Introduction to Trees

• A tree is a perennial plant, typically woody, featuring an elongated stem or trunk that supports branches and leaves

Definition of a Tree

• A tree is any plant with an elongated stem or trunk that supports photosynthetic leaves or branches above the ground
• Trees are also defined by height, with smaller plants from 0.5 to 10 m called shrubs
• A narrower definition specifies a woody trunk formed by secondary growth, which thickens annually
• Herbaceous plants like palms and bananas do not fit this narrower definition
• Some monocots, such as Joshua trees, bamboos, and palms, create "pseudo-wood" through lignified cells from primary growth, lacking true wood and growth rings
• Dracaena species, which are monocots, exhibit secondary growth via meristem activity in their trunks, differing from dicotyledonous trees
• Trees are also defined by use, such as plants that yield lumber

Overview of Tree Characteristics

• The tree growth habit is an evolutionary adaptation that allows plants to compete effectively for sunlight by growing taller
• Trees tend to be tall, long-lived organisms, with some living for thousands of years
• Modified structures, including thicker stems made of specialized cells, provide structural strength and durability
• Trees usually grow larger and have a single main stem, unlike shrubs, which have multiple stems
• The tree form has evolved separately in unrelated plant classes as a result of similar environmental challenges, exemplifying parallel evolution
• There are an estimated 60,000-100,000 tree species worldwide, making up an estimated 25% of all living plant species
• The highest number of tree species grow in tropical regions
• Most tree species are angiosperms (hardwoods), while others are gymnosperms (softwoods) like conifers, cycads, ginkgos, and gnetales
• Gymnosperms produce seeds in open structures such as pine cones and often possess tough, waxy leaves like pine needles
• Angiosperm trees are mostly eudicots, named for having two cotyledons or seed leaves
• Wood provides structural strength to the trunk, while the vascular system distributes water, nutrients, and chemicals
• Trees draw water high up the stem through the xylem from the roots by capillary action, which is essential due to constant transpiration
• The main parts of trees include the root, stem, and leaves, interconnected by the vascular system
• The vascular cambium in woody plants enables expansion of vascular tissue
• Cork cambium develops among the phloem, producing thickened cork cells to protect the surface and reduce water loss
• Trees are either evergreen, retaining foliage year-round, or deciduous, shedding leaves at the end of the growing season and entering a dormant period
• Most conifers are evergreens, while some species like larches and certain cypresses are deciduous
• The crown is the top of a tree, consisting of branches and leaves, while the canopy is the uppermost layer in a forest
• A sapling is a young tree

Distribution of Trees

• Globally, there are an estimated 3.04 trillion trees (2015)
• Of these, 46% are in the tropics/sub-tropics, 20% in temperate zones, and 24% in boreal forests
• Around 15 billion trees are cut down yearly, while 5 billion are planted
• Since the start of human agriculture 12,000 years ago, the number of trees worldwide has decreased by 46%
• There are about 64,100 known tree species
• South America has the highest biodiversity (43%), followed by Eurasia (22%), Africa (16%), North America (15%), and Oceania (11%)
• Forests are the climatic climax community in suitable environments
• Coniferous forests predominate in cool temperate regions, such as the taiga, which forms 29% of the world's forest cover
• Temperate broadleaf and mixed forests, with species like oak, beech, birch, and maple, are found where rainfall is evenly distributed
• In tropical regions with a monsoon climate, forests are dominated by different broad-leaved trees, some of which are deciduous
• Savanna climates in tropical regions have open canopies with grass and scrub, with species like acacia and baobab adapted to these conditions

Parts of Trees: Roots

• Roots anchor the tree, gather water and nutrients, and are used for reproduction, defense, survival, and energy storage
• The radicle is the embryonic root that emerges during germination, developing into a taproot
• Lateral roots branch out horizontally in the upper soil layers, eventually replacing the taproot in most trees
• Root hairs near the tip of finer roots absorb water and nutrients and require oxygen to respire
• Mycorrhiza form mutualistic relationships with tree roots, enhancing mineral uptake for the tree and obtaining carbohydrates from it
• Some trees have a symbiotic relationship with Frankia bacteria, forming actinorhizal root nodules that fix nitrogen

Parts of Trees: Trunk

• The main purpose of the trunk is to raise the leaves above ground for light competition
• It also transports water and nutrients from the roots to the aerial parts and distributes food
• The outermost layer of the trunk is the bark, composed of dead cells that protect the inner tissue
• The bark includes lenticels for oxygen diffusion and is continually replaced by the cork cambium
• The innermost layer of bark, the phloem, transports sugars produced by photosynthesis
• Inside the phloem is the vascular cambium layer, which divides to create phloem and xylem cells

Buds & Growth

• Trees have spurts of active growth followed by rest periods related to climate conditions
• Buds protect the meristem (active growth zone) during inactive periods with scales
• New shoots push out, shedding scales when growing conditions improve

Leaves

• Leaves are structures specialized for photosynthesis, arranged to maximize light exposure
• They are thorny or contain phytoliths, lignins, tannins, or poisons to discourage herbivory
• Trees have various leaf shapes and sizes due to environmental pressures
• Needles of coniferous trees are adapted for low-resource or scarce-water environments
• Broad-leaved trees in temperate regions shed their leaves in winter
• In tropical and subtropical regions, many trees keep their leaves year-round, while others shed them annually

Reproduction

• Trees are pollinated by wind or animals, and use various methods of seed dispersal
• Some rely on wind with winged or plumed seeds, while others rely on animals with edible fruits

Seeds

• Seeds are the primary way trees reproduce, varying greatly in size and shape
• Seed dispersal methods include wind, animals, ejection, and gravity
• Seeds need light to grow into adult trees, and their dispersal prevents competition among saplings and shade from the parent treer

Evolutionary History

• The earliest trees were tree ferns, horsetails, and lycophytes during the Carboniferous period
• Wattieza may have been the first tree
• Gymnosperms evolved in the Triassic period
• The tree forms of flowering plants evolved during the Cretaceous period

Ecology

• Trees are an important part of the terrestrial ecosystem, providing essential habitats including many kinds of forest for communities of organisms.
• Trees stabilize the soil, prevent rapid run-off of rain water, help prevent desertification, have a role in climate control and help in the maintenance of biodiversity and ecosystem balance.
• Many species of tree support their own specialized invertebrates.

Uses: Food

• Trees are the source of many of the world's best known fleshy fruits
• Other commercially important fruit include dates, figs and olives.
• Many trees bear edible nuts which can loosely be described as being large, oily kernels found inside a hard shell.
• Maple syrup comes from North American sugar maples
• Various parts of trees are used as spices like cinnamon
• The leaves of trees are widely gathered as fodder for livestock and some can be eaten by humans

Uses: Fuel and Timber

• Wood has traditionally been used for fuel, especially in rural areas
• Timber, "trees that are grown in order to produce wood" is 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

Uses: Art, Bonsai and Shaping

• Besides inspiring artists down the centuries, trees have been used to create 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 is the practice of growing and shaping small trees
• Tree shaping is the practice of changing living trees and other woody plants into man made shapes for art and useful structures

Uses: Bark

• Cork is produced from the thick bark of the cork oak
• The 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
• At least 120 drugs come from plant sources, many of them from the bark of trees
• The papery bark of the paper birch tree was used extensively by Native Americans
• Nowadays, bark chips, a by-product of the timber industry, are used as a mulch and as a growing medium for epiphytic plants that need a soil-free compost

Ornamental Trees

• 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
• Trees provide shade and cooling through evapotranspiration, absorb greenhouse gasses and pollutants, intercept rainfall, and reduce the risk of flooding

Other Uses

• Latex is a sticky defensive secretion that protects plants against herbivores
• Resin is another plant exudate that may have a defensive purpose
• The camphor tree produces an essential oil and the eucalyptus tree is the main source of eucalyptus oil which is used in medicine, as a fragrance and in industry

Threats and Conservation

• Dead trees pose a safety risk, especially during high winds and severe storms
• During times of drought, trees can fall into water stress, which may cause a tree to become more susceptible to disease and insect problems, and ultimately may lead to a tree's death
• About a third of all tree species, some twenty thousand, are included in the IUCN Red List of Threatened Species

Mythology

• Trees have been venerated since time immemorial
• In Greek mythology, dryads were believed to be shy nymphs who inhabited trees
• The Oubangui people of west Africa plant a tree when a child is born; when the tree flowers it is time for marriage
• In Norse mythology, Yggdrasil is a central cosmic tree whose roots and branches extend to various worlds

Superlative Trees

• Trees have a theoretical maximum height of 130 m; the tallest known specimen is a coast redwood in California named Hyperion, 115.85 m tall
• The largest tree by volume is believed to be a giant sequoia known as the General Sherman Tree in California
• The oldest living tree is a Great Basin bristlecone pine in the White Mountains with an estimated age of 5,079 years
• The tree with the broadest trunk is a Montezuma cypress in Oaxaca, Mexico, known as Árbol del Tule, with a diameter at breast height of 11.62 m