Gathering Moss: A Natural and Cultural History of Mosses

City Mosses

City Mosses  91 City Mosses  I f you’re a city dweller, you don’t have to go on vacation to see mosses. Sure, they’re much more abundant on a mountaintop or in the falls of your favorite trout stream, but they also live alongside us every day. The city mosses have much in common with their urban human counterparts; they are diverse, adaptable, stress-tolerant, resistant to pollution, and thrive on crowded conditions. They are also well traveled. A city offers mosses a multitude of habitats which may otherwise be quite uncommon in nature. Some moss species are far more abundant in the human-made environment than they are in the wild. Grimmia doesn’t discriminate between a granite crag in the White Mountains and a granite obelisk on Boston Common. Limestone cliffs are not abundant in nature, but there’s one on every Chicago street corner and mosses perch contentedly on its pillars and cornices. Statues provide all kinds of water-holding niches where mosses abound. Next time you walk through the park, look in the folds of the flowing coat of whatever general sits mounted on a pedestal, or in the wavy marble locks of Justice’s hair outside the courthouse. Mosses bathe at the edges of our fountains and trace the letters on our gravestones. Ecologists Doug Larson, Jeremy Lundholm, and colleagues have speculated that the stress-tolerant, weedy species that cohabit our urban spaces may have been with us since our earliest days as a species. In their urban cliff hypothesis, they note a striking number of parallels between the flora and fauna of natural cliff ecosystems and the vertical walls of cities. Many weeds, mice, pigeons, house sparrows, cockroaches, and others are all endemic to cliff and talus-slope ecosystems, so perhaps it is Cushion of Grimmia pulvinata 92  Gathering Moss no surprise that they willingly share our cities. The same can be said for urban mosses, many of which are typical of rock outcrops, whether natural or human-made. We tend to devalue the flora of cities as a depauperate collection of stragglers, arising de novo with the relatively recent development of cities. In fact, the urban cliff hypothesis suggests that the association between humans and these species may be ancient, dating from our pre-Neanderthal days when we both took refuge in cave and cliff dwellings. In creating cities, we have incorporated design elements of the cliff habitat and our companions have followed. Admittedly, city mosses are not the soft feathery mats of forest mosses. The harsh conditions of urban life limit them to small cushions and dense turfs as tough as the places they inhabit. The arid conditions of pavements and window ledges cause mosses to dry out quickly. To defend against drying, the moss shoots pack closely together, so that limited moisture may be shared among shoots and held as long as possible. Ceratodon purpureus makes such dense colonies that when dry they resemble small bricks; when wet, green velvet. You’ll find Ceratodon most commonly in gravelly spots, like at the edge of a parking lot or on a rooftop. I’ve even seen it growing on the rusted metal of old Chevys and abandoned railroad cars. Every year it produces a dense crop of unmistakable purplish sporophytes to send its spores off to the next bare patch. The most ubiquitous of mosses, urban or otherwise, is Bryum argenteum, the Silvery Bryum. I have never traveled without encountering Bryum on my journey. It was on the tarmac in New York City and on the tiled roof outside my window in Quito the next morning. Bryum spores are a constant component of aerial plankton, the cloud of spores and pollen which circulates all around the globe. Shoot and sporophyte of Bryum argenteum City Mosses  93 Leaves of Bryum argenteum You’ve probably walked over millions of Bryum without ever realizing it, for it is the quintessential moss of sidewalk cracks. After a rain, or a hosing down by a sanitation worker, water lingers in the tiny canyon of a fissure in the pavement. Mingling with the nutrients provided by the flotsam of pedestrians, the crack becomes ideal for Silvery Bryum. It takes its name from the burnished silver color of the dry plants. Each tiny round leaf, less than a millimeter long, is fringed with silky white hairs, visible with a magnifying glass. The shiny hairs reflect...