Nitrogen deposition :: Montane Habitats

Effects and implications

• Eutrophication: these systems are adapted to low levels of mineral N availability, increasing the availability of N will threaten the competitive balance between species leading to changes in composition and loss of habitat species constants.
• Lichens and mosses are particularly sensitive to nitrogen both from direct effects associated with N accumulation and from shading as a consequence of N stimulated growth of over-storey vegetation.
• Terricolous lichens (lichens growing on the ground): growth of some species is very sensitive to N concentration in cloudwater, occult deposition (Britton & Fisher 2010).
• Species sensitivity to other stresses e.g. grazing pressure, desiccation and pathogens may be enhanced.
• Potentially damaging interaction between N deposition and grazing for Racomitrium and evergreen dwarf shrubs, particularly since the thickness of the Racomitrium matt is reduced (Armitage 2011).

Overview: evidence, processes and main impacts

Montane heaths and scrubs of dwarf shrubs are naturally adapted to low levels of nutrient availability since the generally acid, cold, wet, conditions restrict mineralization and N assimilation. As a consequence also of these often extreme environmental conditions, growing seasons are short. With high levels of precipitation any nitrate may be easily leached, while ammonium is retained in organic matter. These systems are therefore, sensitive to additional N inputs, reflected in the low critical load  (Bobbink & Hettelingh 2011). These environmental characteristics: short growing season, environmental restrictions on growth and stature re exposure and snowmelt concentrating the winter N input, will condition the responses of these ecosystems to N enrichment. Bryophytes and lichens which may be directly impacted by wet N deposition (Pearce and van der Wal 2008),  or outcompeted by more vigorous plants, are most at risk (Thompson and Baddeley 1991, Hornung et al. 1995a, Pearce et al 2003, Britton & Fisher 2007a). These lower plants are particularly at risk from N that has accumulated in snow and is released in high concentrations in spring melt water (Woolgrove and Woodin 1996b).

The fertilising effect of additional N can favour the growth of higher plants, reducing light levels reaching the bryophyte and lichen under -storey (Pearce and van der Wal 2002; van der Wal et al 2005; Britton & Fisher 2007). Lichen diversity declines with increasing N load in prostrate Calluna heaths (Britton & Fisher 2007a). Snowbed species are expected to be particularly sensitive to N deposition (Woolgrove and Woodin 1996 a & b). Montane Calluna heaths (Britton & Fisher 2007a), Racomitrium heaths (Pearce & van der Wal 2002, 2008; Pearce et al 2003; van der Wal et al 2005) and snowbeds (Woolgrove and Woodin 1996a,b) are the communities in the UK we know most about with respect to their N responses, although few long term studies exist. In terms of eutrophication, a long term decline in lichens (presence and diversity), mosses and evergreen dwarf shrubs together with modest increases in grasses and herbs are to be expected. However, transformation into grasslands is much less likely, rather, it will lead to more bare ground. Long term exposure to N deposition may also cause a shift from N to P limitation of growth (Britton & Fisher 2007b) potentially affecting the outcome of competition between species. In a short-term study (3 months) Britton et al (2010) demonstrated that terricolous lichens (growing on soil)  were sensitive to not just the N dose but also N concentration, which can often be higher in montane areas as these are subject to occult deposition.

Pollutant deposition type and risk areas

Indicators of N enrichment

• Decline in cover of matt forming Cladonia lichen species and  Racomitrium moss
• Increase in Carex bigelowii cover
• Increased N concentrations in tissue.
• Increase in bare ground

Below-ground

• Increased rates of decomposition increasing N availability
• Loss of moss sponge increasing N availability
• Reduced soil C:N ratio (Britton et al 2005).
• Increased activity of soil and litter phosphomonoesterases
• Soil acidification and the associated loss of essential plant nutrients.
•  Mobilisation of toxic ions e.g. Al and heavy metals detrimental to terrestrial and aquatic life.

Example evidence of species specific responses

Some examples of specific responses are given in the table below. This does not represent a comprehensive review of all species impacts.