Abstract
The two most successful “pioneer” tree species in an abandoned urban brickyard were cottonwood (Populus deltoidesļ and ailanthus (Ailanthus altissima). Woody plants of 1 6 genera were found in the area 10 years after abandonment and the presence or absence of species is discussed in relation to current planting practices.
What happens when a parcel of urban land is abandoned? Plants begin to invade the area and, if left unchecked, can convert the area into a forest. However, we seldom are permitted to observe the end result of plant invasion, since the period of non-use is usually short and another building or parking lot has soon replaced whatever structures has previously occupied the site.
Long before “ecology” became a much-used (and little-understood) word in our everyday lexicon, foresters and botanists were formulating theories as to why plants grow where they do.’ One of the more intriguing aspects of such research was the change in composition of plant communities on the same site with the passage of time. The universal process of formation development was termed “succession,” and the final stage was a “climax” formation.
Clements (F.E. Clements. 1928. Plant succession and indicators. H.W. Wilson Co., New York, 453 p.) listed 6 stages in the development of a climax formation. The first was “nudation”; the destruction of existing vegetation. The second stage was “migration” of plants to the denuded site. Successful migration depended on seed production and mobility. The adjustment of these plants to their new location was called “ecesis,” and this stage was followed by that of “competition” between plants. The plants of early migrations caused a change in the habitat, and this phase was called the “reaction” stage. The “stabilization” stage involved the replacement of colonizing plants with the least site demands by those with greater demands, leading to a “climax” formation. Obviously these stages do not constitute a straight-line progression, and successive migrations and competitions may be necessary to achieve stabilization.
With this brief introduction, it is of interest to study what has happened to a rather unique urban industrial site 10 years after abandonment, and what lessons on this site may provide for urban tree planters.
The Brickyard Site
At the time of the establishment of the U.S. National Arboretum in Washington, D.C. in 1927, there was considerable industrial activity adjacent to the Arboretum property along New York Avenue. Since 1909, the 30-acre site had been utilized for the manufacture of bricks and had passed through several ownerships and modes of brickmaking. A survey made in 1931 listed the presence of 9 “beehive” brick kilns built between 1927 and 1931. One additional kiln was constructed between 1931 and 1939, and 2 more kilns were built probably between 1939 and 1942. At present there are 12 kilns on the site. The brickyard operation ceased in January 1972 and the U.S. Department of Agriculture purchased the land for the U.S. National Arboretum in 1976.
The kilns are mąde of brick and are circular in shape, varying in inside diameter from 40 to 50 feet. The walls (25 to 41 inches in thickness) are about 8 feet high and surmounted by a circular or parobolic brick “crown.” There is also a flattened area (“shoulder”) around the crown which corresponds roughly to the wall thickness. When in use, most of the kiln crowns were apparently covered with tar.
In the 10 years since the abandonment of the brick factory, the intense heat generated by the brick making process has ceased and the kilns have been subject to natural weathering. The tar coverings have virtually disappeared and the mortar between the bricks has loosened. Cracks and crevices have trapped sufficient atmospheric dust and debris to harbor a kind of “soil” and plants have exploited these niches.
In 1982, we made a survey of the woody plants growing on or around these brick kilns (Fig. 1) to see whether such observations might provide any clues to the successes and failures of urban tree plantings. Surveys of plants on the “crowns” and “shoulders” of the kilns were made from an aerial bucket, since some of the kilns were judged unsafe to walk on. There was no attempt to measure size or determine the age of the various shrubs and trees or to determine the exact species in most cases. The data for numbers of plants recorded in each of the 3 kiln zones are given in Table 1.
The “crown” of an abandoned brick kiln showing (from right); upper portion of ailanthus growing from base, and black locust and cottonwood growing on “shoulder” and “crown.”
Numbers of plants of various genera found growing on or at the base of abandoned brick kilns — 1982.
Plant Migrants
It is likely that the first plant migrants were annual herbs and grasses, and this was the only type of vegetation that was found on the crown of one kiln, where the crown covering had been of concrete, rather than tar, and the weathering process had been retarded. The growth, death, and decomposition of these herbs serve to build up sufficient “soil” for the subsequent establishment of perennial woody plants.
Successful migration and establishment of woody plants depends on seed size and an efficient system of seed dissemination. Thus it is not surprising that the majority of the woody plant genera found on or near the kilns produce small seeds that are disseminated by wind. Populus, Salix, Platanus, and Betula should be the most efficient of these, followed by Acer, Ailanthus, Catalpa, and Ulmus. The seed of Robinia are not particularly well adapted to wind dissemination, but the dry, seed-bearing pods can be carried considerable distances by wind.
Seeds of Celtis, Crataegus, Juniperus, Morus, and Rhus are probably disseminated by birds. Seed distribution of Fagus and Quercus could be by any combination of wind, water, gravity and animals.
Now that we have at least considered the mode of seed dissemination of all of the genera found on the kiln site, let us turn again to Table 1. There were, quite obviously, two highly successful species, and these have become dominant in quite different sites. The hot, dry crowns of the kilns are almost a pure stand of poplar, mainly Populus deltoides Bartram ex Marshall, the eastern cottonwood. The cool, moist areas around the bases of the kilns are largely occupied by the Chinese tree-of-heaven (Ailanthus altissima (Miller) Swingle).
How is the eastern cottonwood, normally a denizen of flood plains or other well-watered sites, performing on the brick kilns? Not too well, but it is surviving. Most of the cottonwoods on the kiln crowns are dwarfed and contorted and only a few of the trees on the shoulder even approximate the upright vigorous growth expected from this species. However, the fact that some trees of cottonwood are found in all 3 zones indicates that this species possesses a wide range of adaptability. The same can be said of Ailanthus which, although it also occurs in all 3 zones, is doing best where moisture conditions are better. Ailanthus is well known as an urban “pioneer,” but we seldom encounter cottonwood as an urban “weed tree.” The answer may be the inability of cottonwood to compete with other species on sites which, although they may appear to be severe, can actually support species with higher site demands. The fact is that poplars can be successful urban trees and planted cottonwoods once lined Constitution Avenue in Washington, D.C.
Other bottomland trees that have taken hold in the more severe kiln locations are the willows (Salix sppj and Platanus, probably derivatives of so-called “London” plane. Black locust (Robinia pseudoacacia L.) and Catalpa species have also exploited all 3 kiln zones, and these have also been previously noted as urban weeds. Among other common urban invaders, the elms (both American and Siberian) and mulberries (Morus spp.) were only found at the bases of the kilns.
As mentioned earlier, there was a paucity of species on the kilns from bird-disseminated seed. The case of Celtis deserves special mention. All three Celtis seedlings in the kiln area are C. sinensis Persoon, a Chinese species represented by only 3 planted trees on the Arboretum grounds a few hundred yards from the kilns.
Why was there not a greater representation of other bird-disseminated species that are growing in great numbers in close proximity to the brickyard site? Where are the Callery pears (Pyrus calleryana Decaisne), flowering dogwoods (Cornus florida L. ), hollies (Ilex spp.) and cherries (Prunus spp.), which include not only the Japanese flowering cherries but our ubiquitous native choke cherry (P. virginiana L.). These are all major items in urban landscape planting.
Among the wind-disseminated species, where are the ashes (Fraxinus sppj, the locally native and abundant tuliptrees (Liriodendron tulipifera L.) and sweetgums (Liquidamber styraciflua L.), and the maples (Acer spp.), presently represented by only 1 struggling seedling of boxelder (Acer negrundo L.)? Likewise, no seedlings of linden (Tilia spp.) crape myrtle (Lagerstroemia indica L.) or Paulownia tomentosa Thunberg (Steudel) were found on or around the kilns. The absence of another very popular urban tree (honeylocust Gleditsia triacanthos L.) can be explained by the current usage of seedless cultivars of that species.
If plant succession proceeds from species which are less site-demanding to those which are more site-demanding, it is obvious that we are not currently using many of these early migrant pioneers in planting the urban forest. Short life span, disease susceptibility, cultural problems, or lack of “esthetics” may be reasons to preclude the widespread use of many of these species. But urban trees have a tenuous survival at any rate, and increased planting of currently favored species will undoubtedly reveal pest and cultural problems similar to some of the pioneer species.
Urban tree planting in the United States is thus based largely on trees with high site demands. The planting of small trees in big holes, and some after care, will assure that these demands are met — for a short time. With increased tree growth and increased site demands — beyond those that the site can satisfy — our favored trees may only live as long as those less-favored pioneers.
Footnotes
↵1 Presented at the annual conference of the International Society of Arboriculture in Indianapolis, Indiana in August, 1 983.
- © 1983, International Society of Arboriculture. All rights reserved.
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