“The trees along this city street,
Save for the traffic and the trains,
Would make a sound as thin and sweet
As trees in country lanes.”
-Edna St. Vincent Millay, “City Trees”
Ecological succession is the change, over time, in the species structure of an ecological community. After a disturbance suddenly and drastically alters an environment, the slate is wiped clean: a new community develops, its constituents suited to the new environment. A progression begins: new species take hold, modify the habitat, and are, in turn, superseded by other species fine-tuned by evolution to exploit the new environment. Early-succession plants, in the wild, are the first to set in after a disturbance: they are the mosses, herbaceous annuals, and hardy perennials that grow up out of nothing or come in on the wind to colonize bare ground.
They are the same species you see in urban ecosystems, marked by frequent and heavy disturbance. The city, with its traffic, concrete, and crowds, makes for a distinctive habitat. Impervious pavement alters water flow. Fragmentation of ecosystems affects biodiversity and population dynamics. Sunlight, soil, and space may be limited. In these conditions, plants that manage to propagate themselves without human help are called weeds—but maybe “warrior” is a more accurate title.
Ailanthus altissima is one such warrior. The “Tree of Heaven” and the central metaphor in A Tree Grows in Brooklyn, is sometimes called “ghetto palm” for its prevalence in vacant lots and rough urban settings. It spreads aggressively by cloning itself at the root; it grows rapidly, even with limited nutrients; it can withstand high concentrations of pollutants, low pH, and high salinity. Ailanthus doesn’t mind the urban heat island effect: in fact, the warmer microclimate of the city is kind to it. And, as early-succession species modify a habitat to make it more receptive to new species, Ailanthus trees alter the ecosystems they invade. Urban vegetation, whether cultivated or spontaneous, sequesters gases and pollutants and decreases the urban heat island effect.
The life of a weed in a sidewalk crack is not the same as the life of its counterpart in a meadow two hundred years ago. Natural selection favors plants adapted to whatever circumstances they face. In urban environments, these circumstances are unique and pressing. Take Crepis sancta as a case study. C. sancta can produce light or heavy fruit. Light fruits are blown by the wind; heavy fruits fall to the ground near their parents. In a fragmented ecosystem where patches of soil are scarce, it takes only a dozen generations for a population of C. sancta to favor the heavy fruit. A fruit that falls quickly to the ground is likely to land in its parent’s dirt patch instead of the street. This adaptive reproductive strategy of urban C. sancta populations helps plants to survive in the short term, but ultimately reduces the gene pool of the isolated population. “As concrete slabs chop up more and more of our urban landscapes, plants are forced to evolve in ways that may not benefit them in the long run.”
Take a careful look around. Spring is coming in the Northeast. As ice melts and salt is washed away from the sidewalks, notice the green things coming up in cracks and ditches. Each one can tell us something about the local ecosystem–and each one can change that ecosystem in myriad ways.
“Oh, little leaves that are so dumb
Against the shrieking city air,
I watch you when the wind has come,—
I know what sound is there.” -Millay
Natasha Balwit is an undergraduate student in the Department of Urban Studies and Planning at MIT.