Acid Deposition :: Inland Rock & Scree

[For Acid Deposition processes see overview link]

Effects and implications

  • Non vascular plants (e.g. lichens and mosses) remain the most sensitive to acid inputs. In the absence of a protective cuticle (skin)they readily absorb precipitation of any pH making them highly vulnerable. Physiological processes are also pH sensitive.
  • Plant communities growing on acid rock are likely to be sensitive whereas calcareous communities should be relatively insensitive due to the acid neutralising capacity of the calcareous rock.

Overview: evidence, processes and main impacts

Limestone pavements

Limestone pavements have not been studied specifically with respect to acid deposition but aspects of responses of communities on calcareous (chalk grasslands) and neutral acid grasslands may be informative (LINK – see acid calcareous grasslands). Mosses and lichens are expected to be the most sensitive components of these systems, with the potential for leaching of  base cations from the cell membranes leading to loss of membrane integrity and damage Scree

This habitat covers a wide range of rock types, varying from acidic to highly calcareous, occurring throughout the uplands, at high altitudes, but also at low altitudes notably in northern Scotland. Screes are typically dominated by Cryptogramma crispa and other ferns, lichens and bryophytes. On cliff ledges, tall herbs such as Sedum rosea and Angelica sylvestris are generally abundant. Bryophytes and lichens also occur in crevices but are able to flourish on the open rock surfaces where there is a lack of competition from vascular plants. Non vascular plants might be affected by acid deposition although species subject to and tolerant of sea salt spray may be relatively insensitive to acid deposition.

There are no specific studies of acid deposition effects. Consequently, concerns and sensitivity are an informed best guess by consideration of other similar habitat types. Mixed herb communities growing amongst scree can most appropriately be considered as similar to montane and sub-alpine grassland.

Inland rock outcrop and scree

Inland rock outcrop and scree habitats are widespread in upland areas of the UK, with more limited occurrence in the lowlands. Acidic rock and scree are especially widespread, whereas calcareous communities are restricted by the underlying geology. Communities growing on acid rock are likely to be sensitive whereas calcareous communities should be relatively insensitive due to the acid neutralising capacity of the calcareous rock.  

Pollutant deposition type and risk areas

Type of acid deposition

Pollutant

Risk areas

Dry deposition

Gaseous

SO2

Significant reductions in sulphur emissions have successfully addressed by International control measures. Areas where exceedances could still occur are around industrial zones and port areas (due to shipping emissions).

 Dry deposition

Gaseous

NOx

None

Wet deposition

precipitation and occult

(cloud, mist)

H+, NO3-, SO42-

Risk areas include those affected by orographic enhancement

Indicators of acid deposition

  • Fall in soil pH
  • Increase in Al3+ concentrations
  • Change in species composition
  • Increase in calcifuges species.

Evidence of species specific responses

None available

What factors modify acid deposition impacts?

None available

Evidence of recovery

  • Increases in soil pH have been recorded over the UK.

Critical Load/Level: 

Critical Load/ Level

No estimate available