Signs of Fall 9: Tree of Heaven

Photo by D. Sillman

In Betty Smith’s novel, “A Tree Grows in Brooklyn,” the sight of a Tree of Heaven pushing its way up through the endless concrete and brick of the Williamsburg tenements is an inspiring metaphor for the robustness and tenacity of life. If a tree can survive all of the vicissitudes of that harsh, urban existence, then so can the protagonists of the novel, and, maybe, so can we all. This is a wonderful book, but I wish that Smith had used a different tree for her focus: maybe a red maple, or a honey locust or a London plane tree. Choosing a Tree of Heaven ennobles an invasive species that is inflicting great damage on our native forests!

Tree of Heaven (Alianthus altissima) has  number of apt, but less exultant names: “stinking sumac” or more simply “stink tree” emphasize the abundant, odiferous chemicals that it emits from its roots, leaves and bark. “Ghetto palm” or “tree of hell” emphasizes its ability to thrive in polluted, stressful environments. Tree of Heaven is native to northern and central China and Taiwan, and the Chinese name for this tree (“Chochun”) translates to into “foul smelling tree.” The name Tree of Heaven was coined because of the species’ very fast vertical growth rates (it “quickly reaches for the heavens!”). Initially, it was touted as a rapidly growing shade tree that could add great quality to a garden or street-side tree lawn.

Tree of Heaven was first introduced to North America in 1784 by a Philadelphia gardener and landscaper named William Hamilton (no relation!) who advocated its planting as a shade and ornamental tree. A second wave of Tree of Heaven introduction occurred in 1820 in Flushing, Long Island.  A large, local tree nursery promoted the species emphasizing its exotic nature and its rapid growth rate. A third introduction occurred in California as Chinese immigrants brought Tree of Heaven with them as both a reminder of their homeland and also as source of traditional, medicinal chemicals. Very quickly, though, the invasive and destructive properties of this species were recognized and active planting and propagation were discontinued.

Free use photo, Pixabay

Once established in an area, Tree of Heaven will both persist and spread. A paper published by Penn State researchers this past August in the journal Forests detailed some aspects of the species’ reproductive potential. A female Tree of Heaven can make up to 325, 000 winged seeds in a single season, and these trees begin to make seeds at a very young age. By age 40 a female Tree of Heaven will have made 10 million seeds! By age 100 (and thankfully most Tree of Heaven don’t live that long) a female Tree of Heaven will have made 52 million seeds! The good news is that these seeds do not persist very long in their seed beds (they are viable for only a year or two), the bad news is that their winged samara can carry them great distances, and they have a very high rate of germination. The seeds from 40 year old trees had 78% germination rates while seeds from 100 year old trees had 66% germination rates. The researchers compared this to the seeds of tulip poplars which have a germination rate of only 9%! Many of the dispersed Tree of Heaven seeds, then, will generate new seedlings!

The Tree of Heaven grows most rapidly in full sun (a vertical growth rate of 3 to 6 feet per year in each of the tree’s first four years of life is typical), but it is able to tolerate shady, forest floor conditions. These are trees, then, that especially fill in the sun gaps and the edges of a forest. Environmental factors that cause the deaths of mature trees (like gypsy moth outbreaks killing oaks, ash borers killing ash trees, and wooly adelgids killing hemlocks, etc.) will accelerate the invasion of Tree of Heaven. Disturbance to an established forest will also facilitate Tree of Heaven invasion. The tree’s ability to grow even in very harsh conditions or in nutrient-poor soils also potentiates its invasive dispersal.

Photo by D. Sillman

Tree of Heaven is also able to propagate itself vegetatively via sprouting from root suckers. An established tree can send up new shoots up to 50 feet away and is, thus, able to quickly fill in a very wide area. Tree of Heaven also produces allelopathic chemicals that powerfully inhibit the growth of potential competitors. An established tree and its clones or offspring alter the fundamental flora in their understory and inhibit the growth of longer-lived, shade producing trees that might allow a forest succession sequence that could exclude it.

Finally, Tree of Heaven is not readily eaten by white-tailed deer. The abundance of noxious chemicals that it synthesizes makes its leaves and shoots unpalatable to most herbivores. Deer, as we have talked about many times before, are a major sculpting force in our developing Eastern forests. By consuming tree seedlings other than Tree of Heaven, the white-tailed deer are opening more and more of our forests to the establishment and perpetuation of this invasive tree.

It is very difficult to get rid of Tree of Heaven! Cutting mature trees leads to stump and root sprouts. Spraying trees with herbicide stimulates vigorous root sprouting. Regular mowing of areas with sprouts and seedlings is an effective way to extirpate the species, but this is not always possible in forested habitats. Planting other, fast-growing trees to generate a shady environment can be a good, long-term plan to eliminate Tree of Heaven, but any edges or sun gaps or spaces in between the shade trees will undoubtedly fill in with the extremely fast growing alien invader.

 

 

 

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One Response to Signs of Fall 9: Tree of Heaven

  1. Patrick K says:

    Sadly we have a large colony of these not so heavenly monsters growing at edge of parking lot near Rachel Carson Shelter ironically, thought to have been planted by the County when Park was first constructed. Your blog makes me ponder why County keeps forcing new trees on us in the park rather than trying to irridicate the trees of heaven that we already have, difficult if not possible as that may be.

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