Question

Discuss the current status of Ulmus americana and the reasons for it status. (answer in detail will get 5 stars)

Answer 1

Reasons: Widespread, still common, but far less common than several decades ago due to depletion from an exotic fungus (Dutch elm disease).

Ulmus americana (American elm) is a species of tree in the family Ulmaceae. It has simple, broad leaves and green flowers. It is a photoautotroph.

Ulmus americana, generally known as the American elm or, less commonly, as the white elm or water elm, is a species native to eastern North America, naturally occurring from Nova Scotia west to Alberta and Montana, and south to Florida and central Texas. The American elm is an extremely hardy tree that can withstand winter temperatures as low as −42 °C (−44 °F). Trees in areas unaffected by Dutch elm disease can live for several hundred years. A prime example of the species was the Sauble Elm, which grew beside the banks of the Sauble River in Ontario, Canada, to a height of 43 m (140 ft), with a d.b.h of 196 cm (6.43 ft) before succumbing to Dutch elm disease; when it was felled in 1968, a tree-ring count established that it had germinated in 1701.

For over 80 years, U. americana had been identified as a tetraploid, i.e. having double the usual number of chromosomes, making it unique within the genus. However, a study published in 2011 by the Agricultural Research Service of the USDA revealed that about 20% of wild American elms are diploid and may even constitute another species. Moreover, several triploid trees known only in cultivation, such as 'Jefferson', are possessed of a high degree of resistance to Dutch elm disease, which ravaged American elms in the 20th century. This suggests that the diploid parent trees, which have markedly smaller cells than the tetraploid, may too be highly resistant to the disease.

In the United States, the American elm is a major member of four major forest cover types: black ash-American elm-red maple; silver maple-American elm; sugarberry-American elm-green ash; and sycamore-sweetgum-American elm, with the first two of these types also occurring in Canada. A sugar maple-ironwood-American elm cover type occurs on some hilltops near Témiscaming, Quebec.

The leaves of the American elm serve as food for the larvae of various lepidopterans (butterflies & moths).

Dutch elm disease is a fungal disease that has ravaged the American elm, causing catastrophic die-offs in cities across the range. It has been estimated that only approximately 1 in 100,000 American elm trees is Dutch elm disease-tolerant, most known survivors simply having escaped exposure to the disease. However, in some areas still not infested by Dutch elm disease, the American elm continues to thrive, notably in Florida, Alberta and British Columbia. There is a particularly beautiful grove of old-growth American Elm in Manhattan's Central Park. The trees there were spared because of the grove's isolation in such an intensely urban setting.

The American elm is particularly susceptible to disease because the period of infection often coincides with the period, approximately 30 days, of rapid terminal growth when new springwood vessels are fully functional. Spores introduced outside of this period remain largely static within the xylem and are thus relatively ineffective.

Elms in forest and other natural areas have been less affected by Dutch Elm Disease than trees in urban environments due to lower environmental stress from pollution and soil compaction and due to occurring in smaller, more isolated populations.

In the 19th and early 20th century, American elm was a common street and park tree owing to its tolerance of urban conditions, rapid growth, and graceful form. This however led to extreme overplanting of the species, especially to form living archways over streets, which ultimately produced an unhealthy monoculture of elms that had no resistance to disease and pests. Elms do not naturally form pure stands and trees used in landscaping were grown from a handful of cultivars, causing extremely low genetic diversity.These trees' rapid growth and longevity, leading to great size within decades, also favored its horticultural use before the advent of Dutch elm disease.

It is this distinctive growth form that is so valued in the open-grown American elms of street plantings, lawns, and parks; along most narrower streets, elms planted on opposite sides arch and blend together into a leafy canopy over the pavement. However, elms can assume many different sizes and forms depending on the location and climate zone. The classic vase-shaped elm was mainly the result of selective breeding of a few cultivars and is much less likely to occur in naturally grown trees.