Canadian poplar ( Populus × canadensis )
The poplars ( Populus ) are a genus of plants in the willow family (Salicaceae). They are widespread in temperate areas in the northern hemisphere in North America and Eurasia . They grow on river banks and in forests and are often grown for the production of wood , paper and energy .
Description and ecology
Poplars form cardiac root systems to horizontal root systems. Taproots are not occupied, but strongly pronounced horizontal roots, from which sinkers branch off on the one hand, and shoots upwards on the other, which are used for vegetative reproduction . Compared to other trees, the fine roots of the poplars are quite long, not very branched and thin. The roots form both ectomycorrhizae and vesicular-arbuscular mycorrhiza .
Wood and bark
The wood of all poplar species is quite similar, the differences due to environmental conditions being greater than the differences between species. Poplar wood has a very high cellulose content , which makes it flexible. The density with a wood moisture content of 15 percent averages 0.45 g cm −3 , with values between 0.41 and 0.60. The quivering poplar is around 0.49. The values of poplar wood are similar to those of the wood of the conifers, Norway spruce and Weymouth pine .
The wood of the poplar is pored ; the vessels of the early wood are no larger than those of the late wood. The wood rays are essentially made up of cells of the same size. The wood parenchyma stores starch, protein and oil. The heartwood, in which there is no longer any living parenchyma, is formed in Populus tremuloides from the fifth year and is not clearly delimited in color in all poplars.
The leaves are triangular, heart- or egg-shaped and either entire or lobed . The petiole is long, the cross-section round or laterally flattened. The leaves on long and short shoots, d. H. the leaves preformed in the winter buds and those formed in summer are often designed differently. The winter buds have several uneven scales.
Like all representatives of the willow family ( Salicaceae ), poplars are dioecious , so there are male and female plants. In all sections of the genus, however, individuals were also exceptionally observed whose inflorescences contained both pistil and stamen flowers.
Flowers and fruits
The inflorescences are stalked, drooping catkins . Male and female kittens are very similar. The flowers are in the axils of a lanceolate or toothed bract and are petiolate. The bracts fall off during the anthesis . Male and female flowers do not have an inflorescence , but the male or female organs are surrounded by a more or less pronounced disc . The male flowers have four to 60 stamens , the stamens are not grown together. In the female flowers, the ovary is sessile and unilocular. The ovary consists of two to four fused carpels and is surrounded by the discus at the base or up to three quarters of its height . The ovules are too many on two to four parietal placentas . The pen is short and carries two to four scars . Nectaries are missing.
The poplars are pollinated by the wind ( anemophilia ), the flowers appear before the leaves.
The fruit is a two- to four-lobed capsule that contains numerous flying seeds . The seeds are surrounded by a long, dense fluff of hair , and the endosperm is barely developed or completely absent. The weight of a seed is usually only a few tenths of a milligram, but a tree can produce over 25 million seeds per year. The seeds are spread by the wind ( anemochory ).
Poplars form phenols as secondary constituents , but not nitrogen-containing secondary plant substances . The most important phenols in terms of quantity are phenol - glycosides , flavonoids and tannins . The phenol glycosides include salicin , salicortin , tremuloids and tremulacin . They are especially found in leaves, twigs and bark. In feeding experiments they reduce the growth of many insects.
In autumn, the sugars sucrose , raffinose and stachyose are formed in the trunk from the stored starch . While starch only occurs in small amounts in winter, raffinose and stachyose can each make up 6 to 7 percent of the trunk dry matter. Fats are also stored in the trunk and in the bark. In contrast to earlier opinions, when the poplars were viewed as a pure fat-storing species, the fats, along with the carbohydrates, are only part of the reserve materials.
The number of chromosomes in all species is 2n = 38. The core genome is comparatively small with 2C = 1.2 picograms.
Poplars bloom between February and April, depending on the geographical latitude. Even within a population, the flowering time of individual trees can differ by two months. The egg cells are fertilized within 24 hours after pollination . The seeds are carried by the wind and water up to a few kilometers spread ; but mostly it is a few hundred meters. The seeds are often only able to germinate for a few weeks, they germinate immediately under favorable conditions, i.e. on moist mineral soil. In the first year the seedlings can reach root depths of 75 to 150 centimeters, while the growth in height is much more modest. The first flowering takes place at the age of about five to ten years. Poplars usually live to be 100 to 200 years old.
The poplars can be very vegetative, i. H. multiply via root rashes. Also broken branches and fallen trees - e.g. B. by flooding - can take root again. Poplars in particular often grow in large clone colonies that have arisen from a single seed. The saplings can appear up to 40 m away from the mother tree. Even if the entire aboveground tree population is destroyed by a fire, the root network survives. So the colony can last for thousands of years. In the American state of Utah, a group of poplars is known that may have existed in this location for 80,000 years.
Poplars are one of the fastest growing trees in temperate latitudes. This property favors their ecological role as pioneer trees. As is the case with most trees in these areas, growth is not restricted to the leaves that are preformed in the winter buds. The poplars continue to grow with new leaves later in the year. The growth is - at least in some species - only stopped by the shortened photoperiod in autumn.
Poplars occur in the temperate areas of the northern hemisphere ( Holarctic distribution). Their distribution ranges from the subtropical areas of China, where they have a center of diversity , to the boreal zone . In America they occur in the south as far as Mexico. Populus ilicifolia occurs in East Africa.
In Central Europe the black poplar ( Populus nigra ), the silver poplar ( Populus alba ), and the trembling poplar ( Populus tremula ) are native; next to it the natural hybrid gray poplar ( Populus canescens ).
Poplars are often found along rivers where they are part of the alluvial forests . Many species such as the black poplar are tolerant of flooding and overtaking, while drought is often poorly tolerated. With willows and alders, they belong to the softwood meadow , the deepest alluvial forest level.
They also grow in temperate, boreal and montane forests. Many species are aggressive colonists on previously disturbed locations due to their rapid growth in their youth stage and vegetative reproduction.
Worldwide there are around 80 million hectares of natural poplar stands, of which 28.3 million hectares in Canada (2001), 21.9 million in Russia and 17.7 million in the USA (2003). The main use here is wood production. China follows as the fourth largest poplar country with 2.1 million hectares. The main use here is in nature conservation through the forests.
Diseases and predators
Over 650 species of fungus have been identified on poplars. Over 300 species of insects and 250 species of fungi have been found on Populus tremuloides . Globally economically important diseases are leaf rust ( Melampsora spp.), Leaf and sprout bleaching ( Venturia spp.), The leaf spot disease Marssonia , the stem cancer Hypoxylon mammatum , the rot pathogen Phellinus tremulae and Septoria
Major diseases in Central Europe are bacterial poplar cancer ( Xanthomonas populi ), bark blight ( Dothichiza populae ), brown speck, leaf rust ( Melampsora spp.), Shoot tip disease ( Pollaccia radiosa ) and leaf spot disease ( Marssonia spp.). As for insects, the large poplar goat ( Saperda carcharias ), the small poplar goat ( Saperda populnea ) and the musk billy goat ( Aromia moschata ) should be mentioned. Against chewing and Fegeschäden are by mice, hares , rabbits , red - Dam and - Roe possible.
Almost a hundred species of butterfly caterpillars settle on poplars.
The genus Populus has always been part of the willow family. It is a monophyletic group and the sister group of the genus Salix .
Hybridizations often occur within the genus. There are different divisions in sections, and the number of species varies depending on the processor between 22 and 89. Especially those who work with the Russian and Chinese poplars tend to have a larger number of species. The following structure comes from Eckenwalder 1996:
- Section Abaso corner w
- Populus mexicana Wesm. , North America
- Turanga Bunge Section : in Eurasia and East Africa
- Section Leucoides Spach : In Eastern Eurasia and North America
- Section Aigeiros Duby - Black Poplars: Holarctic
- Section Tacamahaca Spach - Balsam poplars: Holarctic
- Populus angustifolia E. James : It occurs in North America.
- Balsam poplar ( Populus balsamifera L. )
- Populus ciliata Wall. ex Royle : It occurs in several varieties in Xizang, Yunnan, Bhutan, India, Kashmir, Myanmar, Nepal, Pakistan and Sikkim.
- Bay-leaved poplar ( Populus laurifolia Ledeb. )
- Birch Poplar or Simon's Poplar ( Populus simonii Carriere )
- Populus suaveolens fish. (including Populus cathayana Rehder , Populus koreana Rehder and Populus maximowiczii A. Henry ): It occurs in Asian Russia, Korea, Japan, Mongolia and China.
- Chinese balsam poplar ( Populus szechuanica C.K. Schneid. )
- Western balsam poplar ( Populus trichocarpa Torr. & A. Gray )
- Populus yunnanensis Dode : It occurs in Guizhou, Sichuan and Yunnan.
- Section Populus (Syn .: Section Leuce Duby ) - White and quaking aspen: Holarctic
- Populus adenopoda Maxim. : China.
- Silver poplar ( Populus alba L. )
- Gray poplar ( Populus x canescens (Aiton) Sm. )
- Populus gamblei Dode
- Large-toothed poplar ( Populus grandidentata Michx. )
- Populus guzmanantlensis A.Vázquez & R.Cuevas : Mexico.
- Populus monticola Brandegee : California and Mexico.
- Populus Sieboldii Miq. : Japan.
- Populus simaroa Rzed. : Mexico.
- Quivering poplar ( Populus tremula L. , including Populus davidiana Dode )
- American quivering aspen ( Populus tremuloides Michx. )
Phylogenetic studies based on gene sequence analyzes have essentially confirmed this division into sections based on morphological features.
Natural hybrids are very common among poplars . The individual species of the respective section hybridize with one another. Species of the two sections Tacamahaca and Aigeiros hybridize reciprocally, while species of the Populus section do not hybridize with species of other sections. In contrast to many other plant species, polyploidy does not occur in poplars after hybridization.
The two hybrids were created through natural crossbreeding and are very widespread through cultivation:
- Gray poplar ( Populus × canescens (Aiton) Sm. ): Hybrid of Populus alba × Populus tremula .
- Canadian poplar ( Populus × canadensis Moench ): Hybrid of Populus deltoides × Populus nigra . It arose spontaneously after the American Populus deltoides was introduced to Europe in the 17th century.
Triploid hybrids, which are then sterile, but can be clonally propagated, occur rarely. One example is the Astria convariety, a cross between Populus tremula × Populus tremuloides .
Within the genus, the Abaso section is considered to be the most original, from which the other sections have developed via Leucoides . The Populus section is considered to be the most developed.
Fossil is the genus represented very often by its leaves. The oldest finds come from the late Paleocene of North America and are around 58 million years old. They can be assigned to the Abaso section . In the late Eocene , fossils of the Leucoides section appeared for the first time , as did the first fossils in Eurasia. In the Oligocene , representatives of North America and Eurasia appeared in North America as well as common forerunners of the Tacamahaca and Aigeiros sections. Fossils that can be clearly assigned to the two sections and those of the Populus section did not appear until the Miocene .
Poplars grow quickly, are easy to propagate, and many forms have erect, distinct main stems. These properties make poplar wood a popular timber.
The use of poplars for wood, fuel and litter for animals has been documented for the Mediterranean region to Central Asia since ancient times. At first, Populus nigra and Populus alba were mainly used. After the introduction of the American Populus deltoides to Europe, this species and the hybrid Populus x canadensis were increasingly used. Since the early 20th century, poplar cultivation has continued scientifically. In 1947 the International Poplar Commission was founded under the auspices of the Food and Agriculture Organization of the UN (FAO).
In Germany and the Netherlands, wooden shoes , the Dutch clogs , are made from poplar wood, among other things.
The wood of the poplar is generally whitish to slightly yellowish. It's soft, but still resilient. It is also less flammable than other indigenous woods. It is therefore made into matches, which should not burn down too quickly after being ignited. Heat-treated poplar wood is used for the interior construction of saunas.
Crushed poplar wood is used as wood wool for packaging purposes. This is also well suited for keeping small animals, as the wood does not contain any essential oils (such as pine and other conifers). Poplar wood is finely split to make food packaging (e.g. for camembert ) and chopsticks . Dissolved into cellulose pulp, it is processed into paper. Pallets and plywood in simple qualities are made from poplar wood. The core of snowboards often consists of the very flexible wood of the poplar. Likewise, the body of string instruments, guitars and drums, as it should have good resonance properties.
In Italy, especially during the Renaissance, paintings and panels were painted on poplar panels, including the portrait of the Mona Lisa.
Although the trunk wood on the tree is not very resistant to rot in the event of injuries, the processed wood is somewhat dimensionally stable and weather-resistant, as long as it can dry off quickly after being soaked. Outside, it bleaches to a silver-gray color.
In northwest Russia, poplar wood is traditionally used to make roof shingles.
Like oak , the bark of trees contains tannins, which were used to tan leather.
Because of the phenols found in leaves, buds, twigs and bark, extracts have traditionally been used to treat burns, itching and joint swelling, as well as to make tea for use in treating mild urinary tract infections.
Worldwide there are around 6.7 million hectares of poplar trees planted, of which 3.8 million are for wood production and 2.9 million for environmental protection. 30 percent of the area are agroforestry systems. Of the total area, 4.9 million (73 percent) are in China, 1 million in India. It is followed by France with 236,000, Turkey with 130,000, Italy with 118,800, Argentina with 63,500 and Chile with 15,000 hectares. The timber harvest from natural stocks in 2003 was 100 million m³ in Russia and 16 million in Canada. More than one million m³ were harvested from planted stocks in five countries: Turkey (3.8 million), China (1.85 million), France (1.8 million), Italy (1.4 million) and India (1.2 million).
There are three main types of poplar plantations: production plantations, protective plantings and landscaping plantations.
Use in medicine
The poplar buds were formerly used to produce the poplar ointment (Unguentum) Populeum (also called Unguentum Populi , "unguentum populeon" and "Popolium ointment") used in medicine, in particular the poplar tree buds of Populus nigra and Populus alba . Popular uses of poplar ointment have been proven against hemorrhoids, burns and inflammation.
Monoclonal stocks as fast-growing wood are used in short rotation crops. In Northern and Western Europe as well as in North America, the rotation times of crops with a useful life of 20 to 30 years are 1 to 5 years. In Italy, the poplar plantations are harvested after 10 to 15 years. The wood obtained from plantations is used in the form of wood chips and wood pellets for energy generation, but can also be used as industrial wood for the wood-based materials industry for the production of chipboard , paper and other materials.
Balsam poplars from the Tacamahaca section , on the other hand, tend to be grown in forests, often together with other species. This form is widespread in North America. In pure stands, the rotation period is 40 to 50 years. In Asia, the Middle East and North Africa, columnar forms are often grown at close intervals of one meter in rotation for 10-20 years.
Plantings for protection on river banks and as wind protection have been common for centuries. Because of their rapid growth, poplars are well suited as wind protection. By choosing the poplar variety, the width and height of the crown can also be determined quite well in advance. Columnar forms like the pyramid poplars are often planted. Poplars are widely used as wind protection in Russia, Ukraine and Canada.
Along river banks, poplars not only reduce erosion, but also improve water quality. Because of their rapid growth and undemanding nature, they are used as pioneer tree species in the forestry recultivation of heaps or open-cast mines .
To maintain the landscape, individual trees or small groups are planted, often columnar poplars. Examples of frequently used cultivars are Populus nigra cv. italica in temperate areas and Populus nigra var. thevestina in Asia and the Mediterranean. In Western and Central Europe, the cultivars Robusta, Serotina, Regenerata and Marilandica of Populus canadensis are used . Poplars are often found in rows along ditches, paths and property lines.
Use of the wood
→ Main article: Poplar wood
If the wood is large enough, it is processed into sawn timber and peeled veneers (for making matches). It is also used in the pallet and packaging industry. Further processing areas are chipboard, fiberboard and molded chipboard parts. Poplar wood can be used well in interior construction; it has a limited shelf life under direct weather conditions.
However, a predominant part of the poplar wood is used in the pulp, cardboard and paper industries. The use as firewood and as biomass for energy production is not very important.
Research and breeding
The genome of Populus trichocarpa was the first genome of a tree to be fully sequenced. Between 1994 and 2004, poplars were the most frequently used trees in biotechnology after pines, and with 47 percent of all studies the most frequently used tree genus for genetic engineering. Transgenic poplars are being studied in release trials in the United States , Canada , the European Union, and China . Changed wood quality, pest resistance and improved heavy metal absorption and its accumulation for soil remediation are the focus of research activities.
Since 1998, transgenic poplars that express a Bt toxin have been approved for commercial cultivation in China to prevent feeding damage by insects without spraying insecticides . From 2013 to 2016, 543 hectares of insect-resistant poplars were planted. In field trials, the possibility of outcrossing of the Bt transgene in wild poplar forms is classified as unlikely, since the pollen is not widely distributed and the seeds have a limited survival rate in the dry climate.
Properties of poplars which are favorable for breeding are the easy formation of species hybrids, which often have more favorable properties due to the heterosis effect , and the possibility of keeping the properties of such hybrids constant by clonal propagation. In 1992 the Catalog of Poplar Cultivars listed 280 cultivars, which consist mainly of clones. There has been an official database since 2006, in which 332 cultivars were registered in March 2008.
The name Populus was used before Linnaeus . In Latin , the word pōpulus (feminine in contrast to the masculine populus , which means "people") has always been used since Cato the Elder , always meaning poplar . The etymology of the Latin word is not known. Pliny the Elder already distinguished the three species occurring in southern Europe. The German name Pappel is derived from the Old High German popel , Middle High German papele, papel with the same meaning.
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