Pyrus, the pears, is a genus of about 25 species of usually deciduous trees and shrubs, native to Europe, Asia, and North Africa, several of which are have been cultivated since ancient times for the fruit, and are also used as ornamentals. The major species for fruit are P. communis, the common or European pear, of which there are more than a thousand cultivars all with the traditional pear shape, and P. pyrifolia (sometimes classified P. serotina), the Asian pear, which has a round fruit; P. ussuriensis, P. pyrifolia, and P. bretschneideri are widely cultivated fruit pears in China. Species used as ornamentals include P. betulifolia and P. calleryana, the Callery pear (including the Bradford cultivar), which is widely planted and becoming invasive in North America, and may be used for its wood (which has an attractive grain for furniture), as well as landscaping, in China. Various other species are used as rootstock for the pear species used for fruit production, including P. ussuriensis, P. xerophila, P. betulifolia, and P. phaeocarpa.
Pears are generally small trees or shrubs, sometimes armed with thorns, which have simple, alternative leaves with either serrate (toothed) or entire (smooth) leaf margins. The showy white (rarely pink) 5-petalled flowers are in corymbs (branched, often flat-topped clusters), and often open before leaves have emerged, or in a few species open at the same time. The fruit is a pome (a fleshy fruit with leathery tissue encasing the seeds, like an apple). All pear species contain stone cells or brachysclereids (small bundles of schlerenchyma tissue) that give the fruit a gritty texture, even when it is ripe and juicy.
It is difficult to assess the economic importance of ornamental pears, but the U.N. Food and Agriculture Organization estimates that total commercial production of pears (of all species, but probably mostly P. communis) was 22.64 million metric tons, harvested from 1.53 million hectares worldwide. China was the largest producer, responsible for 15.23 million metric tons—two thirds of world production—valued at $62.27 billion U.S. Other top-ranked producers were Italy, the U.S., Argentina, and Spain, each with less than 3% of the world total.
The Bradford pear cultivar, which has been planted widely in the U.S. after being imported in 1918—it was voted the second most popular tree in the U.S. in 1982 by the National Landscape Association—has escaped cultivation and become naturalized in many states in the eastern U.S. It is now considered invasive in several states, including Maryland and Pennsylvania, where it can spread aggressively with the help of bird-dispersed fruits into old fields and along roadsides, and compete with native early-successional tree species.
(Bailey et al. 1976, Flora of China 2003, Invasives.org 2012, van Wyk 2005.)