Acid deposition :: Montane habitats

[For Acid Deposition processes see overview link]

Effects and implications

• Detrimental effects on lichens and mosses can be sensitive to acid deposition. The leaching of base cations (e.g. Potassium) from cell membranes can lead to the loss of membrane integrity and damage. Species sensitivity to other stresses (e.g. grazing pressure, climatic stress winter and summer desiccation, freezing stress) and pathogens may be enhanced.
• Nutrient limitation will be exacerbated, although this can  mitigate against competition from grass species that favournitrogen (N) enrichment.
• Likely to exacerbate grazing damage to Racomitrium and evergreen dwarf shrubs (Armitage, 2012).
• Acid deposition effects on montane habitats per se have been poorly studied, much more is known about N enrichment (N Dep record).

Overview evidence, processes and main impacts

Sulphur based compounds can be phytotoxic (Sheppard, 1994). Now that concentrations of gaseous SO₂ are much lower, the risk of direct damage to sensitive vegetation from gaseous SO₂ deposition is minimal. The main threats from acid deposition come from nitrogen emissions, although there can be a legacy effect associated with the decades of sulphur emissions at many sites.

At high altitude much of the wet deposition is concentrated through the seeder feeder scavenging effect and ion concentrations can be 10 times that in rainfall. Therefore, montane habitats are particularly at risk from long-range transport of acid pollutants. Montane habitats are also at risk from acid flushes, e.g. when snow melts and deposition is concentrated into one event. This can happen several times during a winter.

Montane environments, where weathering is combined with high rainfall, tend to be naturally acidic unless they occur on basic outcrops.  Montane vegetation, heaths and scrubs of dwarf shrubs have adapted to low levels of nutrient availability since the generally acid, cold, wet, conditions restrict mineralisation and N assimilation.

High levels of precipitation may cause any nitrate to be easily leached while ammonium is retained in organic matter. These systems are probably sensitive to acidity and N inputs (to N Dep record). Bryophytes and lichens which may be directly impacted by wet deposition (Pearce and van der Wal, 2008) are most at risk (Britton and Fisher, 2007).

Pollutant deposition type and risk areas

Indicators of acid deposition

• Fall in soil pH
• Soil acidification and the associated loss of base cations.
• Mobilisation of toxic ions e.g. Al³⁺ and heavy metals detrimental to terrestrial and aquatic life.
• Increase in bare ground
• Increase in calcifuge species

Evidence of species specific responses

What factors modify acid deposition impacts?

• Orographic enhancement, occult deposition and the seeder feeder effect significantly enhance pollutant loadings to these systems.
• Vegetation growing on base rich rocks will be buffered against soil mediated effects, though lower plants will not be protected from direct effects of deposition or snowmelt.