Impact Type: Deposition of pollutant
Many bryophytes rely largely on rainfall for their nutrients (Tamm 1953, Brown & 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 of the plant which is high relative to its volume. Bryophytes are thus closely coupled with the atmosphere and are sensitive to changes in concentrations of potentially damaging pollutants (Makipaa 1995). Damage to cell membranes is the most widespread direct impact (Farmer et al 1992).
On the basis of the field evidence it is difficult to separate the relative effect of acid deposition from that of excess N deposition.
Early studies in the southern Pennines by Tallis (1964), Lee and co-workers (Ferguson & Lee 1983a,b) showed that the decline in ombrotrophic Sphagnum species was related to excess sulphur pollution as SO2 and acid deposition. Although SO2 concentrations have declined since the 1980s, no marked recovery of Sphagnum cover has occurred in the southern Pennines. However, the increasing contributions of nitrogen deposition may be the main factor in determining poor growth of Sphagnum species since the 1990s.
Sensitive epiphytic species can only persist on bark with high buffer capacity (e.g. Acer, Fraxinus and Ulmus). Stemflow can also ameliorate the effects of acid deposition especially on Oak which is a rich source of Ca. (Farmer et al 1992). Declines in Swedish mosses attributed to acid deposition on Oland are described by Sjogren (1995). Solutions below pH4 damaged Boreal forest mosses including Hylocomium splendens and Pleurozium schreberl causing loss of chlorphyll, photosynthetic capacity, membrane potential and reductions in growth. Sheppard (1998) showed that NH4NO3 + H2SO4 pH 2.5 simulated acid mist killed those mosses following >50% base cation depletion. By contrast Plagiothecium undulatum, Polytrichum commune and some Sphagnum species appear to be much more tolerant.
|Critical Load/ Level|
No estimate available