Conifer

For other uses, see Conifer (disambiguation).

Conifer Temporal range: 307–0 Ma PreꞒ Ꞓ O S D C P T J K Pg N Carboniferous–Present
Large conifer forest of silver fir (Abies alba) at Vosges, Eastern France
Scientific classification
Kingdom:	Plantae
Clade:	Tracheophytes
Clade:	Gymnospermae
Division:	Pinophyta
Class:	Pinopsida
Subclasses, orders, and families
Cupressidae Araucariales Araucariaceae Podocarpaceae Cupressales Sciadopityaceae Cupressaceae Taxaceae Pinidae Pinales Pinaceae Cheirolepidiaceae † Arctopityaceae † Gnetidae? Gnetaceae Welwitschiaceae Ephedraceae Palissyales † Voltziales † Cordaitales †
Synonyms
Coniferophyta Coniferae Pinophytina

Conifers (/ˈkɒnɪfər/) are a group of seed plants, a subset of gymnosperms. They are mainly evergreen trees with a regular branching pattern, reproducing with male and female cones, usually on the same tree. They are wind-pollinated and the seeds are usually dispersed by the wind. Scientifically, they make up the division Pinophyta, also known as Coniferae. All extant conifers except for the Gnetophytes are perennial woody plants with secondary growth. There are over 600 living species.

Conifers first appear in the fossil record over 300 million years ago in the Carboniferous. They became dominant land plants in the Mesozoic, until flowering plants took over many ecosystems in the Cretaceous. Many conifers today are relict species, surviving in a small part of their former ranges. Such relicts include Wollemia, known only from a small area of Australia, and Metasequoia glyptostroboides, known from Cretaceous fossils and surviving in a small area of China.

Although the total number of species is relatively small, conifers are ecologically important. They are the dominant plants over the taiga of the Northern Hemisphere. Boreal conifers have multiple adaptations to survive winters, including a conical shape to shed snow, strong tracheid vessels to tolerate ice pressure, and a waxy covering on the needle leaves to minimise water loss. Several fungi form ectomycorrhizal associations with conifers. Other fungi cause diseases such as needle cast, especially harmful to young trees. Conifers are affected by pest insects such as wood-boring longhorn beetles and by bark beetles, which make galleries just under the bark. Conifers are of great economic value for timber and paper production.

Evolution

Fossil history

The earliest conifers appear in the fossil record during the Late Carboniferous (Pennsylvanian) over 300 million years ago. Conifers are thought to be most closely related to the Cordaitales, a group of extinct Carboniferous-Permian trees and clambering plants whose reproductive structures had some similarities to those of conifers. The most primitive conifers belong to the paraphyletic assemblage of "walchian conifers", which were small trees, and probably originated in dry upland habitats. The range of conifers expanded during the Early Permian (Cisuralian) to lowlands due to increasing aridity. Walchian conifers were gradually replaced by more advanced voltzialean or "transition" conifers.^[1] Conifers were largely unaffected by the Permian–Triassic extinction event,^[2] and were dominant land plants of the Mesozoic era. Modern groups of conifers emerged from the Voltziales during the Late Permian through Jurassic.^[3] Conifers underwent a major decline in the Late Cretaceous corresponding to the explosive adaptive radiation of flowering plants.^[4]

• 
  Voltziales: Walchia laxifolia foliage, Cisuralian, Germany
• 
  Araucaria cone, Jurassic, Argentina
• 
  Elatides foliage, Late Cretaceous, N. America
• 
  Base of conifer trunk with roots, Early Miocene, Lesbos, Greece

Relict species

Several extant conifers have relict taxon status, surviving in small areas or in very small numbers where they once may have been common and widespread. One such is Wollemia nobilis, discovered in 1994 in some narrow, steep-sided, sandstone gorges in Australia.^[5] The wild population consisted of under 60 adult trees with essentially no genetic variability, implying a genetic bottleneck some thousands of years ago.^[6] The extant gnetophytes consist of three relict genera, namely Ephedra, Gnetum, and Welwitschia. Fossils definitely of the group date back to the Late Jurassic, with many species in the Cretaceous.^[7] Conifers as a whole, too, declined markedly after the angiosperms (flowering plants) diversified during the Cretaceous, coming to dominate most terrestrial ecosystems. Many conifer species became extinct, leaving 30 out of 80 genera with just one extant species, and 11 more with just two or three species. The popular phrase "living fossils" could, the Dutch botanist Aljos Farjon states, fittingly be applied to many of these. Thus, Metasequoia glyptostroboides, the dawn redwood, is known from fossils of Late Cretaceous and Miocene age, and was found also as an extant tree with a small relict range in China.^[8]

• 
  Wollemia nobilis is a relict taxon known only from a small area in Australia.
• 
  Welwitschia mirabilis is one of the gnetophytes, all relict taxa very unlike other conifers.
• 
  Metasequoia glyptostroboides survives in a small part of China, and is known from fossils from the Late Cretaceous onwards.

External phylogeny

The cladogram summarizes the group's external phylogeny. The conifers are gymnosperms, sister to a clade consisting of the ginkgos and cycads.^{[9][10][11][12]}

Gymnosperms	Ginkgoidae Cycadidae Conifers

Internal phylogeny

The Gnetophyta, despite their distinct appearances, were long viewed as outside the conifer group, but phylogenomic analysis indicates that they are part of the conifer clade, sister to the pine family (the 'gnepine' hypothesis). If so, the gnetophytes once shared the distinctive characters of the conifers, and have lost them.^[13] The cladogram summarizes the conifers' internal phylogeny:^[14]

Pinophyta	Pinaceae pine family Gnetophyta (3 relict genera) Araucariaceae monkey puzzle family Podocarpaceae podocarps Sciadopityaceae umbrella pines Cupressaceae cypress family Cephalotaxaceae plum yew family Taxaceae yew family
(Coniferae)

Taxonomy

The name conifer, meaning 'cone-bearing', derives from Latin laconus (cone) and ferre (to bear).^[15] As recently as 1999, the botanist Aljos Farjon wrote that while the Coniferae had up to the early 20th century been considered "a natural family",^[8] comparable to the Rosaceae, he doubted that the conifers or the gymnosperms formed natural groups (clades).^[8] By 2016, the conifers were recognised as a clade, with six families (not including the gnetophytes),^[16] 65–70 genera, and over 600 living species (c. 2002).^{[17]: 205 [18][19]} Depending on interpretation, the Cephalotaxaceae may or may not be included within the Taxaceae, while some authors recognize Phyllocladaceae as distinct from Podocarpaceae. The family Taxodiaceae is here included in the family Cupressaceae.^[20]

Description

All living conifers (except the gnetophytes) are woody plants, and most are trees with narrow leaves, often needle-like. There are separate male and female reproductive structures, the cones. Pollination is always by wind; the seeds are mostly winged. The trees have a regular branching pattern. Many conifers have distinctly scented resin.^[21] The world's tallest and oldest living trees are conifers. The tallest is a coast redwood (Sequoia sempervirens), with a height of 116.07 metres (380.8 ft).^[22] Among the smallest conifers is the pygmy pine (Lepidothamnus laxifolius) of New Zealand, which is seldom taller than 30 cm when mature.^[23] The oldest non-clonal living tree is a Great Basin bristlecone pine (Pinus longaeva), 4,700 years old.^[24] Boreal conifers have multiple adaptations to survive winters, including the tree's conical shape to shed snow, strong tracheid vessels to tolerate ice pressure, and a waxy covering on the needle leaves to minimise water loss.^[25]

• 
  Tallest: Sequoia sempervirens can reach a height of 116.07 metres (380.8 ft).^[22]
• 
  Oldest: Pinus longaeva can reach an age of 4,700 years.^[24]
• 
  The narrow conical shape of boreal conifers, and their downward-drooping limbs, help them shed snow.^[25]

Foliage

Most conifers are evergreens.^[26] In many species such as pines, firs, and cedars, the leaves are long, thin and needle-like. Others like cypresses have flat, triangular scale-like leaves.^[27] In the majority of conifers, the leaves are arranged spirally, the exceptions being most of Cupressaceae and one genus in Podocarpaceae, where they are arranged in decussate opposite pairs or whorls of 3 or 4. In many species with spirally arranged leaves, such as Abies grandis, the leaf bases are twisted to present the leaves in a very flat plane for maximum light capture. Leaf size varies from 2 mm in many scale-leaved species, up to 400 mm long in the needles of some pines (e.g. Apache pine, Pinus engelmannii). The stomata are in lines or patches on the leaves and can be closed when it is very dry or cold. The leaves are often dark green in colour, which may help absorb a maximum of energy from weak sunshine at high latitudes or under forest canopy shade. Conifers from lower latitudes with high sunlight levels (e.g. Turkish pine Pinus brutia) often have yellower-green leaves, while others (e.g. blue spruce, Picea pungens) may develop blue or silvery leaves to reflect ultraviolet light. In the great majority of genera the leaves remain on the plant for several (2–40) years before falling, but five genera (Larix, Pseudolarix, Glyptostrobus, Metasequoia and Taxodium) are deciduous, shedding their leaves in autumn.^[26] The seedlings of some conifers, including pines, have a distinct juvenile foliage period where the leaves are different from the typical adult leaves.^[28]

• 
  Pinaceae: needle-like leaves of Scots pine (Pinus sylvestris)
• 
  Araucariaceae: awl-like leaves of Cook pine (Araucaria columnaris)
• 
  In Abies grandis and many other species with spirally arranged leaves, each leaf is twisted near its base to maximize light capture.
• 
  Cupressaceae: scale leaves of Lawson's cypress (Chamaecyparis lawsoniana); scale in mm

Wood

Conifer wood consists of two types of cells: parenchyma, which have an oval or polyhedral shape, and strongly elongated tracheids. Tracheids make up more than 90% of timber volume. The tracheids of earlywood formed at the beginning of a growing season have large radial sizes and smaller, thinner cell walls. Then, the first tracheids of the transition zone are formed, where the radial size of cells and the thickness of their cell walls changes considerably. Finally, latewood tracheids are formed, with small radial sizes and greater cell wall thickness. This is the basic pattern of the internal cell structure of conifer tree rings.^[29]

• 
  Vertical (tangential) section of Abies concolor wood (xylem), showing tracheids as long overlapping tubes. Perforation pits (small circles) allow water to move from one tracheid to the next.
• 
  Transverse section of wood, cutting across the tracheid tubes, showing tree rings of fast (big cells, earlywood) and slow seasonal growth

Reproduction

Main article: Conifer cone

Conifers produce their seeds inside a protective cone called a strobilus. Most species are monoecious, with male and female cones on the same tree. All conifers are wind-pollinated. In conifers such as pines, the cones are woody, and when mature the scales usually spread open allowing the seeds, which are often winged, to fall out and be dispersed by the wind. In others such as firs and cedars, the cones disintegrate to release the seeds.^[30] Some conifers produce nut-like seeds, such as pine nuts, which are dispersed by birds, in particular, nutcrackers, and jays, which break up the cones.^[31][32] In fire-adapted pines such as Pinus radiata, the seeds may be stored in closed cones for many years, being released only when a fire opens the cones.^[33] In families such as Taxaceae, the cone scales are much modified as edible arils, resembling berries. These are eaten by fruit-eating birds, which then pass the seeds in their droppings.^[34]

• 
  Pinaceae: unopened female cones of subalpine fir (Abies lasiocarpa)
• 
  Northern nutcracker with nut of Pinus sibirica
• 
  Taxaceae: the fleshy aril that surrounds each seed in the European yew is a highly modified seed cone scale.
• 
  Pinaceae: pollen cone of a Japanese larch (Larix kaempferi)

Life cycle

Life cycle of a pine tree

Conifers are heterosporous, generating two different types of spores: male microspores and female megaspores.^[35] These spores develop on separate male and female sporophylls on separate male and female cones, usually on the same tree.^[36]

In the male cones, microspores are produced from microsporocytes by meiosis. The microspores develop into pollen grains, which contain the male (micro)gametophytes. Large amounts of pollen are released and carried by the wind. Some pollen grains land on female cones, pollinating them. The generative cell in the pollen grain divides into two haploid sperm cells by mitosis, leading to the development of the pollen tube. At fertilization, one of the sperm cells unites its haploid nucleus with the haploid nucleus of an egg cell.^[36]

The female cone develops two ovules, each of which contains haploid megaspores. A megasporocyte is divided by meiosis in each ovule. The female gametophytes grow to produce two or more haploid eggs. The fertilized egg, the (diploid) zygote, gives rise to the embryo, and a seed is produced. The female cone then opens, releasing the seeds which grow into seedlings. Some seedlings survive to grow into trees.^[36]

Conifer reproduction is synchronous with seasonal changes in temperate zones. Reproductive development slows to a halt during each winter season and then resumes each spring. The male strobilus development is completed in a single year. Conifers have one of three reproductive cycles that differ in the time to complete female strobilus development from initiation to seed maturation. The cycle is one year in genera such as Abies, Picea, Cedrus, and Tsuga; two years in most pine species and in Sequoiadendron; and three years in three pine species including Pinus pinea. All three types have a long gap between pollination and fertilization.^[37]

Distribution and ecology

Conifers are the dominant plants over the taiga forest of the Northern Hemisphere,^[26] forming the world's largest terrestrial biome. The taiga consists mainly of larches, pines, and spruces.^[38] Larch is the most common tree in Russia, and by volume of timber, easily the most abundant tree genus worldwide.^[39] The larch species Larix gmelinii is the world's most northerly-growing tree, at 75° north in the Taymyr Peninsula.^[8] Conifers are widespread also in southern Europe, the Middle East, the Himalayas, Southeast Asia, and Japan. Conifers are not confined to the Northern Hemisphere: around 200 conifer species live only in the tropics, and others live in Australasia, Africa (including Madagascar), and Central and South America.^[40] Species richness decreases with latitude; a northern country like Canada has just 9 species, whereas Mexico has 43, and the tropical island of New Caledonia has 42 endemic species.^[8]

Since conifers cannot regrow their leaves rapidly like hardwoods, leaf diseases can seriously damage coniferous plantations, especially dense stands of young trees. Needle cast diseases, often caused by ascomycete fungi in the Rhytismataceae family, result in leaf fall.^[41] Another ascomycete, Rhizosphaera (Sphaeropsidales), causes severe defoliation and shoot blight, for instance in spruces.^[42]

At least 20 species of roundheaded wood-boring longhorn beetles (Cerambycidae) feed on the wood of spruces, firs, and hemlocks.^[43] Bark beetles (Scolytinae, in the Curculionidae) are destructive pests of commercial forestry; major pests of spruce and other conifers include Ips typographus in Eurasia^[44] and Dendroctonus rufipennis in North America.^[45]

The basidiomycete fungus Boletus pinophilus is among the fungi that form an ectomycorrhizal association with conifers, in its case with pines such as Pinus sylvestris.^[46]

Some conifers introduced for forestry including Pinus radiata have become invasive species in New Zealand,^[47] South Africa,^[48] and Australia.^[49][50]

• 
  Taiga coniferous forest, mostly larches, pines, and spruces, covers a large area of Siberia (pictured) and Canada.^[38]
• 
  Galleries of Ips typographus bark beetles weaken conifers such as Norway spruce, and can seriously harm commercial forestry.
• 
  The pine bolete Boletus pinophilus forms an ectomycorrhizal association with several pines.
• 
  Pinus radiata (radiata or Monterey pine) is an invasive species in Australia (pictured), New Zealand, and South Africa.

Economic importance

Further information: Forestry and Silviculture

The softwood derived from conifers is more easily worked than hardwood from broadleaved (angiosperm) trees. This makes it widely used and of great economic value, its many uses including construction, furniture, telegraph poles and fencing.^[51] A large part of production is used for paper.^[51][52] In the United Kingdom, the 48% of the woodland that is coniferous yields over 90% of the timber; the top species is sitka spruce, yielding about half of the timber produced.^[53] Worldwide, wood products reached a value of $100 billion by the end of the 20th century.^[8]

Conifer wood  Pine   Spruce   Larch   Juniper

Conifers such as fir, cedar, cypress, juniper, spruce, pine, yew and false cedar have been selected by plant breeders for ornamental purposes. Plants with unusual growth habits, sizes, and colours are propagated and planted in parks and gardens throughout the world.^[54]

• 
  Commercial forestry using sitka spruce
• 
  Globosa, an ornamental cultivar of Scots pine
• 
  Pinus thunbergii 'Kotobuki'
  as a 65-year-old bonsai

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External links

• Conifers at the Tree of Life Web Project
• Gymnosperms at Michigan State University
• Conifers of the World: Resources for Conifer Research - some 37,000 herbarium records