All About Trees

• A tree is a perennial plant, typically woody, with an elongated stem or trunk that supports branches and leaves above the ground.
• Trees are defined by their height, with smaller plants (0.5 to 10 m) classified as shrubs.
• Trees are not a monophyletic taxonomic group.
• Most tree species are angiosperms (hardwoods) or gymnosperms (softwoods).
• Trees can live for several thousand years.
• Trees evolved approximately 370 million years ago.
• There are an estimated three trillion mature trees worldwide.
• Trees have secondary branches, a trunk composed of woody tissue, and roots that anchor the tree while extracting moisture and nutrients.
• Tree trunks are protected by bark, while shoots bear leaves to capture light energy, which is converted into sugars via photosynthesis.
• Trees reproduce using seeds; flowering plants produce seeds inside fruits, conifers use cones, and tree ferns use spores.
• Trees reduce erosion, moderate climate, store carbon, and provide habitats.

Definition of a Tree

• There is no universal definition, botanically or in common language, for what constitutes a tree.
• A tree is any plant with an elongated stem or trunk supporting leaves or branches.
• A narrower definition requires a woody trunk formed by secondary growth, thus excluding herbaceous plants like palms and bananas.
• Some monocots like Joshua trees, bamboos, and palms produce "pseudo-wood" but lack secondary growth.
• Dracaena species, also monocots, exhibit secondary growth via a unique meristem.
• Trees are also defined by their use, such as plants that yield lumber.

Overview of Trees

• The tree growth habit is an evolutionary adaptation that allows plants to compete for sunlight by growing taller.
• Trees are typically tall, some reaching several thousand years old.
• Trees possess modified structures like thicker stems with specialized cells.
• Trees are differentiated from shrubs by their larger size and single main stem.
• Harsh environmental conditions can reduce tree size.
• The tree form has evolved independently in unrelated plant classes.
• Trees comprise 25% of all living plant species, with the majority growing in tropical regions.
• Most tree species are angiosperms (hardwoods), while conifers, cycads, ginkgophytes, and gnetales are gymnosperms (softwoods).
• Most angiosperm trees are eudicots, with seeds containing two cotyledons or seed leaves.
• Basal angiosperms or paleodicots include Amborella, Magnolia, nutmeg, and avocado, while monocots include bamboo, palms, and bananas.
• Wood provides structural strength to the trunk, and the vascular system distributes water, nutrients, and chemicals.
• Water is drawn up the stem through the xylem from the roots by capillary action.
• The main parts of trees are the root, stem, and leaves, all interconnected by the vascular system.
• The vascular cambium allows expansion of vascular tissue and woody growth, while the cork cambium produces cork cells for protection and water loss reduction.
• Trees are either evergreen (foliage persists year-round) or deciduous (shed leaves at the end of the growing season).
• Conifers are mostly evergreen, but larches are deciduous.
• The crown of a tree includes its branches and leaves, while the canopy is the uppermost layer in a forest.
• A sapling is a young tree.

Distribution of Trees

• There are an estimated 3.04 trillion trees in the world (2015).
• 46% are in the tropics or sub-tropics, 20% in temperate zones, and 24% in coniferous boreal forests.
• Approximately 15 billion trees are cut down annually, while 5 billion are planted.
• The number of trees worldwide has decreased by 46% since the start of human agriculture.
• There are approximately 64,100 known tree species.
• South America has the highest tree biodiversity (43%), followed by Eurasia (22%), Africa (16%), North America (15%), and Oceania (11%).
• Forests are stable climatic climax communities in suitable environments.
• Conifers predominate in cool temperate regions, such as moist taiga or northern coniferous forest (boreal forest), which forms 29% of the world's forest cover.
• In temperate regions with evenly spread rainfall, temperate broadleaf and mixed forests with oak, beech, birch, and maple are common.
• Tropical regions with monsoon climates feature broad-leaved trees, some of which are deciduous.
• Tropical regions with drier savanna climates have open canopies with grass and scrub undergrowth, where acacia and baobab trees are well-adapted.

Roots

• Roots anchor the tree
• Roots gather water and nutrients to transfer to all parts of the tree.
• Roots are also used for reproduction, defence, survival, energy storage
• The radicle develops into a taproot, followed by lateral roots
• Most trees, the taproot eventually withers away and wide-spreading laterals remain
• Near the tip of the finer roots are single cell root hairs
• Roots require oxygen to respire
• Some trees such as Alder, have a symbiotic relationshiop with Frankia species, a filamentous bacterium
• Trees interconnected through their root system, forming a colony.
• The interconnections are made by inosculation
• Tests to demo networking are performed by injecting chemicals, sometimes radioactive, into a tree, and then checking for its presence in neighboring trees
• Some tree species have evolved roots that are aerial
• The common purposes for aerial roots may be of two kinds: to contribute to the mechanical stability of the tree, and to obtain oxygen from air
• The red mangrove develops prop roots that loop out of the trunk and branches and descend vertically into the mud
• Many large trees have buttress roots which flare out from the lower part of the trunk to provide stability

Trunk

• The trunk raises the leaves above the ground
• Transports water and nutrients from the roots to the aerial parts of the tree
• Distributes the food produced by the leaves to all other parts of the tree, including the roots
• The outermost layer of the trunk is the bark, mostly composed of dead cells of phellem (cork)
• It protects the trunk against the elements, disease, animal attack and fire
• It is perforated by a large number of fine breathing pores called lenticels, through which oxygen diffuses
• Large tree-like plants with lignified trunks in the Pteridophyta, Arecales, Cycadophyta and Poales such as the tree ferns, palms, cycads and bamboos have different structures and outer coverings
• The innermost layer of bark is known as the phloem

Buds and Growth

• Trees do not usually grow continuously throughout the year but mostly have spurts of active expansion followed by periods of rest
• Trees form buds to protect the meristem, the zone of active growth
• Before the period of dormancy, the last few leaves produced at the tip of a twig form scales

Leaves

• Leaves are structures specialised for photosynthesis
• Are arranged on the tree in such a way as to maximise their exposure to light without shading each other
• May be thorny or contain phytoliths, lignins, tannins or poisons to discourage herbivory
• Needles of coniferous trees are compact but are structurally similar to those of broad-leaved trees
• Can be broad or needle-like, simple or compound, lobed or entire, smooth or hairy, delicate or tough, deciduous or evergreen

Reproduction

• Trees can be pollinated either by wind or by animals, mostly insects
• Many angiosperm trees are insect pollinated

Seeds

• Seeds are the primary way that trees reproduce
• Vary greatly in size and shape
• For a tree seedling to grow into an adult tree it needs light

Evolutionary History

• The earliest trees were tree ferns, horsetails and lycophytes, which grew in forests in the Carboniferous period
• The first tree may have been Wattieza, fossils of which were found in New York state in 2007 dating back to the Middle Devonian (about 385 million years ago)
• Prior to this discovery, Archaeopteris was the earliest known tree
• The gymnosperms include conifers, cycads, gnetales and ginkgos and these may have appeared as a result of a whole genome duplication event which took place about 319 million years ago
• Ginkgophyta was once a widespread diverse group of which the only survivor is the maidenhair tree Ginkgo biloba
• During the Mesozoic (245 to 66 million years ago) the conifers flourished and became adapted to live in all the major terrestrial habitats

Ecology

• Trees are an important part of the terrestrial ecosystem, providing essential habitats including many kinds of forest for communities of organisms
• Epiphytic plants such as ferns, some mosses, liverworts, orchids and some species of parasitic plants hang from branches
• Leaves, flowers and fruits are seasonally available
• On the ground underneath trees there is shade, and often there is undergrowth, leaf litter, and decaying wood that provide other habitat
• Trees stabilise the soil Trees prevent rapid run-off of rain water
• Trees help prevent desertification
• Trees have a role in climate control
• Trees help in the maintenance of biodiversity and ecosystem balance
• Tree apparency varies with a tree's size and semiochemical content, and with the extent to which it is concealed by nonhost neighbours from its insect pests

Uses

• Trees are the source of many of the world's best known fleshy fruits
• Many trees bear edible nuts which can loosely be described as being large, oily kernels found inside a hard shell
• About 90% of the sap is water, the remaining 10% being a mixture of various sugars and certain minerals
• Leaves of the curry tree (Murraya koenigii) are eaten, those of kaffir lime (Citrus × hystrix) (in Thai food) and Ailanthus (in Korean dishes such as bugak) and those of the European bay tree (Laurus nobilis) and the California bay tree (Umbellularia californica) are used for flavouring food
• Wood is used in the construction of buildings, bridges, trackways, piles, poles for power lines, masts for boats, pit props, railway sleepers, fencing, hurdles, shuttering for concrete, pipes, scaffolding and pallets

Fuel

• Wood has traditionally been used for fuel, especially in rural area
• Modern wood-burning stoves are very fuel efficient and new products such as wood pellets are available to burn

Timber

• "Trees that are grown in order to produce wood"
• Cut into lumber (sawn wood) for use in construction
• Engineered wood products are available which bind the particles, fibres or veneers of wood together with adhesives to form composite materials

Art

• Living trees have been used in bonsai and in tree shaping, and both living and dead specimens have been sculpted into sometimes fantastic shapes

Bonsai

• "Tray planting"
• The practice of growing and shaping small trees, originating in China as penjing and spreading to Japan more than a thousand years ago
• Bonsai practice focuses on long-term cultivation and shaping of one or more small trees growing in a container, beginning with a cutting, seedling, or small tree of a species suitable for bonsai development

Tree Shaping

• Tree shaping is the practice of changing living trees and other woody plants into man made shapes for art and useful structures
• There is a gradual method and there is an instant method

Bark

• Cork is produced from the thick bark of the cork oak (Quercus suber)
• Harvested from the living trees about once every ten years in an environmentally sustainable industry
• More than half the world's cork comes from Portugal and is largely used to make stoppers for wine bottles
• Bark of other varieties of oak has traditionally been used in Europe for the tanning of hides though bark from other species of tree has been used elsewhere

Ornamental Trees

• Create a visual impact in the same way as do other landscape features and give a sense of maturity and permanence to park and garden
• Are grown for the beauty of their forms, their foliage, flowers, fruit and bark and their siting is of major importance in creating a landscape

Other Uses

• Latex is a sticky defensive secretion that protects plants against herbivores
• Originally used to create bouncy balls and for the waterproofing of cloth, natural rubber is now mainly used in tyres for which synthetic materials have proved less durable
• Resin is another plant exudate that may have a defensive purpose
• It is a viscous liquid composed mainly of volatile terpenes and is produced mostly by coniferous trees

Conservation

• About a third of all tree species, some twenty thousand, are included in the IUCN Red List of Threatened Species
• Of those, over eight thousand are globally threatened, including at least 1400 which are classed as "critically endangered"

Mythology

• Trees have been venerated since time immemorial
• To the ancient Celts, certain trees, especially the oak, ash and thorn, held special significance as providing fuel, building materials, ornamental objects and weaponry
• The Oubangui people of west Africa plant a tree when a child is born
• Trees have their roots in the ground and their trunk and branches extended towards the sky

Superlative Trees

• Trees have a theoretical maximum height of 130 m (4
• The tallest known specimen on earth is believed to be a coast redwood (Sequoia sempervirens) at Redwood National Park, California with a height of 115.85 m (380.1 ft)
• The largest tree by volume is believed to be a giant sequoia (Sequoiadendron giganteum) known as the General Sherman Tree in the Sequoia National Park in Tulare County, California
• The oldest living tree with a verified age is also in California, a Great Basin bristlecone pine (Pinus longaeva) growing in the White Mountains and is estimated to be 5,079 years old
• The tree with the broadest trunk is a Montezuma cypress (Taxodium mucronatum) known as Árbol del Tule and its diameter at breast height is 11.62 m (38.1 ft) giving it a girth of 36.2 m (119 ft)