Ecologists have begun to pay close attention to the spontaneous vegetation that appears in disused spaces like the High Line's original railroad beds. Sometimes referred to as "cosmopolitan", these self-seeded plant communities are comprised of unique mixes of native and exotic species. Such plant communities have adapted to thrive alongside humans in urban environments where climate, pollution levels, and soil conditions may differ dramatically from surrounding areas.
As more and more land is developed for human use it is important to learn about the species composition of these plant communities, how they change over time and what ecological services they offer. In the High Line's Interim Walkway, which is only two blocks long, nearly one hundred species have found their niches over the last several decades.
Though they have evolved in different parts of the world, many of the native and exotic plants that succeed here come from similarly harsh environments. Some exotics, like foxtail grass (
Setaria pumila), grow almost exclusively in disturbed sites and pose little threat to healthy North American ecosystems. Dozens of bird species feed on foxtail grass seeds, making it a valuable plant in the urban landscape. On the High Line, foxtail grass and prairie three-awn (Aristida oligantha), a North American native which prefers dry, barren soils have helped lay the groundwork for herbaceous perennials. Each year as these annuals die, they provide organic matter that builds the soil. Pioneer species like these grasses alter site conditions, creating new niches for other plants. This process, called succession, is in full swing on the High Line. Pioneering trees like eastern red cedar (Juniperus virginiana) and black cherry (Prunus serotina) create shade. Their roots break up compacted soils and support beneficial soil organisms.
Human activity often directs species composition. In the photograph below, you will notice a stark difference between the tracks and the bare gravel ballast. Small differences in site conditions translate into major differences in vegetation. For example, water running off the visitor's path has helped vegetation flourish along the path's edge. Grasses and perennials take advantage of water captured between the railroad ties, especially where the wood has rotted and become more porous. However, only a few species have been able to take hold on the gravel slope where water quickly flows over without impediment.
Cosmopolitan plant communities have the capacity to enrich soil, reduce water runoff, cool and clean the air, and provide food and habitat for animals. However, many of the characteristics of exotic species which help them thrive in tough places like vacant lots, roadsides, and railroad beds also enable them to invade healthy ecosystems and out-compete native plants. Invasive species employ a range of strategies to expand territory including rapid growth, prolific seed production, allelopathy, and vegetative reproduction. At the moment, the ratio of natives to exotics along the Interim Walkway is about 1:1. By observing how these plants interact and spread over time, we can begin to evaluate whether some level of management is necessary.
Much has already been written about how the original self-seeded landscape inspired the planting design for the High Line, but in the next post we'll delve into what plant communities, both cosmopolitan and native, can teach us about garden maintenance.
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