Impact Type: Deposition of pollutant
Many bryophytes rely largely on rainfall for a supply of nutrients (Tamm 1953, Brown and Bates 1990). Due to a virtual absence of cutin development, mineral ions from both rain and cloud water are readily absorbed over the whole surface. Bryophytes are, therefore, closely coupled with the atmosphere and sensitive to changes in concentrations of potentially damaging pollutants.
A wide range of studies on bryophyte species have demonstrated that in general, increased N supply adversely affects growth. For example, growth of Sphagnum cuspidatum was reduced by very small increases in N (Press et al. 1986) and growth of Hylocomium splendens and Pleurozium schreberi was strongly reduced by addition of 30-60 kg N ha-1 year-1 (Dirkse & Martakis 1992). However, when these species growing in the understorey of a young spruce forest were treated with NH4 NO3 there were no visible effects. In experiments the ionic strength of the treatment may be driving the response rather than the dose per se. In the field, atmospheric inputs of N are thought to be partly responsible for the loss of certain bryophyte and lichen species and decreases in cover at Moor House NNR, Cumbria between 1950s and 1990s (Pitcairn et al. 1991), the decline in Racomitrium heath in areas south of the Scottish Highlands (Thompson & Baddeley 1991), and the decline in health and survival of Sphagnum communities in areas of Wales and the southern Pennines (Woodin & Farmer 1993).
Woolgrove and Woodin (1996c) carried out a study of the late snowbed bryophyte, Kiaeria starkei, which they collected from 14 sites in the Scottish Highlands. The acid flushes during snowmelt corresponded with the atmospheric N loads to which the bryophyte was exposed. Due to the sensitivity of this species, this could possible pose a threat to its continued survival.
Bryophyte survival can also be indirectly affected by reduced light levels resulting from increased growth of other components of an ecosystem in response to increased N deposition.
Many of the studies described above relate to wet deposited N, but dry deposited ammonia can also affect bryophyte growth either directly, or indirectly through reduced light levels. Bryophytes are largely absent from woodland groundflora close to poultry farms in Scotland where NH3 concentrations and N deposition are large (Pitcairn et al. 1998).
|Critical Load/ Level|
No comparable habitat with established critical load estimate available