Nitrogen Deposition :: Dwarf Shrub Heath

Impact Type: Deposition of pollutant

Key Concerns:

• Changes in species composition with a marked decline in heather (Calluna vulgaris) and ericoids, and an increased dominance of grasses (e.g. Bobbink and Roeloffs 1995b, Pitcairn and Fowler 1991).
• Loss of mosses, liverworts and lichens which receive their nutrients from the atmosphere directly (Fangemeier et al. 1994).
• Decline in R. lanuginosum heath, one of the most extensive terrestrial near- natural plant communities in Britain, and its replacement with grasses (Thompson and Baddeley, 1991).
• Increased risk of heather beetle attacks on Calluna vulgaris, encouraged by higher N levels in foliage
• Initial N stimulated growth for Calluna, increased litter, N return and mineralization.
• Negative effects on ericoid mycorrhiza and increase in drought sensitivity.
• Impacts linked to increased attractiveness to insects pests, and opening up of the canopy due to frost.

Habitat Specific Information

Lowland heaths

The impact on lowland heathland is one of the best documented effects of nitrogen deposition with the results of many experimental studies available (Bobbink et al 1998). The scale of impacts need to be understood in relation to the interactions between deposition and management practices (burning and grazing). Management intervention can reduce impacts of N through removal via burning and sod cutting. Impacts of nitrogen deposition are particularly associated with the Netherlands where livestock densities provide very high levels of ammonia emissions and deposition (e.g. De Graaf et al. 1998, Bobbink and Roelofs 1995a,b), Other areas include the East Anglian Breckland (Pitcairn and Fowler 1991) see ammonia deposition.

Impacts on lowland heathlands are likely where relict heathlands remain surrounded by an intensive agricultural landscape. A wide range of sources apply (e.g. nearness to pig, poultry or cattle farming). Smaller impacts are expected from oxidised nitrogen deposition (NOX) due to the slower rate of deposition, although there is still high uncertainty regarding the scale of impacts on heathlands adjacent to major roads or cities. The nearness to agricultural grassland also increases the likelihood of invasion by more nitrophilic grasses. N inputs will enhance N availability via increased litter production, decomposition and increased N mineralization.

Upland heaths

Upland heaths and moorland occur under cold, wet, acid conditions that favour dwarf shrubs, especially Calluna and bryophytes. Calluna can absorb up to 90% of its nitrogen from the atmosphere. Impacts include increased winter injury (Carroll et al 1999). Effects on soil pH were not observed (Caporn et al 1995). Responses of mycorrhizas depend on the limiting nutrients. There is the possibility of increased winter damage to Calluna especially when high pressure prevails, the ground is frozen and there is little or no snow cover. Interactions between N deposition and pests or stress that lead to openings in the canopy, will increase the risk from grass invasion (e.g. Deschampsia flexuosa and Molinia caerulea).

The interactions between deposition and management practices (specifically burning and grazing) need to be considered.