of Prometheus
  The oldest known tree, Prometheus, was cut down in 1964 by a doctoral student studying glaciers, and the tree was later confirmed to be almost 4,900 years old.^{12Bristlecone Pine · Bryce Canyon National Park, U.S. National Park Service ↗}
• Mid-September 2021
  Protecting the General Sherman Tree
  In mid-September 2021 the General Sherman Tree was wrapped in protective aluminum material as the KNP Complex Fire burned upslope toward Giant Forest.^{21The General Sherman Tree · Sequoia & Kings Canyon National Parks, U.S. National Park Service ↗}