Our Apollo house. 1989. Photo by D. Sillman
Click on link to listen to an audio version of this blog ….Trees in my yard part 3
(Continuing the walk through the trees in my old home in Pennsylvania)
The southern boundary of our field was the narrow woodlot that shielded the view of the field from our street. The woodlot was hodge-podge of fast growing trees that were in an almost constant state of growth and decline. Fallen limbs and fallen trees littered the woodlot and punctuated the ongoing process of intense competition among the closely planted tree species. There were black cherry (Prunus serotina), yellow birch (Betula alleghaniensis), European white birch (B. pendula), red maple (Acer rubrum) , bitternut hickory (Carya cordiformis), shagbark hickory (C. ovata) and box elder (Acer negundo). Most of these trees were incredibly common, locally abundant opportunistic invaders that rushed into this space when it was abandoned to its own successional devices. The European white birch (obviously an exotic species) probably came from a tree planted in someone’s nearby landscaped yard. It, like most European white birches, did not live very long but remained with its distinctive gray bark, standing dead in the center of woodlot for more than a decade.
Red pine
There were four trees, though, that must have been intentionally planted into the space of this woodlot. The other trees listed above, then grew in around them. These four trees were red pines (Pinus resinosa). Red pine is a native, North American species whose natural range is up around , and especially to north of, the Great Lakes. Red pines grow especially well on sandy, well-drained soils and can live for five centuries and reach heights of almost 150 feet with trunk diameters of more that three feet. The four red pines in our street-side woodlot were tall (well over 60 feet), skinny and straight, with no branches until they reached their dense, top knotted masses of needle-covered limbs.
Red pines have been planted as windbreaks and ornamental trees all around the world, and they grow quite readily in almost any type of climate zone or habitat. They often do not, however, continue to grow and thrive if the soils in which they are growing are too damp.
A closely related tree to the red pine is the Austrian (or “European black”) pine (Pinus nigra). These two species are quite difficult to tell apart except for the flexibility of their needles (Austrian pine needles bend without breaking, red pine needles are more brittle) and for a small bump (“prickle”) which is found on the scales of an Austrian pine’s cones (but not on the cone scales of the red pine). The Austrian pine, like the red pine, have been extensively planted around the world especially as an ornamental, landscaping tree and has similar limitations as the red pine especially when planted in soils that stay moist for long portions of the year.
Photo by D. Sillman
Since 1983 I had been watching an extensive planting of Austrian pines on the campus of Penn State New Kensington. I thought for many years, though, that they were red pines. One of my students finally noticed the scale prickle on one of pine cones and made the definitive identification! Clusters of these pines had been planted around the campus back in the 1970’s, and a large area (three or four acres) was also planted as a pine wood lot on the north side of the campus where I eventually built my nature trail. The first third of that nature trail wound through the pine monoculture forest. The densely planted pines heavily shaded the forest floor and that shading, along with a thick accumulation of mulching pine needles on the soil surface, prevented any significant undergrowth of shrubs, flowering plants or other trees.
By the 1990’s, though, the campus’ black pines had begun to fail. Needles yellowed and fell, standing dead trees were blown over by windstorms, and slowly the pine forest changed into a young poplar and ash forest. Increased light coming into the forest floor changed the distribution of wildflowers and shrubs, and by the early 2000’s this part of the trail was an entirely different place!
The isolated clusters of the Austrian pines around campus also failed and were quickly cut down by the maintenance crews. The little wood lots that had provided shade and shelter for many birds and small mammals were transformed into more and more uninteresting and quite ecologically sterile, mowed-grass lawns.
The same process that was killing the campus pines was also affecting the red pines in my street-side woodlot. Their top branch clusters dropped their needles and very quickly the pines were transformed into tall, standing dead trees. I had the two pines closest to the street removed so that they would not fall on the utility wires or , possibly come down and block the street. The other two pines were well tucked back into the surrounding “Allegheny hardwood mix” and would do harm to anyone if they fell, so I elected to leave them be. They were still standing, by the way, when we moved out of the Apollo house in 2020.
Tree of heaven. Photo by D. Markham, Flickr
A fairly recent addition to our mixed hardwood street-side woodlot was the invasion of the site by some exotic, invasive trees. I thought at first these new trees were simply sumacs inching their way into the woody community but looking more closely at these newcomers, I realized that were not sumacs. Instead they were an extremely aggressive invasive species that had the incredibly inappropriate common name, “tree-of-heaven.” This plant (from hell!) is Alianthus altissima.
Tree-of-heaven has number of apt, but less exultant names: “stinking sumac” or more simply “stink tree” emphasizes 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 even the Chinese name for this tree (“Chochun”) translate 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!”), and 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 seed mass, Pixabay
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. In 1820 a second wave of tree-of-heaven introduction was sponsored by a tree nursery in Flushing, Long Island with an emphasis on the exotic nature of the tree and its rapid growth rates. 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.
Once established in an area, tree-of-heaven will persist and spread. A female tree-of-heaven makes up to 325,000 winged seeds in a single season, and the trees begin to make seeds at a very young age. By age 40 a female tree-of-heaven will have made 10 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. Many of the dispersed tree- of-heaven seeds, then, will generate new seedlings!
Tree of heaven. Photo by G. Rex, Flickr
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 at least poorly tolerate shady, forest floor conditions. It can also grow in soils that are very low in nutrients. They are trees, then, that fill in the sun gaps and the edges and poorly vegetated margins of a forest. Factors that cause the deaths of large, mature trees in a forest (gypsy moth outbreaks killing oaks, ash borers killing ash trees, wooly adelgids killing hemlocks, etc.) will accelerate the invasion of tree-of-heaven. Disturbance to an established forest or woodlot will also facilitate tree-of-heaven invasion. The competitive chaos among the trees growing in our street-side woodlot undoubtedly has facilitated the invasion of the site by tree-of-heaven.
Tree-of-heaven is also able to propagate itself 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.
Tree-of-heaven is also the plant on which the invasive insect, the spotted lanternfly reproduces! The recent, rapid spread of this extremely destructive insect across Pennsylvania has been due in large part to the abundance of tree-of-heaven all across the state!
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 deer. Deer, as we have talked about many times before, are a major sculpting force in our Eastern forests. By consuming other tree seedlings, the white-tailed deer are opening more and more of our forests to the establishment and perpetuation of the invasive tree-of -heaven.
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 this extremely fast growing alien invader. Our street-side woodlot with its sunlit edges and constant wind-pruning of limbs and wind-throw of established trees was an ideal habitat for the invasion of this exotic tree.
(Continued next week!)