365 urban species. #266: Tree of Heaven

Photos by urbpan. Location: The Fenway, near Peterborough street.

Urban species #266: Tree of heaven Ailanthus altissima

Everyone loves a survivor, right? Not necessarily, at least, not when we're talking about Ailanthus altissima, the Tree of Heaven. Not long after being brought from China as an ornamental plant, Ailanthus (as it's usually called) has become vilified as a weed, an invasive; destructive, useless, and smelly. Ailanthus seemed like a great choice for urban landscaping, initially. It has an attractive, exotic appearance, it tolerates a wide range of temperatures and water conditions, and puts up with the various stresses of urban living better than any creature this side of the cockroach.

Its persistent nature, and tendency to grow in pavement cracks too meager for even smaller weeds, earned a place as the central symbol in the novel A Tree Grows in Brooklyn. Repeatedly cut back to a stump, the Ailanthus tree in the story continues to resprout new growth in its tenement courtyard habitat.

But Ailanthus is not usually seen in such an inspirational light. One grievance against it is the fact that the male flowers of this tree are pollinated by flies; to this end, the flowers produce an odor that is described in various terms, never positively. Like a huge stinkhorn (but with a funk that is more plant-like, less fecal) this tree's dependence on an insect has shaped its relationship to humans. The smell has caused some municipalities to zone this tree out of existence; reportedly, It is illegal to plant an Ailanthus altissima in Washington D.C.

It is also illegal to plant one in Massachusetts, but for a rather different reason. Its remarkable ability to survive, and reproduce, has put it on the list of prohibited plants. Despite the lack of Chinese flies, this tree is perfectly able to reproduce itself, mainly with the help of honeybees, and, presumably, other native and introduced flies. The seeds of the tree are produced in dense and prolific numbers--up to 300,000 from a single tree. The fruit is a samara (a dry winged seed, like maple trees have) that can travel a great distance from the parent tree by wind or by water. The seed needs only to find purchase in the crack between a building and its foundation, or come to rest at the edge of a chain-link fence, in order to germinate. It grows quickly, looking at first like any old weed, but once ignored, shoots up into a sapling, roots swelling. The Ailanthus tree is tolerant of poor soil quality, and even tolerates salt water. It can stand up to air pollution, and the compaction of the soil that holds its roots. At this point the tree is difficult to remove, and often grows back from its stump. In a decade or so, the little weed is decidedly a tree, and may grow to fifty feet tall, or more, in its 50 year life span. Quick-growing woods are brittle, and Ailanthus sheds branches occasionally--an undesirable characteristic for a street tree. There is also some worry about their roots penetrating sewers and other utilities. The roots, wood, leaves, seeds, and fruit of the tree are mildly toxic. The leaf litter, left in place, will inhibit the growth of neighboring plants, through a nifty bit of chemical warfare called allelopathy. Some humans are sensitive to the tree's sap, and its pollen is a potentially serious allergen. Voracious plant eaters like deer and squirrels are not fond of Ailanthus as a food source.

There are some animals that will eat its foliage, notably caterpillars. In some places in contemporary Europe, there has been some effort to raise silkworms on Ailanthus trees. In China, the caterpillar of the cynthia moth has been used with Ailanthus to produce a low grade silk. An attempt was made to introduce this industry, along with the caterpillar and its tree, to the United States. The industry did not succeed, although the Cynthia moth is now established in New York City, and other parts of the Northeast, a region which has no shortage of host trees.

The value of Ailanthus is in its dogged determination to survive. There are places in the city, where no one will plant a tree, where the tree of heaven plants itself. It has become a part of the urban landscape, despite its faults--a perch for starlings, a bit of green in a gray alley, a reminder of life in a place where people forget it exists. For the urban nature lover, Ailanthus is ambiguous, creating life where there is none, and preventing it where it might otherwise arise.

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