Racomitrium heath is a moss-dominated alpine habitat which is widespread and covers extensive areas in oceanic alpine regions. It is important for the rare bird species that it supports, and the ecosystem services it provides, including carbon storage and regulation of water quality.
Alpine habitats such as this one can, however, experience high levels of nitrogen pollution in rainfall and are increasingly affected by climate change. Understanding the resilience of biodiversity and ecosystem function in these habitats to global change is vital both to guide their management and to predict future change in ecosystem services.
Stage
Work CompletedDirectory of Expertise
Purpose
Racomitrium heath is a moss-dominated alpine ecosystem which is widespread on mountain summits in the UK and the western fringe of Europe. The vegetation is dominated by a single species of moss, Racomitrium lanuginosum, which forms a thick carpet of living and dead shoots over the ground. This moss carpet stores carbon, filters rainfall, and provides a habitat for important mountain bird species including the Dotterel. Racomitrium moss thrives in the cold, damp conditions found on UK mountains, it is adapted to conditions where plant nutrients are naturally very scarce, and it is highly efficient at capturing nutrients such as nitrogen from rainfall.
Figure: Racomitrium heath in the Cairngorm Mountains
(Photograph taken by Dr Andrea Britton)
Racomitrium heath is very vulnerable to the effects of global warming and nitrogen pollution. In the Lake District and north Wales, and in southerly areas of Scotland, high levels of nitrogen pollution have been linked to a reduction in moss cover and increase of grasses on Racomitrium heaths. These changes negatively affect habitat quality and populations of important bird species. Loss of the moss carpet may also negatively impact carbon storage in this habitat and reduce the quality of the water which drains through the moss and soil. A thick moss carpet contains a significant carbon stock and can efficiently remove pollutants such as nitrogen, sulphur and heavy metals from rainfall as it filters through the moss carpet. Since mountain areas are very extensive in Scotland, form the headwaters of many river systems and are the source of around 70% of drinking water resources in the UK, it is important to understand how resilient these habitats are to ongoing climate change and pollutant deposition.
Results
To find out how nitrogen pollution (which arises from animal husbandry, fertilizer use and industrial sources) and climate affect carbon stocks and ecosystem functions in Racomitrium heath, we collected moss and soil from sites across the UK. The amount of nitrogen deposited in rainfall at these sites ranges from 6 – 35 kg N ha-1 y-1 which is representative of the range of nitrogen pollution levels found across the UK and they also vary in terms of temperature and rainfall. The results in this case study (along with other ongoing studies) are being used to help set the acceptable limits for nitrogen pollution.
Moss and soil samples were used to assess above- and below-ground carbon and nitrogen stocks, moss decomposition rate and fungal communities. We also collected large cores of vegetation and soil which we brought into the laboratory to explore how nitrogen pollution affected water quality.
Figure (L-R): Racomitrium lanuginosum moss shoots, Racomitrium heath cores in the controlled environment room and a typical profile through a Racomitrium heath core showing the thick moss carpet and shallow organic soil layer.
(Photographs taken by Dr Andrea Britton)
We found that:
- Moss decomposed almost five times faster2 at sites with the highest nitrogen pollution and warmest temperatures.
- Many more species of fungi were found on sites with faster decomposition rates.
- The carbon stock in the vegetation was reduced by as much as 90%1 at sites with the highest, compared to the lowest, nitrogen pollution levels.
- Above-ground carbon stock was also lower at warmer sites2.
- Pollutant nitrogen (from rainfall) accumulated in the soil, but there was no effect of nitrogen pollution or climate on soil carbon stock2.
- Nitrogen pollution reduced water quality1: water draining from vegetation and soil cores from high nitrogen sites was more acidic and contained large amounts of ammonium and nitrate. However, we found that if nitrogen pollution was reduced, then water quality rapidly improved.
- When exposed to high nitrogen pollution levels, soil from clean sites (with lower nitrogen levels) was much better at retaining nitrogen1, while soil from polluted sites immediately leaked a large amount of nitrogen into the drainage water.
Results have been published in the following Research Papers:
- Britton, A.J., Gibbs, S., Fisher, J.M., Helliwell, R.C. (2019) Impacts of nitrogen deposition on carbon and nitrogen cycling in alpine Racomitrium heath in the UK and prospects for recovery. Environmental Pollution 254: 112986. Doi: 10.1016/j.envpol.2019.112986.
- Britton, A.J., Mitchell, R.J., Fisher, J.M., Riach, D.J., Taylor, A.F.S. (2018) Nitrogen deposition drives loss of moss cover in alpine moss-sedge heath via lowered C:N ratio and accelerated decomposition. New Phytologist 218: 470-478. Doi: 10.1111/nph.15006.
Benefits
Our research shows that nitrogen pollution and climate affect not only the vegetation in alpine moss-heaths but also important aspects of ecosystem structure and function underlying the services that we gain from this ecosystem. Components of the ecosystem vary in their resilience to the effects of nitrogen and climate and our research has allowed us to build a holistic view of global change impacts on this system.
The rapid response of water quality to reductions in nitrogen pollution highlights the sensitivity of this ecosystem component but also the rapid gains that could be made from action to control nitrogen pollution. Since water draining from alpine heaths, such as those studied, impacts on the quality of headwater streams and downslope habitats, any improvements in water quality would benefit the wider upland ecosystem and downstream water users. The reduced ability of soils from polluted sites to retain nitrogen, however, suggests reduced resilience to any future nitrogen pollution.
Above-ground carbon stocks and above- and below-ground biodiversity were sensitive to both nitrogen deposition and climate. In this ecosystem, biodiversity is naturally low and increasing fungal diversity with nitrogen pollution and warming is a negative change, associated with degradation of the ecosystem. Previous work suggests that reducing nitrogen deposition could reverse degradation of the moss carpet, but the warming climate will continue to be a problem for Racomitrium heath. In contrast to above-ground, soil carbon stocks were resilient to the effects of nitrogen and climate. This is good news, as most carbon stocks in this ecosystem are in the soil, but soil carbon changes very slowly, so changes may appear in the longer term.
Project Partners
Research Papers
- Impacts of nitrogen deposition on carbon and nitrogen cycling in alpine Racomitrium heath in the UK and prospects for recovery
- Nitrogen deposition drives loss of moss cover in alpine moss-sedge heath via lowered C:N ratio and accelerated decomposition
- Soil microarthropod-plant community relationships in alpine moss sedge heath
- Relative importance of local- and large-scale drivers of alpine soil microarthropod communities
- The relative importance of nitrogen deposition as a driver of Racomitrium heath species composition and richness across Europe
- Nitrogen deposition enhances moss growth, but leads to an overall decline in habitat condition of mountain moss-sedge heath
- Grazing exclusion and phosphorus addition as potential local management options for the restoration of alpine moss-sedge heath
- Assessing the recovery potential of alpine moss-sedge heath: reciprocal transplants along a nitrogen deposition gradient