Mason Heberling

This specimen of Callery pear (Pyrus calleryana) was collected on October 11, 1979 by W.Z. Fang in Jiangsu, China. Callery pear is native to East Asia (China, Vietnam).
 
But...Callery pear can be found in the United States. It was (and is) widely planted as an ornamental landscape tree.  Along streets, in residential yards, in parking lots  - it was a prized plant for well-groomed anthropogenic landscapes. It uniformly grows in low resource conditions, explodes with many beautiful blossoms each spring, provides shade, and has a decent foliage display in the autumn. As many introduced plants go, it went from prized ornamental to an unwanted “invasive species,” spreading across the landscape and affecting the environment. It is now widely recognized invasive species in many states or closely watched as a species likely to become invasive. That said, beyond the legacy of over half a century of mass plantings across the country, it is still common planted and old and new cultivars are commercially available.  USDA estimated over $23 million in sales in the US in 2009 alone.
 
How’d Callery pear get to the US? The story behind the introduction of Callery pear is a fascinating one.  Like many of our cultivated plants, seeds were collected on special expeditions in search of plants useful to horticulture, agriculture, or just because. Pyrus calleryana was first introduced to the US in the early 1900s, though not for its attractive blossoms as you might expect. Instead, it was first introduced for its disease resistance. It was successfully used in horticulture as a root stock for European pear fruit production. At the time, European pears in the Pacific northwestern US were being hit hard, grafting to a Callery pear rootstock dramatically decreased crop losses to disease. Callery pear does not produce edible fruit.

​The tree was widely planted starting in the 1960s, when it became commercially available and promoted by the nursery industry as a hardy ornamental tree.  Before that, it was planted by the US Department of Agriculture (USDA) for testing at Plant Introduction Stations in Corvallis, Oregon, and Glenn Dale, Maryland. One of these Maryland planted trees was targeted for its special traits and became the source of the hugely popular ‘Bradford’ cultivar (“Bradford pear”). In 1952, one tree was used to graft to rootstock, a plant propagation method making all plants genetically identical.  Other cultivars have since been commercialized, but ‘Bradford’ were/are exceptionally popular. Though intended to be sterile (non-reproducing), it turned out the trees were capable of setting viable seed.   The cultivars themselves are not invasive, but because multiple cultivars exist, together they can cross-pollinate to become invasive.

Herbarium specimens have been critical to understanding the spread of this species.  In a 2005 study, Dr. Michael Vincent (Miami University in Ohio) found that 50% all specimens examined over the range of 39 years were collected between 2000-2003.  Though some specimens  were collected in the 1960s in natural areas, it became widely “escaped” from cultivation in many natural  areas in the 1990s.
 
You may have noticed that most of our specimen images to date are those collected in Pennsylvania and surrounding states.  That’s because these specimens are being digitized as part of a multi-institutional project, the Mid-Atlantic Megalopolis Project, with the overarching goal to mobilize herbarium specimens across the region  to understand the effects of urbanization on plant life.  However, this project is a step towards digitizing the entire Carnegie Museum Herbarium.  The herbarium is worldwide in scope, and specimens in the Mid-Atlantic region accounting for only 35% of the 540,000 specimens.
 
As more specimens become digitized as part of the Mid-Atlantic Megalopolis Project, we’ll have a more complete understanding of the introduction and spread of Callery pear in the region.
 
Herbarium specimens are collected in the native range too! We’ll be able to compare specimens collected in the invaded range to those in its native China. The use of cultivated specimens and those collected in the native range are underutilized  but can provide critical information.

Escaped from cultivation. This specimen  (above) from Muhlenberg College Herbarium (MCA) was collected along the roadway in Montgomery county, Pennsylvania by Bayard Long in 1962. This specimen is probably the earliest specimen collected in the wild in Pennsylvania.

Many more specimens like this will be brought to light through the Mid-Atlantic Megalopolis Project.
 
Read more about this species introduction history in the popular press, published in the Washington Post  last year, and in an excellent overview by Dr. Theresa Culley (University of Cinncinati) in Arnoldia and another in BioScience.
 
See all the Pyrus calleryana specimens being made available online from the Mid-Atlantic Megalopolis project here: http://midatlanticherbaria.org/portal/collections/list.php?db=277%2C328%2C334%2C329%2C333%2C320%2C330%2C40%2C410%2C316%2C335%2C331%2C332%3B11&includecult=1&taxa=Pyrus+calleryana&usethes=1&taxontype=2
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 Still have any chocolate or roses from Valentine’s day last month?  The herbarium has both, but from nearly 200 years ago. 

Above: ​Specimen of the cacao tree (Theobroma cacao) from Jacob Wolle’s herbarium, collected in Jamaica around 1840.  Some of the herbarium’s oldest specimens come from Jamaica.  Why?  Jacob Wolle was a botanist and the grandfather of William Holland, one of the first directors of the Carnegie Museum (from 1901-1922). Holland was born in Jamaica, where his father was a Moravian missionary. The CM herbarium has 2,514 specimens from Wolle’s collection, dating as far back as 1819.

Below: Wild field rose (Rosa arvensis) collected in England in 1820.