Oxygen isotope ratio cycle
Oxygen isotope ratio cycles are cyclical variations in the ratio of the abundance of oxygen with an atomic mass of 18 to the abundance of oxygen with an atomic mass of 16 present in some substances, such as polar ice or calcite in ocean core samples, measured with the isotope fractionation. The ratio is linked to water temperature of ancient oceans. Cycles in the ratio mirror climate changes in geologic history. Oxygen has three occurring isotopes: 16O, 17O, 18O, where the 16, 17 and 18 refer to the atomic mass; the most abundant is 16O, with a small percentage of 18O and an smaller percentage of 17O. Oxygen isotope analysis considers only the ratio of 18O to 16O present in a sample; the calculated ratio of the masses of each present in the sample is compared to a standard, which can yield information about the temperature at which the sample was formed - see Proxy for details. 18O is two neutrons heavier than 16O and causes the water molecule in which it occurs to be heavier by that amount. The addition of more energy is required to vaporize H218O than H216O, H218O liberates more energy when it condenses.
In addition, H216O tends to diffuse more rapidly. Because H216O requires less energy to vaporize, is more to diffuse to the liquid surface, the first water vapor formed during evaporation of liquid water is enriched in H216O, the residual liquid is enriched in H218O; when water vapor condenses into liquid, H218O preferentially enters the liquid, while H216O is concentrated in the remaining vapor. As an air mass moves from a warm region to a cold region, water vapor condenses and is removed as precipitation; the precipitation removes H218O. This distillation process causes precipitation to have lower 18O/16O. Additional factors can affect the efficiency of the distillation, such as the direct precipitation of ice crystals, rather than liquid water, at low temperatures. Due to the intense precipitation that occurs in hurricanes, the H218O is exhausted relative to the H216O, resulting in low 18O/16O ratios; the subsequent uptake of hurricane rainfall in trees, creates a record of the passing of hurricanes that can be used to create a historical record in the absence of human records.
The 18O/16O ratio provides a record of ancient water temperature. Water 10 to 15 °C cooler than present represents glaciation; as colder temperatures spread toward the equator, water vapor rich in 18O preferentially rains out at lower latitudes. The remaining water vapor that condenses over higher latitudes is subsequently rich in 16O. Precipitation and therefore glacial ice contain water with a low 18O content. Since large amounts of 16O water are being stored as glacial ice, the 18O content of oceanic water is high. Water up to 5 °C warmer than today represents an interglacial, when the 18O content of oceanic water is lower. A plot of ancient water temperature over time indicates that climate has varied cyclically, with large cycles and harmonics, or smaller cycles, superimposed on the large ones; this technique has been valuable for identifying glacial maxima and minima in the Pleistocene. Limestone is deposited from the calcite shells of microorganisms. Calcite, or calcium carbonate, chemical formula CaCO3, is formed from water, H2O, carbon dioxide, CO2, dissolved in the water.
The carbon dioxide provides two of the oxygen atoms in the calcite. The calcium must rob the third from the water; the isotope ratio in the calcite is therefore the same, after compensation, as the ratio in the water from which the microorganisms of a given layer extracted the material of the shell. A higher abundance of 18O in calcite is indicative of colder water temperatures, since the lighter isotopes are all stored in the glacial ice; the microorganism most referenced is foraminifera. Earth’s dynamic oxygenation evolution is recorded in ancient sediments from the Republic of Gabon from between about 2,150 and 2,080 million years ago. Responsible for these fluctuations in oxygenation were driven by the Lomagundi carbon isotope excursion. Δ18O Isotope fractionation Encyclopædia Britannica under Climate and Weather, Pleistocene Climatic Change Craig Harmon. "Isotopic variations in meteoric waters". Science. 133: 1702–1703. Doi:10.1126/science.133.3465.1702. PMID 17814749. Epstein S.. "Variation of O18 content of waters from natural sources".
Geochimica et Cosmochimica Acta. 4: 213–224. Doi:10.1016/0016-703790051-9. Veizer Ján. "Evidence for decoupling of atmospheric CO2 and global climate during the Phanerozoic eon". Nature. 408: 698–701. Doi:10.1038/35047044. PMID 11130067. NASA Earth Observatory: The Oxygen Balance Scripps O2 Global Oxygen Measurements
Milankovitch cycles describe the collective effects of changes in the Earth's movements on its climate over thousands of years. The term is named for Serbian geophysicist and astronomer Milutin Milanković. In the 1920s, he hypothesized that variations in eccentricity, axial tilt, precession of the Earth's orbit resulted in cyclical variation in the solar radiation reaching the Earth, that this orbital forcing influenced climatic patterns on Earth. Similar astronomical hypotheses had been advanced in the 19th century by Joseph Adhemar, James Croll and others, but verification was difficult because there was no reliably dated evidence, because it was unclear which periods were important. Now, materials on Earth that have been unchanged for millennia are being studied to indicate the history of Earth's climate. Though they are consistent with the Milankovitch hypothesis, there are still several observations that the hypothesis does not explain; the Earth's rotation around its axis, revolution around the Sun, evolve over time due to gravitational interactions with other bodies in the solar system.
The variations are complex. The Earth's orbit varies between mildly elliptical; when the orbit is more elongated, there is more variation in the distance between the Earth and the Sun, in the amount of solar radiation, at different times in the year. In addition, the rotational tilt of the Earth changes slightly. A greater tilt makes the seasons more extreme; the direction in the fixed stars pointed to by the Earth's axis changes, while the Earth's elliptical orbit around the Sun rotates. The combined effect is. Milankovitch studied changes in these movements of the Earth, which alter the amount and location of solar radiation reaching the Earth; this is known as solar forcing. Milankovitch emphasized the changes experienced at 65° north due to the great amount of land at that latitude. Land masses change temperature more than oceans, because of the mixing of surface and deep water and the fact that soil has a lower volumetric heat capacity than water; the Earth's orbit approximates an ellipse. Eccentricity measures the departure of this ellipse from circularity.
The shape of the Earth's orbit varies between nearly circular and mildly elliptical Its geometric or logarithmic mean is 0.0019. The major component of these variations occurs with a period of 413,000 years. Other components have 125,000-year cycles, they loosely combine into a 100,000-year cycle. The present eccentricity is decreasing. Eccentricity varies due to the gravitational pull of Jupiter and Saturn. However, the semi-major axis of the orbital ellipse remains unchanged; the orbital period is invariant, because according to Kepler's third law, it is determined by the semi-major axis. The semi-major axis is a constant. Therefore, when Earth's orbit becomes more eccentric, the semi-minor axis shortens; this increases the magnitude of seasonal changes. The relative increase in solar irradiation at closest approach to the Sun compared to the irradiation at the furthest distance is larger than four times the eccentricity. For Earth's current orbital eccentricity, incoming solar radiation varies by about 6.8%, while the distance from the Sun varies by only 3.4%.
Perihelion presently occurs around January 3, while aphelion is around July 4. When the orbit is at its most eccentric, the amount of solar radiation at perihelion will be about 23% more than at aphelion. However, the Earth's eccentricity is always so small that the variation in solar irradiation is a minor factor in seasonal climate variation, compared to axial tilt and compared to the relative ease of heating the larger land masses of the northern hemisphere; the seasons are quadrants of the Earth's orbit, marked by the two equinoxes. Kepler's second law states; the Earth spends more time near aphelion. This means. Perihelion occurs around January 3, so the Earth's greater velocity shortens winter and autumn in the northern hemisphere. Summer in the northern hemisphere is 4.66 days longer than winter, spring is 2.9 days longer than autumn. Greater eccentricity increases the variation in the Earth's orbital velocity; however the Earth's orbit is becoming less eccentric. This will make the seasons more similar in length.
The angle of the Earth's axial tilt with respect to the orbital plane varies between 22.1° and 24.5°, over a cycle of about 41,000 years. The current tilt is 23.44° halfway between its extreme values. The tilt last reached its maximum in 8,700 BCE, it is now in the decreasing phase of its cycle, will reach its minimum around the year 11,800 CE. Increased tilt increases the amplitude of the seasonal cycle in insolation, providing more solar radiation in each hemisphere's summer and less in winter. However, these effects are not uniform everywhere on the Earth's surface. Increased tilt increases the total annu
Taxus is a small genus of coniferous trees or shrubs known as yews in the family Taxaceae. They are slow-growing and can be long-lived, reach heights of 2.5–20 metres, with trunk girth averaging 5 metres. They have reddish bark, flat, dark-green leaves 10–40 millimetres long and 2–3 mm broad, arranged spirally on the stem, but with the leaf bases twisted to align the leaves in two flat rows either side of the stem; the seed cones are modified, each cone containing a single seed 4–7 mm long surrounded by a modified scale which develops into a soft, bright red berry-like structure called an aril, 8–15 mm long and wide and open at the end. The arils are mature 6–9 months after pollination, with the seed contained are eaten by thrushes and other birds, which disperse the hard seeds undamaged in their droppings; the male cones are globose, 3–6 mm across, shed their pollen in early spring. Yews are dioecious, but occasional individuals can be variably monoecious, or change sex with time. All of the yews are closely related to each other, some botanists treat them all as subspecies or varieties of just one widespread species.
Other sources, recognize 9 species, for example the Plant List. The most distinct is the Sumatran yew, distinguished by its sparse, sickle-shaped yellow-green leaves; the Mexican yew is relatively distinct with foliage intermediate between Sumatran yew and the other species. The Florida yew, Mexican yew and Pacific yew are all rare species listed as threatened or endangered. All species of yew contain poisonous taxine alkaloids, with some variation in the exact formula of the alkaloid between the species. All parts of the tree except the arils contain the alkaloid; the arils are edible and sweet. This can have fatal results. Grazing animals cattle and horses, are sometimes found dead near yew trees after eating the leaves, though deer are able to break down the poisons and will eat yew foliage freely. In the wild, deer browsing of yews is so extensive that wild yew trees are restricted to cliffs and other steep slopes inaccessible to deer; the foliage is eaten by the larvae of some Lepidopteran insects including the moth willow beauty.
All parts of a yew plant are toxic to humans with the exception of the yew berries. These pollen granules are small, can pass through window screens. Male yews release abundant amounts of pollen in the spring. Yews in this genus are separate-sexed, males are allergenic, with an OPALS allergy scale rating of 10 out of 10. Female yews have an OPALS rating of 1, are considered "allergy-fighting". Yew wood is reddish brown, is springy, it was traditionally used to make bows the longbow. Latin taxus "yew tree," is borrowed, via Greek, from Taxša, the Scythian word for yew. British yews tend to be too gnarly, thus the wood for English longbows used at the Battle of Agincourt was imported from Spain or northern Italy. Ötzi, the Chalcolithic mummy found in 1991 in the Italian Alps, carried an unfinished bow made of yew wood. It is not surprising that in Norse mythology, the abode of the god of the bow, had the name Ydalir. Most longbow wood used in northern Europe was imported from Iberia, where climatic conditions are better for growing the knot-free yew wood required.
The yew longbow was the critical weapon used by the English in the defeat of the French cavalry at the Battle of Agincourt, 1415. It is suggested that English parishes were required to grow yews and, because of the trees' toxic properties, they were grown in the only enclosed area of a village – the churchyard; the yew tree can be found in church graveyards and is symbolic of sadness. Such a representation appears in Lord Alfred Tennyson's poem "In Memoriam A. H. H.". The yew can be long-lived; the Fortingall Yew has been considered to be the oldest tree in Europe, at something over 2,000 years old. Tradition has it that Pontius Pilate slept under it while on duty before 30 AD; this has been topped by a tree in the churchyard of a small Welsh village called St Cynog. It has been dated to 5,000 years old by dendrologist Janis Fry; such old trees consist of a circular ring of growths of yew, since their heart has long since rotted away. The Eihwaz rune is named after the yew, sometimes associated with the "evergreen" world tree, Yggdrasil.
Yews are used in landscaping and ornamental horticulture. Over 400 cultivars of yews have been named, the vast majority of these being derived from European yew or Japanese yew; the hybrid between these two species is Taxus media"Taxus" ×"media". A popular fastigiate selection of the European yew (Taxus baccata'
The Canadian Shield called the Laurentian Plateau, or Bouclier canadien, is a large area of exposed Precambrian igneous and high-grade metamorphic rocks that forms the ancient geological core of the North American continent. Composed of igneous rock resulting from its long volcanic history, the area is covered by a thin layer of soil. With a deep, joined bedrock region in eastern and central Canada, it stretches north from the Great Lakes to the Arctic Ocean, covering over half of Canada. Human population is sparse, industrial development is minimal, while mining is prevalent; the Canadian Shield is a physiographic division, consisting of five smaller physiographic provinces: the Laurentian Upland, Kazan Region, Davis and James. The shield extends into the United States as the Superior Upland; the Canadian Shield is U-shaped and is a subsection of the Laurentia craton signifying the area of greatest glacial impact creating the thin soils. The Canadian Shield is more than 3.96 billion years old.
The Canadian Shield once had jagged peaks, higher than any of today's mountains, but millions of years of erosion have changed these mountains to rolling hills. The Canadian Shield is a collage of Archean plates and accreted juvenile arc terranes and sedimentary basins of the Proterozoic Eon that were progressively amalgamated during the interval 2.45 to 1.24 Ga, with the most substantial growth period occurring during the Trans-Hudson orogeny, between ca. 1.90 to 1.80 Ga. The Canadian Shield was the first part of North America to be permanently elevated above sea level and has remained wholly untouched by successive encroachments of the sea upon the continent, it is the Earth's greatest area of exposed Archean rock. The metamorphic base rocks are from the Precambrian and have been uplifted and eroded. Today it consists of an area of low relief 300 to 610 m above sea level with a few monadnocks and low mountain ranges eroded from the plateau during the Cenozoic Era. During the Pleistocene Epoch, continental ice sheets depressed the land surface creating Hudson Bay, scooped out thousands of lake basins, carried away much of the region's soil.
When the Greenland section is included, the Shield is circular, bounded on the northeast by the northeast edge of Greenland, with Hudson Bay in the middle. It covers much of Greenland, most of Quebec north of the St. Lawrence River, much of Ontario including northern sections of the Ontario Peninsula, the Adirondack Mountains of New York, the northernmost part of Lower Michigan and all of Upper Michigan, northern Wisconsin, northeastern Minnesota, the central/northern portions of Manitoba away from Hudson Bay, northern Saskatchewan, a small portion of northeastern Alberta, the mainland northern Canadian territories to the east of a line extended north from the Saskatchewan/Alberta border. In total, the exposed area of the Shield covers 8,000,000 km2; the true extent of the Shield is greater still and stretches from the Western Cordillera in the west to the Appalachians in the east and as far south as Texas, but these regions are overlaid with much younger rocks and sediment. The Canadian Shield is with regions dating from 2.5 to 4.2 billion years.
The multitude of rivers and lakes in the entire region is caused by the watersheds of the area being so young and in a state of sorting themselves out with the added effect of post-glacial rebound. The Shield was an area of large tall mountains with much volcanic activity, but over hundreds of millions of years, the area has been eroded to its current topographic appearance of low relief, it has some of the oldest volcanoes on the planet. It has over 150 volcanic belts; each belt grew by the coalescence of accumulations erupted from numerous vents, making the tally of volcanoes reach the hundreds. Many of Canada's major ore deposits are associated with Precambrian volcanoes; the Sturgeon Lake Caldera in Kenora District, Ontario, is one of the world's best preserved mineralized Neoarchean caldera complexes, 2.7 billion years old. The Canadian Shield contains the Mackenzie dike swarm, the largest dike swarm known on Earth. Mountains float on the denser mantle much like an iceberg at sea; as mountains erode, their roots are eroded in turn.
The rocks that now form the surface of the Shield were once far below the Earth's surface. The high pressures and temperatures at those depths provided ideal conditions for mineralization. Although these mountains are now eroded, many large mountains still exist in Canada's far north called the Arctic Cordillera; this is a vast dissected mountain range, stretching from northernmost Ellesmere Island to the northernmost tip of Labrador. The range's highest peak is Nunavut's Barbeau Peak at 2,616 metres above sea level. Precambrian rock is the major component of the bedrock; the North American craton is the bedrock forming the heart of the North American continent and the Canadian Shield is the largest exposed part of the craton's bedrock. The Canadian Shield is part of an ancient continent called Arctica, formed about 2.5 billion years ago during the Neoarchean era. It was s
Holocene climatic optimum
The Holocene Climate Optimum was a warm period during the interval 9,000 to 5,000 years BP, with a thermal maximum around 8000 years BP. It has been known by many other names, such as Altithermal, Climatic Optimum, Holocene Megathermal, Holocene Optimum, Holocene Thermal Maximum and Mid-Holocene Warm Period; this warm period was followed by a gradual decline until about two millennia ago. For other temperature fluctuations, see temperature record. For other past climate fluctuation, see paleoclimatology. For the pollen zone and Blytt-Sernander period, associated with the climate optimum, see Atlantic; the Holocene Climate Optimum warm event consisted of increases of up to 4 °C near the North Pole. Northwestern Europe experienced warming; the average temperature change appears to have declined with latitude and so no change in mean temperature is reported at low and middle latitudes. Tropical reefs tend to show temperature increases of less than 1 °C. In terms of the global average, temperatures were warmer than now.
While temperatures in the Northern Hemisphere were warmer than average during the summers, the Tropics and parts of the Southern Hemisphere were colder than average. Out of 140 sites across the western Arctic, there is clear evidence for conditions warmer than now at 120 sites. At 16 sites, where quantitative estimates have been obtained, local HTM temperatures were on average 1.6±0.8 °C higher than now. Northwestern North America had peak warmth first, from 11,000 to 9,000 years ago, the Laurentide ice sheet still chilled the continent. Northeastern North America experienced peak warming 4,000 years later. Along the Arctic Coastal Plain in Alaska, there are indications of summer temperatures 2–3 °C warmer than present. Research indicates. Current desert regions of Central Asia were extensively forested due to higher rainfall, the warm temperate forest belts in China and Japan were extended northwards. West African sediments additionally record the African Humid Period, an interval, between 16,000 and 6,000 years ago, when Africa was much wetter.
This was caused by a strengthening of the African monsoon by changes in summer radiation, resulting from long-term variations in the Earth's orbit around the Sun. The "Green Sahara" was dotted with numerous lakes, containing typical African lake crocodile and hippopotamus fauna. A curious discovery from the marine sediments is that the transitions into and out of the wet period occurred within decades, not the previously-thought extended periods, it is hypothesized that humans played a role in altering the vegetation structure of North Africa at some point after 8,000 years ago, when they introduced domesticated animals. This introduction contributed to the rapid transition to the arid conditions found in many locations in the Sahara. In the far Southern Hemisphere, the warmest period during the Holocene appears to have been 8,000 to 10,500 years ago following the end of the last ice age. By 6,000 years ago, the time associated with the Holocene Climatic Optimum in the Northern Hemisphere, they had reached temperatures similar to present ones, they did not participate in the temperature changes of the north.
However, some authors have used the term "Holocene Climatic Optimum" to describe the earlier southern warm period, as well. A comparison of the delta profiles at Byrd Station, West Antarctica and Camp Century, Northwest Greenland, shows the post glacial climatic optimum. Points of correlation indicate that in these two locations the Holocene climatic optimum occurred at the same time. A similar comparison is evident between the Dye 3 1979 and Camp Century 1963 cores regarding this period; the Hans Tausen Iskappe in Peary Land was drilled in 1977 with a new deep drill to 325 m. The ice core contained distinct melt layers all the way to bedrock indicating that Hans Tausen Iskappe contains no ice from the last glaciation. From the delta-profile, the Renland ice cap in the Scoresby Sound has always been separated from the inland ice, yet all the delta-leaps revealed in the Camp Century 1963 core recurred in the Renland 1985 ice core; the Renland ice core from East Greenland covers a full glacial cycle from the Holocene into the previous Eemian interglacial.
The Renland ice core is 325 m long. Although the depths are different, the GRIP and NGRIP cores contain this climatic optimum at similar times; the climatic event was a result of predictable changes in the Earth's orbit and a continuation of changes that caused the end of the last glacial period. The effect would have had maximum Northern Hemisphere heating 9,000 years ago, when the axial tilt was 24° and the nearest approach to the Sun was during the Northern Hemisphere's summer; the calculated Milankovitch Forcing would have provided 0.2% more solar radiation to the Northern Hemisphere in summer, tending to cause greater heating. There seems to have been the predicted southward shift in the global band of thunderstorms, the Intertropical Convergence Zone. However, orbital forcing would pre
The Pleistocene is the geological epoch which lasted from about 2,588,000 to 11,700 years ago, spanning the world's most recent period of repeated glaciations. The end of the Pleistocene corresponds with the end of the last glacial period and with the end of the Paleolithic age used in archaeology; the Pleistocene is the first epoch of the Quaternary Period or sixth epoch of the Cenozoic Era. In the ICS timescale, the Pleistocene is divided into four stages or ages, the Gelasian, Middle Pleistocene and Upper Pleistocene. In addition to this international subdivision, various regional subdivisions are used. Before a change confirmed in 2009 by the International Union of Geological Sciences, the time boundary between the Pleistocene and the preceding Pliocene was regarded as being at 1.806 million years Before Present, as opposed to the accepted 2.588 million years BP: publications from the preceding years may use either definition of the period. Charles Lyell introduced the term "Pleistocene" in 1839 to describe strata in Sicily that had at least 70% of their molluscan fauna still living today.
This distinguished it from the older Pliocene epoch, which Lyell had thought to be the youngest fossil rock layer. He constructed the name "Pleistocene" from the Greek πλεῖστος, pleīstos, "most", καινός, kainós, "new"; the Pleistocene has been dated from 2.588 million to 11,700 years BP with the end date expressed in radiocarbon years as 10,000 carbon-14 years BP. It covers most of the latest period of repeated glaciation, up to and including the Younger Dryas cold spell; the end of the Younger Dryas has been dated to about 9640 BC. The end of the Younger Dryas is the official start of the current Holocene Epoch. Although it is considered an epoch, the Holocene is not different from previous interglacial intervals within the Pleistocene, it was not until after the development of radiocarbon dating, that Pleistocene archaeological excavations shifted to stratified caves and rock-shelters as opposed to open-air river-terrace sites. In 2009 the International Union of Geological Sciences confirmed a change in time period for the Pleistocene, changing the start date from 1.806 to 2.588 million years BP, accepted the base of the Gelasian as the base of the Pleistocene, namely the base of the Monte San Nicola GSSP.
The IUGS has yet to approve a type section, Global Boundary Stratotype Section and Point, for the upper Pleistocene/Holocene boundary. The proposed section is the North Greenland Ice Core Project ice core 75° 06' N 42° 18' W; the lower boundary of the Pleistocene Series is formally defined magnetostratigraphically as the base of the Matuyama chronozone, isotopic stage 103. Above this point there are notable extinctions of the calcareous nanofossils: Discoaster pentaradiatus and Discoaster surculus; the Pleistocene covers the recent period of repeated glaciations. The name Plio-Pleistocene has, in the past, been used to mean the last ice age; the revised definition of the Quaternary, by pushing back the start date of the Pleistocene to 2.58 Ma, results in the inclusion of all the recent repeated glaciations within the Pleistocene. The modern continents were at their present positions during the Pleistocene, the plates upon which they sit having moved no more than 100 km relative to each other since the beginning of the period.
According to Mark Lynas, the Pleistocene's overall climate could be characterized as a continuous El Niño with trade winds in the south Pacific weakening or heading east, warm air rising near Peru, warm water spreading from the west Pacific and the Indian Ocean to the east Pacific, other El Niño markers. Pleistocene climate was marked by repeated glacial cycles in which continental glaciers pushed to the 40th parallel in some places, it is estimated. In addition, a zone of permafrost stretched southward from the edge of the glacial sheet, a few hundred kilometres in North America, several hundred in Eurasia; the mean annual temperature at the edge of the ice was −6 °C. Each glacial advance tied up huge volumes of water in continental ice sheets 1,500 to 3,000 metres thick, resulting in temporary sea-level drops of 100 metres or more over the entire surface of the Earth. During interglacial times, such as at present, drowned coastlines were common, mitigated by isostatic or other emergent motion of some regions.
The effects of glaciation were global. Antarctica was ice-bound throughout the Pleistocene as well as the preceding Pliocene; the Andes were covered in the south by the Patagonian ice cap. There were glaciers in New Tasmania; the current decaying glaciers of Mount Kenya, Mount Kilimanjaro, the Ruwenzori Range in east and central Africa were larger. Glaciers existed to the west in the Atlas mountains. In the northern hemisphere, many glaciers fused into one; the Cordilleran ice sheet covered the North American northwest. The Fenno-Scandian ice sheet rested including much of Great Britain. Scattered domes stretched across Siberi
Elms are deciduous and semi-deciduous trees comprising the flowering plant genus Ulmus in the plant family Ulmaceae. The genus first appeared in the Miocene geological period about 20 million years ago, originating in what is now central Asia; these trees flourished and spread over most of the Northern Hemisphere, inhabiting the temperate and tropical-montane regions of North America and Eurasia, presently ranging southward across the Equator into Indonesia. Elms are components of many kinds of natural forests. Moreover, during the 19th and early 20th centuries many species and cultivars were planted as ornamental street and park trees in Europe, North America, parts of the Southern Hemisphere, notably Australasia; some individual elms reached great age. However, in recent decades, most mature elms of European or North American origin have died from Dutch elm disease, caused by a microfungus dispersed by bark beetles. In response, disease-resistant cultivars have been developed, capable of restoring the elm to forestry and landscaping.
There are about 30 to 40 species of Ulmus. Oliver Rackham describes Ulmus as the most critical genus in the entire British flora, adding that'species and varieties are a distinction in the human mind rather than a measured degree of genetic variation'. Eight species are endemic to North America, a smaller number to Europe; the classification adopted in the List of elm species, varieties and hybrids is based on that established by Brummitt. A large number of synonyms have accumulated over the last three centuries. Botanists who study elms and argue over elm identification and classification are called pteleologists, from the Greek πτελέα; as part of the sub-order urticalean rosids they are distant cousins of cannabis and nettles. The name Ulmus is the Latin name for these trees, while the English "elm" and many other European names are either cognate with or derived from it; the genus is hermaphroditic, having apetalous perfect flowers. Elm leaves are alternate, with simple, single- or, most doubly serrate margins asymmetric at the base and acuminate at the apex.
The fruit is a round wind-dispersed samara flushed with chlorophyll, facilitating photosynthesis before the leaves emerge. The samarae are light, those of British elms numbering around 50,000 to the pound. All species are tolerant of a wide range of soils and pH levels but, with few exceptions, demand good drainage; the elm tree can grow to great height with a forked trunk creating a vase profile. Dutch elm disease devastated elms throughout Europe and much of North America in the second half of the 20th century, it derives its name'Dutch' from the first description of the disease and its cause in the 1920s by the Dutch botanists Bea Schwarz and Christina Johanna Buisman. Owing to its geographical isolation and effective quarantine enforcement, Australia has so far remained unaffected by Dutch Elm Disease, as have the provinces of Alberta and British Columbia in western Canada. DED is caused by a micro-fungus transmitted by two species of Scolytus elm-bark beetle which act as vectors; the disease affects all species of elm native to North America and Europe, but many Asiatic species have evolved anti-fungal genes and are resistant.
Fungal spores, introduced into wounds in the tree caused by the beetles, invade the xylem or vascular system. The tree responds by producing tyloses blocking the flow from roots to leaves. Woodland trees in North America are not quite as susceptible to the disease because they lack the root-grafting of the urban elms and are somewhat more isolated from each other. In France, inoculation with the fungus of over three hundred clones of the European species failed to find a single variety possessed of any significant resistance; the first, less aggressive strain of the disease fungus, Ophiostoma ulmi, arrived in Europe from the Far East in 1910, was accidentally introduced to North America in 1928, but was weakened by viruses and had all but disappeared in Europe by the 1940s. The second, far more virulent strain of the disease Ophiostoma novo-ulmi was identified in Europe in the late 1960s, within a decade had killed over 20 million trees in the UK alone. Three times more deadly, the new strain arrived in Europe from the US on a cargo of Rock Elm.
There is no sign of the current pandemic waning, no evidence of a susceptibility of the fungus to a disease of its own caused by d-factors: occurring virus-like agents that debilitated the original O. ulmi and reduced its sporulation. Elm phloem necrosis is a disease of elm trees, spread by leafhoppers or by root grafts; this aggressive disease, with no known cure, occurs in the Eastern United States, southern Ontario in Canada, Europe. It is caused by phytoplasmas. Infection and death of the phloem girdles the tree and stops the flow of water and nutrients; the disease affects cultivated trees. Cutting the infected tree before the disease establishes itself and cleanup and prompt disposal of infected matter has resul