The Ecosystem Overviews provide a summary introduction to the main habitat types covered in APIS and the main air pollutant pressures to those habitats in the UK. In specific locations, e.g.. close to a major source, other pollutants may be a concern for a habitat and the user should use the searches by location or by habitat/pollutant in these cases.
11.5% of Great Britain is covered with trees. However, only 1.2% of Great Britain is ancient semi-natural woodland. Woodlands are valued for their high conservation status, landscape value, the ability to store carbon and provision of recreation and amenity opportunities.
BAP Habitats: Upland oakwood (priority); Lowland mixed deciduous woodland (priority); Lowland beech and yew woodland (priority); Upland mixed ashwoods (priority); Upland birchwoods (priority); Native pine woodlands (priority); Lowland wood-pasture and parkland (priority); Broadleaved, mixed and yew woodland; Coniferous woodland
Main Pollutant Impacts
Nitrogen deposition and Ammonia
95% of the area of woodlands in the UK (managed and unmanaged) exceed the nitrogen critical load.. This is primarily due towoodlands and forests being able to scavenge air pollutants more effectively than shorter semi-natural vegetation, with the result that inputs of nitrogen deposition to woodlands are generally larger than for other habitat types. Nitrogen deposition on forest ecosystems can lead to increased sensitivity to natural stress, impacts on roots, reduced species diversity of the ground vegetation, reduced growth, and an unbalanced nutritional status due to eutrophication and acidification (Erisman and de Wries 2000). Changes in forest ground flora have been clearly documented as a result of enhanced N deposition near farms (Pitcairn et al. 1998) and are also expected to occur in regions with high wet deposition of ammonium and nitrate.
Atlantic Oakwoods are of high conservation value containing many rare lichens and bryophytes. Nitrogen deposition has been shown to influence the bark pH of trees which in turn can influence epiphytic communities. Lower bark pH caused by nitrogen deposition can have an effect on the composition of these epiphytic lichens (Mitchell et al 2005)
Deposition of acidifying air pollutants is primarily seen as affecting the soils of woodland habitats, where effective inputs of sulphuric and nitric acids lead to leaching of base cations. The resulting soil acidification can lead to mobilisation of naturally occurring aluminium in the soil, which may have toxic effects on plant roots, leading to problems of tree health (UKCLAG 1994). Although a base, ammonia may also lead to acidification, since its oxidation by soil bacteria also produces nitric acid.
Other effects of acid deposition on woodlands include:
Ozone in general decreases tree growth, reduces leaf area and accelerates leaf ageing and abscission (Royal Society, 2008). These impacts of ozone on forests are predicted to be widespread in the UK, due to the exceedance of the critical level for forests. Furthermore, Ashmore and Keelan (2006) investigated the sensitivity to ozone of woodland ground flora species from deciduous woods. The study found that ozone induced a change in species composition, with the cover and biomass of typical woodland species being reduced compared with non-shade adapted, invasive species.