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Where were mosses
when concrete was invented by the early Romans? Were their
roads covered with green? Well, we don't know, but in moist
climates moss are common residents on concrete. Most of
these species originally came to our cities from areas with outcrops
of limestone. Concrete, which is made of cement containing limestone
and clay, combined with aggregates such as crushed stone, sand,
and gravel, provides a calcareous substrate attractive to some
mosses.
But only certain
moss  species favor concrete.
Which ones grow on concrete sidewalks in the Pacific Northwest?
We identified all
mosses found at each of five central sites in the city of Corvallis
in western Oregon, latitude 44° 34' N, longitude 123°
15' W, elevation about 61 m. (200 ft.), and annual rainfall 101
cm (40 in.). Study sites included two residential sidewalks,
a small apartment complex (walkways and steps between buildings),
sidewalks at the downtown post office, and the plaza between
Central Park and the Corvallis Arts Center. Representative
mosses were taken to Oregon State University laboratories for
identification to species. Voucher specimens are stored
in the OSU
Herbarium.
For results of our study, click
here.
The sites were chosen
for their  relatively luxuriant growth
of moss. They varied in percent shade, pedestrian traffic,
and associated vegetation. Except for accumulation of soil
in spots, all substrates were concrete, with varying sizes of
aggregates (largest at the apartment complex), and all were solid,
with one exception. At the Corvallis Arts Center, curvy
concrete tiles form an attractive, patterned surface with green
moss growing in the spaces in between. The moss serves
a useful purpose on this sort of walkway or plaza, as it naturally
creates a clean, cool, and aesthetic filler and soil holder between
the tiles. (Photo: Barbula vinealis forms a
reddish green carpet on a little used sidewalk. When moist the
carpet turns a deep green) |