- Top of page
- Materials and Methods
- Effects of N addition on Racomitrium
- Critical load of N for montane heath
High latitude montane ecosystems are traditionally nitrogen (N) limited with low rates of elemental cycling and N deposition. However, rates of atmospheric N deposition over western Europe have increased dramatically in recent decades (United Kingdom Review Group on Acid Rain, 1997). Increasing anthropogenic activity, particularly fossil fuel combustion and intensive agriculture, has greatly increased gaseous emissions of N and over large areas of Europe deposition rates now range to an upper level of over 60 kg N ha−1 yr−1 (Pitcairn et al., 1995). Although areas remote from urban and intensive agricultural pollution sources generally have N deposition rates towards the lower end of the scale (National Expert Group on Transboundary Air Pollution, 2001), montane vegetation can be exposed to higher atmospheric deposition than surrounding lowland sites because of increased precipitation with altitude (United Kingdom Review Group on Acid Rain, 1997). Pollutant concentration of this wet deposition can also be enhanced at high altitude firstly because of the ‘seeder-feeder effect’ of precipitation, in which cloud water from orographic cloud is washed out by rainfall from higher level cloud, and secondly by long periods of orographic cloud cover causing occult deposition, typically containing 2–5 times the pollutant concentrations of rain (Grace & Unsworth, 1988). Therefore montane areas are potentially at a greater risk of severe and episodic pollution events than surrounding lowlands.
There is a growing body of evidence that increasing levels of atmospheric N deposition have negative impacts on seminatural habitats (Woodin & Farmer, 1993; Bobbink et al., 1998; Lee, 1998), especially in upland and arctic areas (Carroll et al., 1999; Gordon et al., 2001). These habitats are typically nutrient-poor, with low soil N mineralisation rates, and long-term increases in atmospheric N deposition can result in eutrophication of the ecosystem. This is likely to cause changes in community structure and composition, with the loss of species sensitive to increases in N supply (Lee et al., 1987; Potter et al., 1995; Press et al., 1998) and replacement by more N tolerant or competitive species (Jonasson, 1992; Bobbink et al., 1998; Robinson et al., 1998; Jonasson et al., 1999).
Most bryophyte species have no root systems and relatively few species have well-developed rhizoids. They acquire nutrients directly from the atmosphere, making them particularly vulnerable to atmospheric pollution (Bates, 2000). Few experiments have addressed the effect realistic doses of N addition have on physiological responses of montane or upland bryophyte species. Soares & Pearson (1995) found increases in tissue N content, and a reduction in inducible activity of the N assimilating enzyme nitrate reductase, in Racomitrium lanuginosum subjected to short-term field misting with 3 mol NH4+ m−3. An ‘acid flush’ of nitrate and sulphate in snowmelt after prolonged snowlie was observed to cause physiological damage to another montane bryophyte species, Kiaeria starkei (Woolgrove & Woodin, 1996c). Whilst indicating high sensitivity to brief pollution events, neither of these studies investigated long-term effects. Habitats such as arctic tundra or montane heath, in which mosses often dominate, are likely to be highly prone to degradation because of the impacts of N pollution. The increase in both the concentration and total deposition of N pollutants with altitude may therefore pose significant harm to montane bryophytes, creating potential for long-term ecological change.
Montane Racomitrium heath, dominated by the ectohydric moss Racomitrium lanuginosum, is the most extensive near-natural terrestrial community in the UK (Thompson & Baddeley, 1991). However, its cover has been declining in recent decades and in upland areas of England and Wales it has now been replaced by grass dominated communities (Thompson et al., 1987; Ratcliffe & Thompson, 1988). As it cannot directly regulate its nutrient uptake, Racomitrium is particularly sensitive to changes in N deposition, and its tissue N content can reflect amounts deposited to it from the atmosphere (Baddeley et al., 1994; Pitcairn et al., 1995). Montane Racomitrium heath therefore provides an ideal model system for investigating the effect of N pollution on sensitive, moss-dominated communities.
Studies manipulating doses of N addition on Racomitrium have demonstrated a deleterious effect on the moss’ survival when exposed to deposition levels equivalent to 20 kg N ha−1 yr−1 or above (Jones et al., 2002; Pearce & van der Wal, 2002; van der Wal et al., 2003). As deposition estimates for many montane areas in the UK exceed this rate, we have cause for concern that current N pollution is actually damaging sensitive montane vegetation. And indeed, past increases in N deposition have been correlated with deterioration and loss of Racomitrium heath across the UK (Thompson & Baddeley, 1991; Bunce et al., 1999).
This paper reports effects of 5 yr of N addition to Racomitrium in the field, demonstrating the longer term outcome of initial observations (2-yr effects are reported in Pearce & van der Wal (2002)), and making the first contribution to an explanation of physiological mechanisms responsible for the observed loss of Racomitrium within montane heath. Background rates of wet N deposition at the site were estimated. The influence of both total N load and ion type (oxidised and reduced forms of N) on Racomitrium performance are examined using cover and growth measures, as well as underlying physiological changes in the moss. The implications for estimation of the critical load of N for montane heath are discussed. Results also enable us to suggest an effective biological indicator for monitoring occurrence of nitrogenous pollutant damage to Racomitrium.