Heath and montane scrub

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.

Introduction

Heathlands are typically dominated by a range of dwarf shrubs such as heather Calluna vulgaris, bilberry Vaccinium myrtillus, crowberry Empetrum nigrum, bell heather Erica cinerea and, in the south and west, gorse Ulex gallii. Dry heaths typically occur on freely-draining, acidic to circumneutral soils with generally low nutrient content. Lowland heathlands support many Red Data Book species of plants and birds, as well as all 12 species of native amphibians and reptiles in the UK. Montane heathlands and scrub comprise a range of dwarf-shrub, moss and lichen dominated communities occurring above the potential tree-line.

BAP habitats: Mountain heaths and willow scrub(priority); Montane habitats. Lowland Heathland (priority); Upland Heathland (priority); Dwarf Shrub Heath (broad)

Main Pollutant Impacts

Nitrogen deposition

Since heathlands and montane scrubs are usually naturally poor in nutrients, they are particularly sensitive to additional atmospheric nitrogen inputs. Most at risk are high altitude habitats (such as montane heaths and scrubs), which are subject to high levels of wet deposition from long-range atmospheric transport of oxidised and reduced nitrogen, and lowland heaths where these occur in the vicinity of intensive agricultural activities or major roads. In the UK nearly 30% of dwarf shrub heaths and 97% of montane habitats exceed their critical loads for nitrogen (based on 2006-2008 deposition data). For montane habitats the situation is worse where 97% of montane habitats exceed their critical load for nitrogen (based on 2006-2008 deposition data).

Changes in species composition of lowland heaths, with a marked decline in heather Calluna vulgaris and an increased dominance of grasses, have been observed widely in the Netherlands (Bobbink and Roeloffs 1995a,b) and also in the East Anglian Brecklands (Pitcairn and Fowler, 1991). Reduction in moss cover may lead to soil erosion or allow expansion of grasses and sedges able to use the increased N supply. Attention has also been focussed on the interactions between deposition and management practices (specifically burning and grazing) in term of influencing vegetation changes.

Nitrogen deposition to upland heaths can also lead to changes in vegetation structure and enhanced litter accumulation resulting in reductions in the abundance of the bryophyte and lichen species (Carroll et al 2002).

Ammonia

Similar to impacts of nitrogen deposition, ammonia pollution can lead to the direct damage of sensitive species such as lichens, mosses and Calluna, including senescence and leaf loss. Ammonia exposure tends to cause life cycle acceleration so that Calluna becomes woody and 'leggy' earlier. This in turn can encourages invasion by grasses, eg Molinia and Deschampsia. Changes in the composition of groundflora, bryophyte and lichen communities are also experienced.

Acid deposition). Lower plants (mosses, lichens and liverworts) will be at risk from acid leaching of base cations from cell membranes (Farmer et al 1991; Pearce & van der Wal 2002). In the UK ~ 37% of dwarf shrub heaths exceed their critical loads for acidity (based on 2006-2008 deposition data); for montane the percentage exceedance is 88%.

Ozone

Mean concentrations increase with altitude, since increased elevation reduces likelihood of being in the nocturnal atmospheric boundary layer, which leads to ozone depletion. As a result, concentrations at high altitude sites can build up to much higher levels during photochemical episodes. This makes high altitude ecosystems such as alpine heaths and scrubs particularly prone to the effects of ozone.

There are no experimental data currently available on the possible community responses of heathland to ozone, but Millset al., (2007) have estimated that lowland shrub heathland contains 51.7% ozone sensitive species. Mills et al., (2007) have estimated that Arctic, Alpine and Sub-alpine scrub will contain a considerably greater proportion of ozone sensitive species (72.4%). Whilst this may be an overestimate for upland heath in the UK, it is probable that these habitats will be more sensitive to ozone than their lowland counterparts.

Further information and resources:

JNCC habitat pages
UK Research on the Eutrophication and Acidification of Terrestrial Ecosystems.

References: 

Bobbink, R.; Roelofs, J.G.M. 1995 Nitrogen critical loads for natural and semi-natural ecosystems: the empirical approach Water, Air and Soil Pollution 85 2413-2418
Farmer, A.M.; Bates, J.W.; Bell, J.N.B.; Bates, (Eds) J.W.; Farmer, (Eds) A.M. 1992 Ecophysiological effects of acid rain on bryophytes and lichens Bryophytes and lichens in a changing environment
Pearce, I.F.K.; Van der Wal, R. 2002 Effects of nitrogen deposition on growth and survival of montane Racomitrium lanuginosum heath Biological Conservation 104 83-89