The Callery Pear (Pyrus calleryana) is native to eastern Asia but planted widely as an ornamental in urban and suburban residential and commercial areas in the United States. Numerous cultivars have been developed and are clonally propagated. Wild populations of Callery Pear can now be found throughout much of the United States, generally in open or disturbed habitats.
The Callery Pear was originally brought to the United States to address the problem of fire blight, a bacterial disease spread by pollinators, that was affecting Common Pears (Pyrus communis). In the early 1900s, the cultivated pear industry in the western United States was suffering enormous losses due to fire blight. Callery Pears were used in breeding programs and as rootstocks in efforts to develop resistant cultivars. The ornamental value of the Callery Pear was soon recognized and by the early 1960s the "Bradford" cultivar was commercially available. The Bradford Pear quickly caught on and became a widely planted street tree. Many other cultivars followed--and it was realized that Bradfords had a problem in that the architecture of the branches often caused individual trees to split under their own weight after around 15 to 20 years of growth. The shortcomings of the Bradford Pear increased the popularity of other varieties and the diversity of Callery Pears being planted.
In recent years, Callery Pear trees have begun to appear in many natural areas in the eastern United States. By 2005, wild trees had been found in more than two dozen states. Interestingly, the Callery Pear was not long ago viewed as unlikely to become an invasive species in part because of its self-incompatibility. Like many plants, Callery Pear exhibits a gametophytic incompatibility whereby when pollen is transferred pollen tubes begin to grow down the styles of both compatible and incompatible flowers, but if the haploid pollen grain shares the same self-incompatibility allele as the diploid maternal tissue, the pollen tube is prevented from reaching the ovule. It is now apparent, however, that as a consequence of the increasing diversity of the cultivars being planted, and the sprouting and flowering of rootstocks that are from different genetic stock than the scion, this incompatibility is often circumvented. Numerous other traits (e.g., seed dispersal by birds, broad environmental tolerance, few pests. rapid growth, early reproduction, heavy fruit set) facilitate the rapid spread of the Callery Pear. Naturalized Callery Pears often form dense thickets and these are often thorny since even thornless cultivars apparently retain genes for thorniness that may be expressed as genes recombine in their progeny. Although there is significant concern about the ecological impact of the Callery Pear in the United States, it remains to be seen whether it will ultimately persist as a minor non-native component of the ecosystem or will become a more serious problem
(Vincent 2005; Culley and Hardiman 2007 and references therein; Hardiman and Culley 2010)