Acid deposition :: Coniferous woodland

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

Effects and implications

• Epiphytic non vascular plants e.g. lichens and mosses that grow in coniferous woodlands, 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 acid deposition and its effect on the bark substrate. Most lichen species have well defined tolerance ranges for substrate pH. However, lichens of conifers are better adapted to lower pH substrates than lichens on broadleaved tree species.
• 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).  However, management will also play a significant role in determining ground flora composition.  Ground storey bryophytes can be sensitive to acid deposition and consequential leaching of base cations from cell membranes, leading to loss of membrane integrity and death in sensitive species.
• 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).
• Poor general tree health caused by acidification will increase the likelihood of secondary stress damage, both biotic (pests and pathogens) and abiotic (climatic). Increased nitrogen (N) deposition, especially with sulphur (S) or where S is not limiting will increase the likelihood of amino acids accumulating in foliage making it more attractive to insect pests (Whittaker 1994).There is a paucity of research into possible effects of current acid deposition levels on biotic stress.  But if trees are weakened they will be more susceptible. 
• Rarely, visible decline symptoms may be observed e.g. branch dieback, abnormal branching patterns, reddening of needles, reduced crown density and leaf discoloration.
• Many effects reflect below-ground damage, particularly to fine roots.
• Root damage, stunting especially from Al³⁺ (aluminium) 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.
• Loss or reduction in mycorrhizal infection, which help protect roots from heavy metals and improve nutrient foraging and uptake on nutrient poor soils (Munzenberger 1996).
• 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 as it is bound up by aluminium. Base cation availability and uptake, which can buffer against acidification, will also be low in such mineral soils.
• The effects vary with prevailing climatic patterns (exposure effects), as well as distribution of acid soils (ecosystem sensitivity). In many cases, individual trees or groups of trees, rather than whole forests or stands are affected.

Overview: evidence, processes and main impacts

The main threats from acid deposition now come from nitrogen emissions. Nitrogen deposition can both acidify and eutrophy a site, making it more difficult to assign direct effects of acidification (Nitrogen deposition :: Coniferous woodland)

Conifers compared to broadleaves intercept the most precipitation (Cannell 1999) and can thus concentrate pollutants at sites where they grow. Coniferous forests are aerodynamically rough all year round and thus experience the largest pollutant deposition loading of all vegetation.

For coniferous woodlands the adverse effects of the acid deposition legacy are likely to include:  elevated Al³⁺ concentrations, low levels of P and base cation availability in soil, particularly on acid mineral soils (Erisman et al 1997). These chemical changes have adverse effects on the trees and can ultimately together with acid stripping of calcium (Ca²⁺) ions from cell membranes lead to needle loss (Nojd & Reames 1996). 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 ammonium (NH₄⁺) ions. The source of the majority of exchangeable acidity in forest soils is aluminium ions, which are strongly phytotoxic and damaging to roots. Conifers e.g. Abies, Picea and Pinus tend to be more tolerant of acid soils than broadleaf trees but the acid soils where they grow may have low base cation buffering making them more sensitive to acid deposition and low levels of available phosphate (bound by aluminium).

The main threat from acid deposition will be to conifers growing at the cloud base subject to occult deposition. In the UK fewer conifers grow here by comparison with Europe. In Poland and Czechoslovakia damaged trees are still very visible in this region.

Adverse effects were most pronounced during the 1980s.. At that time there was good evidence of detrimental effects on tree growth (Crossley et al 1997). 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

Pollutant deposition type and risk

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
• More open canopy with fewer needle age classes present on branches
• Reddening of foliage

Examples of species specific responses

Native Caledonian pine forest

In general, Scots Pine, Pinus sylvestris, should have similar responses to acid deposition whether it is native or commercially managed. The only difference might be the soil characteristics and locality. Scots Pine is commonly associated with humus iron podzols, peaty podzols, brown forest soils and occasionally peaty gleys and shallow peat, all of which are naturally acid (Robertson 1984). Such soils may be particularly vulnerable to acidic deposition, which can lower the pH, and move from Ca to Al buffering (De Vries et al 1989), leading to the potential for increases in soluble Al3+ and lower base cation availabilities. High Al³⁺ can be toxic to fine roots and restrict phosphorous and base cation availability and uptake (Erisman et al 1997). Negative effects are most likely to happen below ground via changes in soil chemistry and soil biota. Reduction in fine root growth will restrict nutrient and water uptake and may leave trees susceptible to wind-throw and drought.

Effects are often shown up on old trees, and may be more visible in native woodland which is not felled on a prescribed rotation length. The distinction between native pine and planted coniferous woodland is reflected in its management: in the former woods are managed to help promote the existence of other plant/animal species. Wood ash application has been found to be effective in reversing acidification under Scots Pine in southern Sweden (Bramryd and Fransman 1995).

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.