Current and historical relationships between the tissue nitrogen content of a snowbed bryophyte and nitrogenous air pollution

Snowbed vegetation is under threat from atmospheric pollution. Most of the late lying snowbeds in Britain are in the central highlands of Scotland, coinciding with an area of very high deposition of nitrogenous air pollutants. Snow is a very efficient scavenger of atmospheric pollution and, due to the dynamics of snowmelt, much of the pollution load of a snow pack is released at very high concentrations in episodes known as 'acid flushes'. This study demonstrates the existence of a positive relationship between duration of snow-lie and tissue nitrogen content of Kiaeria starkei, a bryophyte characteristic of late snowbeds. An increase in the tissue nitrogen content of this bryophyte over this century is also shown, reflecting increasing air pollution. Maximum tissue nitrogen concentration in K. starkei is up to 50% greater than that recorded in other upland bryophyte species, demonstrating the exceptional threat of pollution to snowbed bryophytes. This has implications for the critical loads approach to pollution emission controls, as it indicates that some mountain communities are receiving higher pollution loadings than previously realised and therefore current exceedence of critical loads is probably greater than recognised at present.