Despite their perceived ‘pristine’ nature and remoteness from major urban centres, alpine regions in Europe and elsewhere are becoming increasingly impacted by human activity. Diffuse pollution (including acidification and nitrogen deposition) and poor land management (amongst other drivers) have the potential to cause major impacts on the biodiversity and functioning of alpine ecosystems world-wide. Alpine regions represent significant reservoirs of biodiversity as, globally, the alpine life zone covers 3% of the land area and holds 4% of higher plant species (Körner 1995), while on a European scale the richness of alpine areas is even more marked, with 20% of species occupying 3% of the land surface (Väre et al. 2003). In addition, the alpine zone performs a vital functional role in the storage and supply of water to lowland regions, as it contains the headwaters of many major rivers. Up to the present time, studies of the impacts of nitrogen deposition have been focused on lowland and upland habitats, while very little is known about the sensitivity of alpine communities (Bobbink, Hornung & Roelofs 1998). Deposition of nitrogen is thought to be potentially detrimental to alpine vegetation, which has generally developed under conditions of low nutrient availability. The few studies that have been conducted suggest that the cryptogram element of the vegetation may be particularly sensitive (Bobbink, Hornung & Roelofs 1998; Fremstad, Paal & Möls 2005). Nitrogen addition has been shown to cause damage to bryophytes and increased dominance of graminoids in Racomitrium heath (Pearce, Woodin & van der Wal 2003; van der Wal et al. 2003). This sensitivity has been reflected in the critical load of nitrogen for arctic and alpine heaths, which is currently set at 5–15 kg N ha−1 year−1 (Achermann & Bobbink 2003). Interactions with management factors such as grazing and fire have been shown to have the potential to modify community responses to nitrogen deposition in some systems (van der Wal et al. 2003; Barker et al. 2004) and so the impacts of nitrogen should not be considered in isolation.
Alpine vegetation in the UK shows evidence of the impacts of both nitrogen deposition and poor land management (heavy sheep grazing and fire; Thompson & Brown 1992). Although generally remote from point sources of pollution, high levels of rainfall and prolonged cover with orographic cloud mean that alpine areas in the UK are exposed to relatively high levels of pollutant deposition. Nitrogen deposition in UK mountain areas currently varies between approximately 7 and 56 kg N ha−1 year−1 (Hall, Heywood & Smith 2004). This is a large range of deposition compared with the rest of Europe and, while conditions in southern Britain are similar to those in continental Europe, the Scottish pollution climate is closer to that in Scandinavia. Grazing has a long history in the alpine regions of Europe and is thought to have caused locally severe community changes and diversity reductions (Erschbamer, Virtanen & Nagy 2003). In the UK, grazing and trampling by domestic herbivores have been blamed for the complete absence of alpine dwarf shrub heaths from areas south of the Scottish Highlands that otherwise appear suitable for this habitat (Thompson & Brown 1992), and continue locally to have detrimental impacts on vegetation composition (Britton, Pearce & Jones 2005). Management of subalpine vegetation, especially heathlands, often takes the form of rotational burning in the UK, and this too can cause damage to communities in the low-alpine zone when fire spreads up from lower altitudes. Strong climatic limitations on growth mean that low-alpine communities may take a long time to recover from such disturbances. While grazing and land management may be locally altered to achieve conservation goals, pollution emissions and impacts occur at a larger spatial scale and must be influenced by application of control policies at a national and international level.
In the UK the low- to mid-alpine zone occupies around 3% of the land surface and contains the most extensive remaining areas of near-natural plant communities (Thompson & Brown 1992; Nagy 2003). British alpine vegetation consists primarily of dwarf shrub-, moss- and grass-dominated heathlands with significant areas of fell field and snowbed vegetation. The largest and best-preserved areas of alpine vegetation occur in Scotland, while smaller areas are present in England and Wales. The alpine plant communities contain elements of both arctic and alpine floras but are strongly influenced by the oceanic climate. This combination of characters results in a number of plant communities that are either unique to or particularly well represented in the UK. Although the higher plant flora is relatively small (around 224 species recorded above the tree line; Nagy 2003) the lichen and bryophyte floras are rich (700 and 212 species above the tree line, respectively; Fryday 1997; Usher 1997). Alpine areas also support important populations of bird species, such as dotterel Charadrius morinellus and ptarmigan Lagopus mutus.
Calluna–Cladonia heathland (NVC H13; Rodwell 1991) is a climax community co-dominated by prostrate Calluna vulgaris (nomenclature for higher plants follows Stace 1991) and terricolous macrolichens, and is almost unique to the UK. This type of vegetation occurs just above the potential treeline and occupies large areas of the low-alpine zone in eastern Scotland (Metcalfe 1950; Averis et al. 2004). The community has similarities with prostrate dwarf shrub vegetation in Scandinavia, particularly in terms of the lichen component (Averis et al. 2004), and is the main habitat for a number of species that are rare in Britain, such as the Red Data Book lichen Alectoria ochroleuca (nomenclature for lichens follows Coppins 2002). Although the effects of nitrogen deposition on upland Calluna heathlands have been well studied, little is known about how the response of alpine Calluna heathland might differ. This community is a climatically controlled climax vegetation, unlike the upland and lowland heaths, which are successional communities maintained by management. Hence the vegetation is subject to much stronger climatic limitations than that at lower altitude, including a short growing season, high wind speeds and exposure to low winter temperatures, because it occurs in areas with little snow cover. Based on current knowledge of related systems, Bobbink, Hornung & Roelofs (1998) have hypothesized that alpine heaths may be particularly sensitive to nitrogen deposition. In this study we investigated the medium-term effects of nitrogen deposition, burning and grazing, and their interactions, on the species diversity and composition of these alpine Calluna heathlands.