Population status and ecology of Micromitrium tenerum Austin in England

Introduction. The moss Micromitrium tenerum is rare in Europe and threatened with extinction. The aim of this study was to investigate its population status and ecology in England. Methods. Surveys were undertaken at most of the sites from where Micromitrium tenerum has been reported in England. Geographical coordinates of colonies were recorded with a GPS unit and used to derive counts of occupied Ordnance Survey grid cells at resolutions of 1, 10 and 100 m. Habitat and community composition were recorded by relevés. Key results. Micromitrium tenerum was found at three locations, one in North Devon and two adjacent sites in East Sussex, with a total of 25 occupied 1 m grid cells. It is likely to survive at a fourth site in West Sussex. Powdermill Reservoir (East Sussex) supported the majority (84%) of the population censused. The species has been lost from at least 11 sites, all in southeast England. Most current and historical locations of the moss were within the drawdown zone of seasonally fluctuating freshwater bodies, a notable exception being on muddy ground along tracks in an adjacent woodland. Vegetation comprised an open community of vascular plants and bryophytes, which most typically included Physcomitrium patens and Pseudephemerum nitidum. Soils were silty clays and strongly acidic (pH 4.1–5.1). Conclusions. Micromitrium tenerum has undergone a historic decline in England, due to a mixture of wetland loss, involving drainage and built development, and habitat transformation driven by change in wetland use. Presently, competitive exclusion by the non-native invasive New Zealand pigmyweed, Crassula helmsii, is the biggest threat to its future survival.