Harvard biologist comes to the defense of the much reviled tree of heaven
Monday, June 14, 2010
The misnamed tree of heaven has been outlawed in the District for more than a century. No one has planted it in decades. Still it grows. Alongside railroad tracks, in the cracked blacktops of abandoned lots, in dark, stagnant alleys.
With its capacity to spring up seemingly anywhere without water or barely any soil, Ailanthus altissima is considered a bullying weed that upsets the ecological order. Peter Del Tredici sees it differently.
The Harvard biologist regards this fast-growing beast (five feet a year) not as a thug but as a savior of the most forlorn pockets of Washington and other East Coast cities. "It's actually a pretty significant plant in terms of the services it's providing: shade, fixing carbon and producing oxygen," he said. Nor is it alone; dozens of outcast species grow where no others will in the vegetatively hostile precincts of urban America. They are not, in Del Tredici's mind, weeds; they are wild, not invasive but "spontaneous."
"The fact they are growing, they are making a contribution," he said. Del Tredici, who has spent six years writing a field guide he calls "Wild Urban Plants of the Northeast," finds himself squarely at odds with native-plant fans who see ailanthus, bittersweet and other expansive plants as alien species that have invaded, choked and degraded natural areas such as Rock Creek Park.
Del Tredici accepts that some are brutes, but to see them generally as ecologically harmful and invasive aliens is a view driven by philosophy, not biology.
These plants are our camp followers. "They have been following people around for hundreds, if not thousands, of years, adapting to what people do to the environment, Del Tredici said. "So just labeling them invasive is a very narrow view of it."
Efforts by conservationists to remove exotics "is a form of gardening," he said. "Restoration requires ongoing maintenance; it's not just ripping out invasives, planting natives and walking away. We know now that that just doesn't work."
But he is taken not so much by the capacity of these plants to muscle in at the expense of others as by their ability to survive and flourish in the most forsaken places: cracks in asphalt, gutters, mortared joints, and compacted, poor soils.
They are "preadapted to the urban environment," he said, with such survival mechanisms as tap roots, low nutrient needs and an ability to sprout from severed roots. Some are coastal plants that tolerate the road salt that has been building up in cities over the past 70 years. These include the common reed that is a dominant plant of the New Jersey Meadowlands. The reed, Phragmites australis, enjoys the road salt and the tidal buffering provided by the New Jersey Turnpike. Other wasteland flora find in the concrete jungle an echo of the soil chemistry and site challenges of their origins in limestone cliff habitats.
Del Tredici, and his landscape architecture students at the Harvard Graduate School of Design, studied land use in the city of Somerville, next to Cambridge, Mass., and found that "spontaneous vegetation" dominated almost 10 percent of the city. This is typical of many cities in the United States but can rise to as much as 40 percent in an economically depressed city, such as Detroit.
Most of these plants are from other continents, arriving accidentally or by design during the past four centuries through human commerce or migration. Of the perennials and grasses examined in his field guide, 59 percent came from Europe and 5 percent from eastern Asia. Among the woody plants, about half are from North America, 32 percent from Asia and 17 percent from Europe. The tree of heaven was introduced in 1784 from China and was valued for its rapid growth in poor soil in an age when trees provided vital shade in summer to people and livestock. "It was a hot plant in the 1840s, people started saying bad things about it in the 1870s, and nobody liked it by the end of the 19th century," he said.
Other plants also are "totally tied up with our culture," he said, even reviled weeds like dandelions, once considered a decorative herb, and noxious grasses such as crabgrass and quackgrass, used to fuel the engines of commerce -- horses and mules.
Today, these plants are merely filling the vacuum of the environmental degradation of city construction and constant rebuilding, and they will become more dominant with climate change, pollution and growth in urban populations, Del Tredici said.
"Urbanization, globalization and climate change have worked together to produce this ecology," he said.
In some cases, tree-choking vines like the bittersweet "are doing terrible harm, but it's a plant that's adapted to the mess we have made of the planet."
Bittersweet, tree of heaven and dandelion are fundamentally "symptoms of degradation, not the causes of it," Del Tredici said.