Good Intentions - Bad Tree

Although they have never been on my A-list of trees, I must admit that the local display from the ornamental pear trees (Pyrus calleryana) has been remarkable this year. The flowers have been blooming now for close to three weeks and have managed to escape the blossom browning freezes that mar them in some years. They have also not been subjected to sustained spells of early heat that can cause the flowers to fall too early and the leaves to appear too soon. They are so stunning this year that you can't help but notice the sheer number of them that have been planted. From corporate office parks, to school yards, to street side plantings, to subsidised housing projects; ornamental pears trees are everywhere. With the exception of crape myrtle (Lagerstroemia), a local favorite - P. calleryana would be the most widely planted ornamental tree. Another thing made obvious by this year's display of blooms - just how many pears seem to have escaped the bounds of cultivation. There are more than a few places here where Pyrus is becoming the dominant tree species. While these wild stands are very pretty right now, the showy blossoms hide an ugly truth. When native ecosystems are altered by invasive species, plants and the animals that evolved with them are pushed out. The story of how this species has gotten to the point of invasiveness reminds me of Dr. Seuss's The Cat in the Hat Comes Back, where each effort to solve the problem of the pink cake stain only creates another pink problem.
Pyrus calleryana is native to China, Japan and Korea, and was brought to this country in the second decade of the 1900's. Researchers were hoping that its resistance to fire blight could benefit the edible European pear (Pyrus communis), which at the time was being decimated by the bacteria. Later in the 1950's it was noticed that this species could make a contribution to the horticultural as well as the agricultural world, and one particularly cultivar stood out - 'Bradford'. Like most P. calleryana, 'Bradford' exhibited disease and insect resistance, it flowered profusely starting at a young age, had a rapid growth rate, clean dark green foliage and had amazing late fall color. The overall shape of the tree was very tight and symmetrical and individual specimens showed a great degree of similarity with each other. It was also tolerant of pollution, heat, drought and the occasional flood. Hardy from zones 5 to 8, it was the perfect street, parking lot and residential tree for a good part of the nation. A landscape architect's dream come true, and if it never existed it would have to have been invented. They were planted everywhere by the tens of thousands.
There was one not so little problem with the perfect tree. As it aged those tight dense branches that grow at a very narrow angle were weak. Entire sections of the tree would collapse to the ground in wind and ice, ruining the shape which no pruning could ever restore. As this problem became apparent, fewer 'Bradford' were planted, but more of the varieties with a stronger branching structure were. 'Aristocrat' and 'Chanticleer' (aka 'Cleveland Select') became the pears of choice. When the streets were full of 'Bradford' pears there was little problem with them being invasive because they are self infertile. Now that these new varieties are being planted, often near older stands of 'Bradford' all of the cultivars are mingling with each other producing fertile fruit. These are being eaten and disseminated by birds, and now pears are coming up everywhere. These new pears seem particularly prone to growing in disturbed soil where most other vegetation has been removed. The same qualities that make the pear such a widely durable street tree, also make it uniquely suited to adapting to life outside of cultivation. As if all of this was not disturbing enough, now thorns are appearing on the trees after not being seen on the cultivated varieties. Perhaps this will provide more secure shelter for their friends the birds and possibly keep the deer and the groundskeepers away.
Should anything be done with species, like the ornamental pear or the Canada goose whose DNA makes it possible for them to thrive in the ecological opportunities humans (perhaps nature's most adaptable species) have inadvertently created for them?

For more detailed, scholarly information about Pyrus calleryana and its invasiveness you can read:
The Beginning of a New Invasive Plant
by Theresa M. Culley and Nicole A. Hardiman
or
On the Spread and Current Distribution of Pyrus calleryana in the United States
by Michael A. Vincent

For different perspectives on different trees you can visit:
Festival of the Trees - a monthly blog carnival for all things arboreal, which will be hosted next month at The Marvelous in Nature.