Additive impacts of nitrogen deposition and grazing on a mountain moss-sedge heath

High levels of nitrogen (N) deposition and long-term heavy grazing pressures are thought to have caused severe degradation and loss of alpine habitats in the UK. Reductions in stocking density following agricultural reform provide an opportunity for restoration but it is unclear to what extent this may be limited by continuing N deposition. We investigated the effects of spatial variability in grazing pressure on the tissue chemistry and growth of the key bryophyte Racomitrium lanuginosum at a moss-heath site in north Wales exposed to high N deposition. Racomitrium N content and cell membrane damage both clearly reflected the high atmospheric deposition and the additional effect of N deposited by grazers, but this was not translated into negative effects of either factor on growth. Racomitrium grew fastest where a well-developed moss mat was present, regardless of grazer presence, and grew slower in degraded areas where the mat was absent or fragmentary. Absence of a moss mat may produce conditions where Racomitrium is more frequently desiccated, resulting in low growth rates and perpetuating the poor condition of the moss heath. Removal of grazing may result in an improvement in Racomitrium condition, even under existing levels of N deposition, where the moss mat remains, but restoration of the most damaged areas may require further intervention.