ECOS 38 (4)

Winner - ECOS Student Article 2017 Competition, supported by Conservation Careers and Green&Blue

Why in the light of the overwhelming ‘atmospheric pollution’ evidence does the ‘overgrazing and over burning’ narrative persist and dominate in the uplands of Britain?

The tragedy of the Dartmoor commons

I spent over a decade in the 21st century sitting in my office on Dartmoor trying to solve a riddle: why when the sheep and cattle numbers on the moor had been reduced was the heather continuing to disappear whilst the purple moor grass took over and dominated the commons with its eerie and stifling ‘raffia’?  Ecologists and conservationists have not provided satisfactory answers despite spending much time and effort looking for them.

The standard narrative suggests that following the introduction of the 1947 Agriculture Act, production goals in the uplands and increased sheep stocking rates clashed with environmental objectives and as a result wildlife declined. Sheep numbers rose dramatically between 1950 and the 1980s and moorland burning produced fresh flushes of grass growth and reduced areas of gorse dominated moorland. Although headage payments were stopped during the 1980s, major reform did not take place until the early 1990s when agri-environment schemes became widely available. As a result stocking numbers dropped dramatically.

The trouble was that the detailed land management prescriptions that came with the agri-environment schemes did not deliver the desired outcomes i.e. the recovery of upland habitats and ‘favourable condition’ on the Sites of Special Scientific Interest.

Wildlife continued to deteriorate and purple moor grass began to expand. According to English Nature in 1990, purple moor grass raffia comprised less than 1% of Dartmoor’s commons and today it is universally agreed that this figure has increased considerably. 1 It would appear that at the end of the period when the commons were most intensively grazed, purple moor grass cover was negligible but since the reduction of grazing pressures in the mid 1990s the area of Dartmoor dominated by the grass has increased significantly.

Moorland dominated by purple moor grass.
Photo: Adrian Colston

Burning, grazing and atmospheric pollution – untangling the impacts

The restoration management prescriptions from English Nature and Nature England focused on reducing the grazing pressure and reducing burning. Whilst such prescriptions aided the blanket bog it was very much less successful at reducing the dominance of purple moor grass on the mires. Natural England in its 2013 Evidence Review on the impact of moorland grazing and stocking rates 2 concluded: “There remains concern and disagreement about the effects of grazing on the upland landscape and biodiversity, in particular about stocking rates, different livestock types and the timing and spatial pattern of grazing regimes”. This is  a rather damning indictment.

This narrative suggests that the unfavourable condition of our upland habitats was created by historic over grazing and unsustainable burning. As a result of the extent of the damage caused by such practices recovery will take decades before it fully occurs. Indeed, I was a supporter of this narrative when I wrote about the deterioration of the National Trust’s land in the Upper Plym in 2015.3

However, there is a second narrative which may help to explain why the restoration of upland habitats across the UK is proving to be so difficult and that involves the role played by atmospheric pollution. Ecologists and conservationists have been aware of the problems caused by atmospheric pollution, particularly nitrogen deposition but they have seen it as an impediment to restoration rather than the driving force.

In 2002 Des Thompson, one of the UK’s most respected upland ecologists said: “Some of us are beginning to form the view that some of the grass-dominated vegetation types of the southern uplands may be the product of nitrogen deposition and heavy grazing pressures. Hence, a reduction in grazing pressures alone may not necessarily result in an improvement in habitats”.4 But these views did not translate into policy changes and the ‘overgrazing- over burning’ narrative persisted.

Understanding nitrogen pollution

In the UK, there are two distinct forms of nitrogen pollution: nitrogen oxides (NOx) and ammonia (NH3). Emissions from vehicles and power stations are largely responsible for NOx, whilst emissions from agriculture account for the majority of NH3.5

NOx emissions peaked around 1990 and by 2015 had fallen 69% compared to the 1970 level. Ammonia levels have by comparison fallen just 9.9% between 1980 and 2015.

However, despite the large falls in nitrogen emissions, the deposition of all nitrogen compounds has hardly fallen at all. This unexpected situation has arisen as the atmospheric chemistry over Britain has been altered leading to more rapid oxidisation of nitrogen. This rapidly oxidised nitrogen is deposited in the UK when previously it would have been exported to Continental Europe. As a result, many parts of the UK have been receiving high levels of nitrogen deposition for decades. English Nature showed that between 1989-92 93% of Dartmoor exceeded nitrogen deposition critical loads.

There is an extensive scientific literature on the impact of nitrogen deposition on vegetation. A number of studies have shown that species diversity in semi-natural habitats declines with increasing nitrogen deposition, other work has shown that lichen and moss species decline while grasses increase.6 Another study showed that heather became more palatable to sheep when grown in enhanced nitrogen conditions and purple moor grass gained a competitive advantage over heather when the soil conditions moved from being nitrogen limited to phosphate limited.7

Research has shown that heather will not be displaced by coarse grasses even under high levels of nitrogen deposition unless the heather is severely damaged either by weather conditions or defoliation by heather beetles.8 Heather beetle outbreak frequencies are also linked to enhanced nitrogen deposition. When this happens, the coarse grasses replace the heather and create conditions where the heather is unable to regenerate.9

Low level ozone (O3) is also a problem especially in the Uplands. O3 is increasing by around 0.2 ppb per annum. O3 is a well-known phyto-toxic gas and it causes significant adverse effects on human health, forest growth, species composition and damage in semi-natural vegetation. For example, 80.4% of species in raised and blanket bog, 60% in mires and 51.7% in temperate shrub heathland are O3 sensitive.10

Research found that after 28 days exposure to increased O3 concentrations purple moor grass showed significantly increased shoot weights and increased root to shoot ratios. The effects of increased O3 concentrations on heather were tested and it was reported that heather can be adversely affected by prolonged O3 episodes particularly if these are followed by or coincide with frosting temperatures.11

Atmospheric pollution – the new priority

The evidence that atmospheric pollution has detrimentally impacted on the vegetation of the uplands is compelling. This narrative explains the rise in extent and vigour of purple moor grass at the expense of heather especially in the light that heather beetle outbreaks have been and are widespread and destructive. Additionally, even in an era of reduced sheep numbers, heather shoots are vulnerable to selective grazing by sheep as nitrogen deposition has enhanced their nutritional value.

Belatedly Natural England has published an Atmospheric nitrogen theme plan.12 The plan reported that in England 80% of sensitive Special Areas of Conservation and 70% of sensitive Special Protection Areas are estimated to exceed the critical load.

Conservationists need to take atmospheric pollution more seriously as it is a driver of change and not just an inconvenience and the two narratives of the Uplands need to be merged into one. We also need to campaign to improve air quality in the UK for both wildlife and people (11,900 premature deaths per annum6) as the Government’s current plans are woefully inadequate and only focus on the urban to the exclusion of the rural. 13,14

Given the interplay of these different pressures and influences on vegetation, it begs the question of what ‘favourable condition’ actually means.

A riddle resolved and an elephant exposed?

References

  1. 1. Mercer I. (2009) Dartmoor: a statement of its time. HarperCollins. London.
  2. Natural England (2013) Impact of moorland grazing and stocking rates. Natural England Evidence Review NEER006. http://publications.naturalengland.org.uk/publication/5976513
  3. Time to go wild with George Monbiot? https://adriancolston.wordpress.com/2015/11/04/time-to-go-wild-with-george-monbiot/
  4. Burt T.P., Thompson D.B.A. & Warburton J. (2002) The British Uplands: Dynamics of Change. Joint Nature Conservation Committee Report No. 319
  5. Atmospheric Nitrogen Pollution and its impact on Dartmoor – the other elephant in the countryside https://adriancolston.wordpress.com/2017/03/31/atmospheric-nitrogen-pollution-and-its-impact-on-dartmoor-the-other-elephant-in-the-countryside/
  6. Stevens C.J., Dupre C., Dorland E., Gaudnik C., Gowing D.J.G., Bleeker A., Diekmann M., Alard D. Bobbink R., Fowler D., Corcket E., Mountford J.O., Vandvik V., Aarrestad P.A., Muller S. & Dise N.B. (2010) Nitrogen deposition threatens species richness of grasslands across Europe. Environmental Pollution 158: 2940-2945.
  7. Kirkham F.W. (2001) Nitrogen uptake and nutrient limitation in six hill moorland species in relation to atmospheric nitrogen deposition in England and Wales. Journal of Ecology 89: 1041-1053.
  8. Bobbink R., Hicks K., Galloway J., Spranger T., Alkemade R., Ashmore M., Bustmante M., Cinderby S., Davidson E., Dentener F., Emmett B., Erisman J.-W., Fenn M., Gilliam F., Nordin A., Pardo L. & De Vries W. (2010) Global assessment of nitrogen deposition effects on terrestrial plant diversity: a synthesis. Ecological Applications 20 (1): 30-59.
  9. The problem with Heather Beetles https://adriancolston.wordpress.com/2017/03/21/the-problem-with-heather-beetles/
  10. Atmospheric pollution from Ozone – an elephant in the Uplands? https://adriancolston.wordpress.com/2017/03/30/atmospheric-pollution-from-ozone-an-elephant-in-the-uplands/
  11. Franzaring J., Tonneijck A.E.G., Kooijman A.W.N. & Dueck Th.A. (2000) Growth responses to ozone in plants species from wetlands. Environmental and Experimental Botany 44: 39-48.
  12. Natural England (2015) Atmospheric Nitrogen Theme Plan. Developing a strategic approach for England’s Natura 2000 sites. Improvement Programme for England’s Natura 200 Sites – Planning for the future. http://publications.naturalengland.org.uk/ publication/6140185886588928
  13. https://www.theguardian.com/environment/2016/nov/23/uk-has-second-highest-number-of-deaths-from-no2-pollution-in-europe
  14. Tackling nitrogen dioxide in our towns and cities (2017) Defra https://consult.defra.gov.uk/airquality/air-quality-plan-for-tackling-nitrogen-dioxide/supporting_documents/Consultation%20Document.pdf

ADRIAN COLSTON

The author has worked for the Northants Wildlife Trust and most recently for the National Trust on Dartmoor. He is now a PhD researcher at the University of Exeter.

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