Orthotrichum is a cosmopolitan genus of small acrocarpous mosses in the family Orthotrichaceae, order Orthotrichales, class Bryopsida (true mosses, division Bryophyta). With approximately 125 recognised species it is one of the larger moss genera and is found on every continent, colonising the bark of trees, exposed rock surfaces, walls, and occasionally soil.
Plants form compact cushions or tufts, typically only a few millimetres to around a centimetre tall. The leaves are lanceolate to oblong-lanceolate, often crisped when dry and erect-spreading when moist — a characteristic shared with many members of Orthotrichaceae. Sporophytes are erect and partially immersed in the perichaetial leaves, with distinctive ribbed or striate capsules; the peristome structure — with its elaborate pattern of teeth and cilia — is a key diagnostic character used to distinguish species within the genus.
Many species are epiphytic, growing on the bark of broad-leaved and coniferous trees in temperate and boreal regions, and are sensitive indicators of air quality: their abundance and diversity have long been used in biomonitoring studies of atmospheric sulfur dioxide and nitrogen pollution. Several European species declined sharply during the industrial era and have partially recovered since emissions controls came into force.
Distribution
Orthotrichum is distributed worldwide, occurring on every continent including temperate, boreal, arctic-alpine, and tropical montane zones. Species are most diverse in the Northern Hemisphere temperate zone and in montane regions of the tropics, where they colonise tree bark and rock surfaces.
Ecology
Most species are epiphytic corticolous mosses (growing on tree bark) or epilithic (growing on rocks). They are notably sensitive to air pollution, particularly sulfur dioxide and nitrogen oxides, and are widely used as bioindicators of atmospheric quality in Europe and North America. Several species are calcicole, favouring calcareous substrates.
Taxonomy Notes
Orthotrichum Hedw. is the type genus of the family Orthotrichaceae and the order Orthotrichales. Multiple checklist treatments at GBIF recognise the genus under Orthotrichales, Bryopsida, Bryophyta. The genus was established by Johann Hedwig and is named for the distinctive straight (ortho-) hairs (trichum) of the calyptra.
