UPDATE: See our NEW publication list below for Callery Pear **
Why would a species commonly planted as a ornamental tree for decades without a problem suddenly begin to spread and become invasive? See our paper published in Arnoldia for the complete story, or just keep reading here.
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 two decades, wild Pyrus calleryana have begun appearing and multiplying in natural areas across the United States. 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 European Starlings and perhaps even American Robins. 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 (see below), 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 and seed dormancy on the spread of the species in the US.
UPDATE: The Callery pear is increasingly being recognized as an Invasive species. In 2018, Ohio was the first state in the United States to place Pyrus calleryana on its list of invasive species regulated for commercial sale and distribution. It was given a 5-year phase out period at that time, in recognition of its economic importance as a horticultural tree. As of January 2023, Callery pear and all of its cultivars will be banned from commercial sale and distribution in the state of Ohio. South Carolina is now following suit as the second state and will also ban P. calleryana from commercial sale in 2024.
Callery Pear Resources
Scientific Literature (including our own lab’s papers):
Woods, Michaela J., Grace Dietsch, and Ryan W. McEwan (2022) Callery pear invasion in prairie restorations is predicted by proximity to forest edge, not species richness. Biological Invasions 24: 3555–3564.
Hartshorn, Jessica A. J. Forest Palmer, and David, R. Coyle (2022) Into the wild: Evidence for the Enemy Release Hypothesis in the invasive Callery pear (Pyrus calleryana) (Rosales: Rosaceae). Environmental Entomology 51: 216–221, https://doi.org/10.1093/ee/nvab136
Coyle, David. R., Brayden M. Williams, Donald L. Hagan (2021) Fire can reduce thorn damage by the invasive Callery pear tree. HortTechnology 31:625-629.
Feldman, Tzipora and Theresa M. Culley (2019) Seed germination and seedling survival of invasive Callery Pear (Pyrus calleryana Decene.) 11 years after fruit collection. Castanea 84(1): 47-52.
McMillen, Heather, Lindsay K. Campbell, and Erika S. Svendsen (2019) Weighing values and risks of beloved invasive species: The case of the survivor tree and conflict management in urban green infrastructure. Urban Forestry & Urban Greening 40:44-52.
Warrix, Adam R. and Jordan M. Marshall (2018) Callery pear (Pyrus calleryana) response to fire in a managed prairie ecosystem. Invasive Plant Science and Management 11: 27-32. https://doi.org/10.1017/inp.2018.4
Warrix, Adam R., Andrea L. Myers, Jordan M. Marshall (2017) Estimating invading Callery pear (Pyrus calleryana) age and flowering probability in an Indiana managed prairie. Proceedings of the Indiana Academy of Science 126(2):153–157.
Merritt, Benjamin J., Joshua B. Jones, Nicole A. Hardiman, and Theresa M. Culley (2013) Comparison of photosynthetic characteristics in cultivated and wild offspring of the invasive Callery pear (Pyrus calleryana). Biological Invasions. doi: 10.1007/s10530-013-0528-6
Culley, Theresa M., Nicole A. Hardiman, and Jennifer Hawks (2011) The role of horticulture in plant invasions: how grafting in cultivars of Callery pear (Pyrus calleryana) can facilitate spread into natural areas. Biological Invasions 13(3) 739-746. doi:10.1007/s10530-010-9864-y.
Hardiman, Nicole A. and Theresa M. Culley (2010) Reproductive success of cultivated Pyrus calleryana (Rosaceae) and establishment ability of invasive, hybrid progeny. American Journal of Botany 97(10): 1698-1706.
Culley, Theresa M. and Nicole A. Hardiman (2009) The role of intraspecific hybridization in the evolution of invasiveness: A case study of the ornamental pear tree Pyrus calleryana. Biological Invasions 11:1107–1119.
Culley, Theresa M., and Nicole A. Hardiman (2007) The beginning of a new invasive plant: A history of the ornamental Callery Pear tree in the United States. BioScience 57(11):956-964.
Hardiman, Nicole A. and Theresa M. Culley (2007) Genetic analysis of Callery Pear cultivars to determine the origin of invasive populations. In: Nicole Cavender, ed. Ohio Invasive Plant Research Conference Proceedings, Ohio Biological Survey, Columbus, OH. pp. 59-66. [Peer-reviewed book]
Vincent, Michael A. (2005) On the spread and current distribution of Pyrus calleryana in the United States. Castanea, 70(1):20-31 (2005). https://doi.org/10.2179/0008-7475(2005)070[0020:OTSACD]2.0.CO;2
Rubin, Sean (2021) This Very Tree: A Story of 9/11, Resilience, and Regrowth. Henry Holt and Company, New York [Children’s book about the Survivor Tree, a Callery Pear that survived the 9/11 attack in New York]
Survivor Tree Website: https://www.911memorial.org/visit/memorial/survivor-tree (see also link above to McMillen et al. 2019 article)