collected on this day

From wild to cultivated to invasive

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 commonly 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% of 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 account 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.

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

Check back for more! Botanists at the Carnegie Museum of Natural History share digital specimens from the herbarium on dates they were collected. They are in the midst of a three-year project to digitize nearly 190,000 plant specimens collected in the region, making images and other data publicly available online. This effort is part of the Mid-Atlantic Megalopolis Project (mamdigitization.org), a network of thirteen herbaria spanning the densely populated urban corridor from Washington, D.C. to New York City to achieve a greater understanding of our urban areas, including the unique industrial and environmental history of the greater Pittsburgh region. This project is made possible by the National Science Foundation under grant no. 1801022.

Mason Heberling is Assistant Curator of Botany at the Carnegie Museum of Natural History. Museum employees are encouraged to blog about their unique experiences and knowledge gained from working at the museum.

Loved and hated: An urban plant with history

​”There’s a tree that grows in Brooklyn. Some people call it the Tree of Heaven. No matter where its seed falls, it makes a tree which struggles to reach the sky. It grows in boarded up lots and out of neglected rubbish heaps. It grows up out of cellar gratings. It is the only tree that grows out of cement. It grows lushly…survives without sun, water, and seemingly earth. It would be considered beautiful except that there are too many of it.“

-from Betty Smith’s classic novel, A Tree Grows in Brooklyn (1943)

(Fact check: the species is impressively resilient but does indeed require light and water. But we’ll let it slide with an artistic license here.)

This specimen of tree of heaven (Ailanthus altissima) was collected on October 4, 1940 by D. Berkheimer near Klapperthal in Berks county, Pennsylvania.

Tree of heaven (Ailanthus altissima) is a species with a fascinating and complex cultural history.  It was once glorified as a beautiful urban street tree in the United States. It was tolerant of insects, pollution, and poor growing conditions. The earliest recorded planting in the US (via England) was at William Hamilton’s estate (currently the “Woodlands Cemetery”) in Philadelphia around 1784.  It was also in Bartram’s famous garden nearby. It became popular in the plant nursery industry after 1820. But that feeling didn’t stay long, and within decades it was vilified as an unwanted weed – it had an unpleasant smell, produced prolific seeds, and resprouted from suckers causing it to spread.  Today, it is a common urban “weed” found in urban and non-urban areas across the eastern US.  It is considered invasive in Pennsylvania (and many other states). It is also considered invasive in many parts of Europe.

But where’s it from? Ailanthus altissima is native to East Asia, including China, Taiwan, and Korea. It has a deep ethnobotanical history in China, where it has been used in various ways in traditional Chinese medicine, with written records of its use dating to 732 AD! It also has deep roots in Chinese literature and culture. Among other uses, it has also been used in silk production, as it is a food host to a silkworm.

The oldest herbarium specimen from the Eastern US is undated but inferred to date from 1815-1831 from Philadelphia.  It is in the herbarium of the Academy of Natural Sciences of Drexel University (PH).  The second oldest Pennsylvania specimen is dated 1841, also from Philadelphia.

It is around this time of year that the abundance of Tree of Heaven in the Pittsburgh area becomes especially obvious, especially along roadsides. With large, compound leaves and found in disturbed, “weedy” areas, Ailanthus can be easily confused with the similar looking native tree, staghorn sumac (Rhus typhina). Staghorn sumac is in a different plant family (poison ivy family, Anacardiaceae), but this confusion goes way back. Linnaeus even described the species as in the same genus as sumac, so don’t feel too bad if you make the same mistake! (Side note – the taxonomic history of the species is also intense. It has been given many different scientific names over the past three centuries, with three people independently naming it at around mid-1700s!) The leaves are noticeably different upon closer inspection. The fruits are even more  clearly different. Ailanthus has brown clusters of winged seeds (called samaras, like that of maple trees’ “helicopter” seeds). These seeds can be clearly seen now on trees as their leaves drop along most highways around Pittsburgh.

Given its introduction history in the Mid-Atlantic and its intentional planting and affinity for urban areas, the Mid-Atlantic Megalopolis Project will be important and fascinating to understanding more about this plant. The Mid-Atlantic Megalopolis Project is a specimen digitation effort involving more than 12 herbaria (including Carnegie Museum herbarium) funded by a National Science Foundation grant to database (put in computer), image (high res. pictures), and georeference (put on map) all specimens in the region. The region is significant because it is one the oldest densely populated urban corridors in the US, from New York City to Washington, D.C. The Mid-Atlantic Megalopolis Project is producing a critical dataset to look at the introduction history and invasive success (and failure) of species in urban and non-urban areas across this connected region – including Tree of Heaven and many other species.

This specimen image (and many other Tree of Heaven specimens in the region) are available online: http://midatlanticherbaria.org/portal/collections/list.php?db=328&taxa=Ailanthus+altissima&usethes=1&taxontype=2

The earliest specimen in southwestern PA at the museum was collected in 1881 in Beaver county.

For more on the species’ fascinating history and biology in our region, check out this detailed study by Dr. Matt Kasson and colleagues done at Penn State published in 2013 in Northeastern Naturalist.

Check back for more! Botanists at the Carnegie Museum of Natural History share digital specimens from the herbarium on dates they were collected. They are in the midst of a three-year project to digitize nearly 190,000 plant specimens collected in the region, making images and other data publicly available online. This effort is part of the Mid-Atlantic Megalopolis Project (mamdigitization.org), a network of thirteen herbaria spanning the densely populated urban corridor from Washington, D.C. to New York City to achieve a greater understanding of our urban areas, including the unique industrial and environmental history of the greater Pittsburgh region. This project is made possible by the National Science Foundation under grant no. 1801022.

Mason Heberling is Assistant Curator of Botany at the Carnegie Museum of Natural History. Museum employees are encouraged to blog about their unique experiences and knowledge gained from working at the museum.