Semi- natural Grassland habitats

The Ecosystem Overviews provide a summary introduction to the main habitat types covered in APIS and the main air pollutant pressures to those habitats in the UK. In specific locations, e.g.. close to a major source, other pollutants may be a concern for a habitat and the user should use the searches by location or by habitat/pollutant in these cases.

Introduction

Grasslands are the most extensive semi-natural plant communities in the UK countryside, with over 20% of our native species associated with these habitats (Preston et al., 2002). Unfertilised acidic and calcareous grasslands contain over 300 of the 540 grassland-associated native plant species (UKREATE). These two types of grassland have exceptional conservation and amenity value as a result of their floristic diversity. Other types of grassland in the UK include neutral grasslands which include all semi-improved and unimproved grassland occurring on circumneutral soils (e.g. hay meadows and pastures, and wet grassland types where the vegetation is dominated by grasses, and tall and unmanaged grassland)

BAP habitats: Lowland calcareous grassland (priority); Upland calcareous grassland (priority); Lowland dry acid grassland (priority); Lowland meadows (priority); Upland hay meadows (priority); Neutral grassland; Improved grassland; Acid grassland; Calcareous grassland

Main Pollutant Impacts

Ozone

The critical level for ozone and semi-natural grasslands is exceeded for large parts of the UK, making this an issue of particular concern. Expected effects include the development of ozone tolerant sub-species (Davison and Barnes 1998), and this may lead to altered competitive abilities between plants. Studies on both acid (Hayes, 2006) and calcareous grasslands showed an increased senescence in plants with the peak ozone treatments. Species composition changes were also observed in some studies (Thwaites et al., 2006). Where they have been studied, lowland and upland hay meadows (neutral grassland types) have not been demonstrated to be sensitive to ozone (ref). However, both lowland and upland acid grasslands are sensitive to ozone particularly at the community level. While studies on calcareous grasslands have shown a reduction in plant growth (Morrissey et al, 2007). Ozone also affects agricultural grassland.

Nitrogen deposition

Nitrogen deposition is of particular concern for semi-natural grasslands that are not fertilised. In these situations, plant species composition is adapted to nutrient-poor conditions, with low productivity. Enhanced nitrogen supply from atmospheric deposition tends to favour the growth of some grasses at the expense of other herbs, bryophytes and lichens, which may be of more conservation interest (e.g. Bobbink and Roelofs 1995). Extensive studies carried out by Stevens (2004) and Dupre (2010) on acid grasslands across the UK and Europe, showed a decline in species richness caused mainly by increasing N deposition. Caroll et al, 2003 showed, at a survey site in the Peak District, that acidic and calcareous grasslands, plant community responses to N deposition have been relatively slow but a general increase in grass cover with declines in forbs has begun to emerge. At the same study site large reductions in forb flowering occurred, which were greater than effects on vegetative growth (O'Sullivan, 2008)

Management regimes may obscure or modify some of the relationships between atmospheric deposition and habitat change. Intensive management can offset higher N inputs to a certain extent from high N inputs and by removal through grazing, mowing or harvesting.

Acid deposition

Critical loads are estimated for the effects of acid deposition on to grasslands, depending on soil type. Most at risk are grasslands which are already moderately acidic, while base-rich calcareous grasslands are resistant to acid deposition, due to a high weathering potential. A particular concern is where small base-rich areas occur in otherwise acid grasslands, as it has been suggested that these, and the associated species communities, may be rather sensitive to acid inputs (e.g. Bobbink and Roelofs 1995, UNECE 2004).

Further information and resources:

JNCC habitat pages

UK Research on the Eutrophication and Acidification of Terrestrial Ecosystems

References: 

Bobbink, R.; Roelofs, J.G.M. 1995 Nitrogen critical loads for natural and semi-natural ecosystems: the empirical approach Water, Air and Soil Pollution 85 2413-2418
Davison, A.W.; Barnes, J.D. 1998 Effects of ozone on wild plants New Phytologist 139 135-151
Dupre, C.; Stevens, C.J.; Ranke, T.; Bleeker, A.; Peppler-Lisbach, C. ; Gowing, D.J.G. ; Dise, N.B.; Dorland, E.; Bobbink, R.; Diekmann, M. 2010 Changes in species richness and composition in European acidic grasslands over the past 70 years: the contribution of cumulative atmospheric nitrogen deposition Global Change Biology 16 344-357
Hayes, F.; Mills, G.; Williams, P.; Harmens, H. ; Buker, P. 2006 Impacts of summer ozone exposure on the growth and overwintering of UK upland vegetation. Atmospheric Environment 40 4088-4097
Morrissey, T.; Ashmore, M.R.; Emberson, L.D.; Cinderby, S.; Buker, P. 2007 The impacts of ozone on nature conservation JNCC 403
Preston, C.D.; Pearman, D.A.; Dines, (Eds) T.D. 2002 New Atlas of the British and Irish Flora
Stevens, C.J.; Dise, N.B.; Mountford, J.O.; Gowing, D.J. 2004 Impact of nitrogen deposition on the species richness of grasslands Science 303 1876-1879
Thwaites, R.H. ; Asmore, M.R. ; Morton, A.J. ; Pakeman, R.J. 2006 The effects of tropospheric ozone on the species dynamics of calcareous grassland. Environmental Pollution 144 500-509