This article was originally featured on The Conversation.
When people think of spring, they often imagine flowers and trees blooming. And if you live in the US Northeast, Midwest, or South, you’ve probably seen a medium-sized tree with long branches covered in small white flowers – the Callery pear (Pyrus calleryana).
For decades, the Callery pear—which comes in many varieties, including “Bradford,” “Aristocrat” and “Cleveland Select” pears—has been among the most popular trees in the U.S. for ornamental plantings. Today, however, it is widely recognized as an invasive species. Land managers and plant ecologists like myself work to eradicate it to preserve biodiversity in natural habitats.
As of 2023, it is illegal to sell, plant, or grow a Callery pear in Ohio. Similar bans will take effect in South Carolina and Pennsylvania in 2024. North Carolina and Missouri will give residents free native trees if they cut Callery pears on their property.
How did this tree, once in high demand, get named the US Forest Service’s “Weed of the Week”? The devil is in the biological details.
A nearly perfect tree
Botanists brought the Callery pear to the US from Asia in the early 1900s. They intentionally bred the horticultural variety to enhance its ornamental qualities. In doing so, they created an arboricultural wunderkind. As the New York Times observed in 1964:
“Few trees possess all the desirable characteristics, but the Bradford ornamental pear comes unusually close to the ideal.”
Modern Callery pear cultivars produce a burst of white flowers in spring, followed by deep green summer foliage that turns deep red and brown in fall. They are also very hardy in urban soils, which can be very compact and difficult for roots to penetrate. The trees grow quickly and have a rounded shape, which made them suitable for planting in rows along streets and roads.
During the boom in suburban development after World War II, Callery pears became extremely popular in homes. In 2005 the Society of Municipal Arborists named the variety ‘Chanticleer’ the urban street tree of the year. But the breeding process that created this and other Callery pear varieties produced unexpected results.
Cloning to produce an American prototype
To ensure that each Callery pear had bright blooms, red foliage and other desirable characteristics, gardeners created identical clones through a process known as grafting: creating seedlings from cuttings of trees with the desired characteristics.
This approach eliminated the messy complexity of mixing genes during sexual reproduction and ensured that when each tree matured, it would have the traits desired by the homeowners. Each tree of a particular variety was a genetically identical clone.
Vaccination also meant that the pear trees could not set fruit. Some fruit trees, such as peaches and tart cherries, can pollinate their flowers with their own pollen. In contrast, the Callery pear is self-incompatible: pollen on a single tree cannot fertilize the flowers on that tree. And since all Callery pears of a particular variety planted in a neighborhood would be identical clones, they would essentially be the same tree.
If a tree cannot produce fruit, it cannot disperse into natural habitats. Gardeners and landscapers thought it was perfectly safe to plant the Callery pear near natural habitats such as grasslands because the species was locked into place by its reproductive biology. But the tree would break free from its isolation and spread seeds everywhere.
THE GREAT ESCAPE
University of Cincinnati botanist Theresa Culley and her colleagues discovered that as gardeners tinkered with Callery pears to produce new versions, they made the individuals different enough to escape the fertilization barrier. If a neighborhood only had ‘Bradford’ pears, then no fruit could be produced – but once someone added an ‘Aristocrat’ pear tree to their yard, then those two varieties could cross pollinate each other and produce fruit.
When Callery pear trees in gardens and parks began to set seed in nearby areas, wild populations of the trees were created. These wild trees could pollinate each other, as well as the trees in the neighborhood.
In today’s landscape, the Callery pear is surprisingly prolific. The prolific flowering that gardeners deliberately bred into these varieties now yields huge crops of pears every year. Although these small pears are generally not edible by humans, birds feed on the fruit, then fly away and excrete the seeds in natural habitats. The callery pear has become one of the most problematic invasive species in the eastern United States.
A thorny problem
Like other invaders, Callery pears crowd out native species. Once Callery pear seedlings spread from the edges of the habitat into grasslands, they have advantages that allow them to dominate the area.
In my research lab, we discovered that the Callery pear comes out very early in the spring and drops its leaves in the late fall. This allows it to absorb more sun than native species. We also discovered that during the invasion, these trees alter the soil and release chemicals that suppress the growth of native plants.
The calleri pear is particularly resistant to natural disturbances. In fact, when my graduate student Meg Maloney tried to kill the trees using prescribed fires or applying liquid nitrogen directly to the stumps after the trees were cut, her efforts failed. Instead, the trees grew aggressively and seemingly gained strength.
Once the Callery pear has escaped into natural areas, its seedlings produce very sharp, stiff thorns that can puncture shoes or even tires. This makes the trees a threat to people working in the area as well as native plants. Another annoying factor is that when Callery pears bloom, they produce a strong odor that many people find unpleasant.
Currently, direct application of herbicides is the only known control for a Callery pear invasion. But the trees are so successful at spreading that poisoning their seedlings may simply make room for other Callery pear seedlings to establish. It is unclear how habitat managers can escape a convoluted ecological cycle of invasion, herbicide application, and reinvasion.
Banned but not gone
In response to the work of the Ohio Invasive Plant Council and other experts, Ohio has taken the extraordinary step of banning the Callery pear to prevent its ecological invasion of natural habitats. But the trees are common in residential areas throughout the state and have established vigorous populations in natural habitats. Conservationists will work well into the future to maintain openness and biodiversity in areas where the Callery pear invades.
In the meantime, homeowners can help. Gardeners recommend that people who have a Callery pear on their property remove it and replace it with something that is not an invasive species. Few trees possess all the desirable characteristics, but many native trees have visually appealing characteristics and do not threaten the ecosystems in your area.
This article is republished from The Conversation under a Creative Commons license. Read the original article.