Acid deposition :: Broadleaved, Mixed and Yew Woodland

[Acid deposition: For Acid Deposition processes see overview link]

Effects and implications

  • Epiphytic (growing on trees) non vascular plants e.g. lichens and mosses remain the most sensitive to acid inputs. In the absence of a protective cuticle (skin) lichens readily absorb precipitation of any pH making them highly vulnerable. Physiological processes are also pH sensitive.
  • Species composition, especially on tree branches is mostly determined by the acidity of the wet and dry deposition and its effect on the bark substrate. Most lichen species have well defined tolerance ranges for bark pH. Lichens of hardwoods are likely to be less well adapted to lower bark pH substrates than lichens on conifers.
  • Ground flora in acid impacted woodlands is likely to be less species rich, although the level of effect will depend on the over-storey tree species (Brunet et al 1996). Management will also play a significant role in determining ground flora composition.Many sites will continue to be affected by the legacy of high inputs of acidified S where effects are mediated via the soil (Kennedy 2011). Although, the tendency for broadleaf species to grow on less acid (more mineralotrophic soils) may have reduced the level of impact in these systems.
  • Visible decline symptoms may be observed in trees: branch dieback, abnormal branching patterns, reduced crown density and leaf discoloration.
  • Poor general tree health which will increase the likelihood of secondary stress damage, both biotic, pests and pathogens and abiotic, climatic.
  • Many of these effects reflect below-ground damage, particularly to fine roots.Root damage especially from aluminium (Al3+) toxicity resembles short stubby sometimes blacked tips. This is generally found where acidification of the soil has released soluble Al3+ into the soil solution. Damaged roots can predispose trees to drought stress and windthrow.
  • Increased risk of nutrient imbalance which will lead to stunted growth e.g.phosporus (P) availability is likely to decline in soils with low pH.. Base cation availability and uptake, which can buffer against acidification, will also be low in such mineral soils.

Overview: evidence, processes and main impacts

For broadleaved woodlands, adverse effects are likely to include low levels of P and base cation availability, particularly on acid mineral soils. Current acidification from deposited N compounds may also lead to reduced base cation availability, via leaching and elevated concentrations of H+ ions and potentially toxic NH4+ ions. 

It is a challenge to establish whether effects of S based acidity derive from an acid induced response or a response to S. Acid deposition effects are most likely to be mediated through indirect effects on soil chemistry, e.g. soil pH (falling) which in turn affects the solubility, mobility (increasing them) of toxic metal cations e.g. Al3+, and the availability and uptake of key nutrients e.g. phosphorus and calcium.  Direct effects can occur, such as lesions on leaves, but these are mostly associated with infrequent acid episodes / events. Acid deposition can also lead to enhanced leaching, loss of base cations from foliage with implications for abiotic stress tolerance.

Adverse effects were most pronounced during the 1970s and 1980s. During this period it was estimated that 60% of yew trees in the UK had been negatively affected. Today, with the decline in S deposition, effects of acid deposition are more difficult to attribute. However, there is a substantial time lag between chemical change associated with reduced inputs and biological recovery i.e. woodlands are likely to be show a legacy of the previous high S inputs for decades. Kennedy (2001) estimated up to 66 years would be required for Oak woodland to fully recover.

Sulphur dioxide affects lichens by disrupting important physiological processes – see also Sulphur Dioxide :: Broadleaved, Mixed and Yew Woodland

The bryophyte flora associated with Atlantic Oak Woodlands may be particularly at risk (Farmer et al. 1991b). Although if the rain acidity has interacted with the canopy before it runs over these epiphytes its harmful effects may be significantly reduced as stemflow is often enriched with base cations (Brown 1974). Tyler (1987) observed changes in the ground flora composition of oak woodland attributed to both acidification and eutrophication from sulphur and nitrogen deposition.

Gilbert (1986) studied evidence for an acid rain effect on lichens at two sites in Northern England. Well established populations of Lobaria pulmonaria on oak, and Sticta limbata on ash trees in remote rural areas were observed to decline to the point of extinction. This was accompanied by bark acidification. Only lichens containing a blue-green algal component and growing in habitats poorly buffered on the acid side were affected.

Pollutant deposition type and risk

Type of acid deposition


Risk areas

Dry deposition



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).



Woodlands close to sources like roads and power stations

Wet deposition

precipitation and occult

(cloud, mist)

H+, NO3- SO42-

Montane woodland growing close to cloud base, where ion concentrations are highest. Coastal woodlands that may be subject to high concentration episodes.

Indicators of Acid deposition

  • Change in composition of epiphytic lichens, absence of acid sensitive species.
  • Fall in soil pH
  • Increase in Al3+ concentrations once soil pH falls below ~ 4.4
  • Stunted fine roots and loss of mycorrhiza

Examples of species specific responses




Lobaria pulmonaria on oak


Gilbert 1986

Sticta limbata on ash


Gilbert 1986

What factors modify acid deposition impacts?

  • Chemical composition of the soil: Sensitivity is highest for acid soils overlying acid bedrock with low weathering rates e.g. granites, sandstones, greywackes and schists. Geologically sensitive areas include Dartmoor, Exmoor, Snowdonia and the Cambrian mountains of central Wales, the Pennines, the North York moors, Lake District, Galloway, the Trossachs, Scottish Grampian mountains and the mountains of Mourne.
  • High rainfall, which promotes leaching can exacerbate effects associated with base cation loss.

Evidence of recovery

  • Reductions in the deposition of acidifying pollutants have reduced UK exceedances of critical loads of acidity from 84% of the total area of sensitive Broad Habitats in 1986-88, to 54% in 2006-08 (RoTAP, 2012)
  • Direct evidence is not available, but generally biological recovery lags behind changes in soil chemistry.
  • Increases in soil pH have been recorded over the UK through the period of the 1970s to the current decade over a range of soil types and habitats (RoTAP, 2012)
  • Where SO2 has been the main driver of acid deposition changes there is good evidence of acid sensitive lichens returning in response to the decline in SO2 concentrations.
Critical Load/Level: 
Critical Load/ Level

No estimate available

Brown, A.H.F.; Morris, (Eds) M.G.; Perring, (Eds) F.H. 1974 The British Oak Nutrient Cycles in Oak wood Ecosystems in NW England 141-161
Brunet, J. ; Falkengren-Grerup, U.; Tyler, G. 1996 Herb layer vegetation of south Swedish beech and oak forests - effects of management and soil acidity during one decade. Forest Ecology and Management 88 259-272
Gilbert, O.L. 1986 Field evidence for an acid rain effect on lichens Environmental Pollution 40 227-231
Kennedy, F. ; Rowell, D. ; Moffat, A.J. ; Singh, B. 2001 An analysis of the structure of the simple mass balance equation: implications for testing national critical loads maps. Water, Air and Soil Pollution 1 281-298