Acid deposition :: Dwarf Shrub Heath

[For Acid Deposition processes see overview link]

Effects and implications

• Reduction of acid sensitive bryophyte species, where acid deposition is high or concentrated.
• Change in species composition and frequency of ground floor bryophytes. Mosses can be sensitive to acid deposition which leaches base cations from cell membranes, leading to loss of membrane integrity and damage.
• Low soil pH which means lower base cations and higher metal cation concentrations in the soil solution (especially aluminium Al³⁺) and potentially lower P availability.
• Below-ground damage, particularly to fine roots. Stunted fine roots and loss of ericoid mycorrhiza; probably requires significant acidification to bring pH towards 3 before such effects would be expected.
• Root damage may increase sensitivity of Calluna to winter desiccation.

Overview: Evidence, processes and main impacts

Dwarf Shrub Heaths are characterised by vegetation dominated (>25%) by members of the heath family (Ericaceae: e.g. heathers, blaeberry, cowberry) with some grasses (e.g. purple moor-grass and deer grass). The exact mix depends on the soil type and amount of rainfall on the area, whether they are in upland or lowland areas, as well as the history of burning and browsing.

There can be legacy effects associated with the decades of sulphur (S) emissions, at many sites.  Soils were significantly acidified in areas of high deposition, e.g. north-west Wales (Kuylenstierna and Chadwick, 1991).Today effects of acidified S deposition on dwarf shrub heath are likely to be mediated via the soil, rather than direct toxic effects to the foliage, and represent the legacy of past (plus current) deposition.

Currently nitrogen pollutants dominate acid deposition. Nitrogen deposition can both acidify and enrich a site with nutrients, making it more difficult to attribute effects of acidification (see Nitrogen Deposition :: Dwarf Shrub Heath)

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). 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 (Houdijk et al. 1993). Aluminium ions are the main source of exchangeable acidity in heathland soils and are strongly phytotoxic and damaging to roots. Ericaceous shrubs are relatively acid tolerant, but these soils often have low base cation buffering, making them more sensitive to acid deposition and low levels of available phosphate (bound by aluminium).

Generally species inhabiting this ecosystem are acid tolerant, however, their roots may still be sensitive to mineral acids and the increase in ammonium ions. Species that are only moderately acid tolerant may be sensitive.

In the absence of a protective cuticle  lichens readily absorb rainfall of any pH making them highly vulnerable to acidification, leading to adverse effects on pH sensitive physiological processes. Cladonia spp., the main lichens growing in heathlands, have been shown to accumulate S.

Effects vary with prevailing climatic patterns (exposure effects), as well as distribution of acid soils (ecosystem sensitivity). In many cases, patches, rather than whole Calluna stands are affected. Damaged roots can predispose vegetation to drought stress and nutrient limitation. While an increased risk of nutrient imbalance will reduce growth. Phosphorus (P) availability is likely to decline in soils with low pH and high Al concentrations through precipitation. Base cation availability and uptake will also be low in such mineral soils. Acid deposition may help to restrict eutrophication effects that can accompany N and also S deposition, i.e. restrict encroachment  by some potentially rank, undesirable weed species that are acid sensitive, and help maintain the ecosystem community composition.

Pollutant deposition type and risk

Indicators of Acid deposition

• Change in composition of non-vascular plants (e.g. lichens) and reduction/absence of acid sensitive species e.g.. Antennaria dioica and Succisa pratensis
• Scarceness of species typical of intermediate pH and NH₄⁺ sensitive
• Fall in soil pH
• Increase in Al³⁺ concentrations once soil pH falls below ~ 4.4

Examples of species specific responses