Why would a species commonly planted as a ornamental tree for decades without a problem suddenly begin to spread and become invasive?

The Callery Pear (Pyrus calleryana) was originally introduced into the United States from China in the early 1900’s as a rootstock species and as a source for fireblight resistance. It has quickly become one of the most popular ornamental tree species planted in urban areas. Known as the ‘Bradford’, ‘Aristocrat’, ‘Cleveland Select’, etc. (see list), these cultivars are highly desired for their springtime display of white showy flowers, vibrant fall leaf color, and tolerance of excessive drought and pollution.

Within the last decade, wild Pyrus calleryana have begun appearing and multiplying in natural areas. Furthermore, many cultivated Callery Pears in urban yards have also begun producing fruits even though each cultivar is self-incompatible. Many of these fruits are dispersed by introduced birds such as starlings. In our lab, we are studying why Pyrus calleryana is beginning to spread and becoming invasive.

Based on a combination of ecological and genetic studies, we now know that the recent expansion of Callery pear is due to intraspecific hybridization between different cultivars as well as rootstock. We are continuing to study this intriguing species to understand what impacts it may have on ecosystem processes as well as to develop practical suggestions for management of invasive populations. Our most recent work explores the effect of photosynthetic ecophysiology on the spread of the species in the US.