Long-term predictions of ecosystem acidification and recovery
Graphical abstract
Introduction
The effects of acid deposition remain a significant environmental problem, even though deposition has reduced considerably in many parts of the world. Although there is still much to learn, we now have a good understanding of the major acidification and recovery mechanisms for sensitive soils and waters, encapsulated in models of various kinds. We also have some good long-term data, which is becoming commensurate with the timescales of ecological change induced by changes in atmospheric deposition. The time seems right to explore the long-term consequences of acid deposition given current knowledge, and to ask whether the data we have are sufficient to constrain our predictions of those consequences so that meaningful statements can be made. This paper is an attempt to answer those questions using a calibrated acidification model and long-term data from a catchment in the UK. A single catchment can only serve as an example given the huge variety of climates, vegetation, soils, deposition changes etc., but its reactions can still provide useful insights. For instance, Skeffington and Brown (1992), used this approach to predict that one consequence of declining sulphur deposition would be lower base cation concentrations in recovering surface waters, and that this could cause biological problems. Controversial at the time, this prediction has subsequently been borne out by observations in many places (e.g. Battarbee et al., 2014, Stoddard et al., 1999).
There have been many acidification modelling studies involving projections into the future (e.g. Evans et al., 1998, Ferrier et al., 2003, Helliwell et al., 2003, Jenkins et al., 1990). Usually the time period concerned is 50 years or less, because of a well-justified feeling that predictions become too uncertain over longer periods. This paper is different: it extrapolates for 500 years. It uses data from a small (0.93 km2) forested catchment in southern England. The MAGIC acidification model was calibrated to the catchment for the year 2000, run back in time for 200 years to assess pre-industrial conditions and forward in time for 500 years to explore future trajectories, given current (2016) knowledge of deposition trajectories. Extrapolating for 500 years may seem unduly speculative in view of the uncertainty over likely environmental change during that time, but the aim of the work is not to make predictions, but rather to explore the implications of current knowledge, and the credibility of current assumptions in the long term. Because observed data on the catchment span 36 years with dramatic changes in deposition, it is possible to apply validation tests to check whether the model predicts the right direction and magnitude of observed change. As well as changes in stream chemistry, a set of soil samples taken in a precise location in 1978 and again in 2013 is used for this purpose. The results are used to discuss the long-term implications of continued low levels of acid deposition for such unmanaged catchments.
Section snippets
Study site
The study was conducted in the Tillingbourne Catchment in SE England. The catchment is described in detail in Hill et al. (2002): a brief summary follows. The Tillingbourne catchment is located 7 km south west of Dorking in SE England, Latitude 51° 11′ N., Longitude 0° 22′ W. (Fig. 1). It consists of a valley sloping northwards at an angle of about 3°, cut into the dip slope of Leith Hill (297 m), the highest point of SE England. There are no motor roads or habitations in the catchment, which is
Hindcast simulation
Fig. 3 shows the modelled sulphate and nitrate concentrations in the stream from 1800 to 2000, together with averaged observed annual means for the two study periods.
Averaging reduces the variation due to differences in rainfall between years, and hence provides a better comparison with modelled values. Stream sulphate concentration is a smoothed version of atmospheric S deposition, with the peak occurring in 1984, 16 years after the deposition peak in 1968. The reason for this behaviour is the
Discussion and conclusions
These results should be seen not as a prediction of what will happen to this particular catchment, but as an exploration of the consequences of current understanding of acidification and recovery processes given present commitments to emission reduction. A number of caveats need to be taken into account, because the model formulation has been kept as simple as possible to maintain a tight focus on understanding of acidification and recovery. Firstly, soil carbon content has been kept unchanged,
Acknowledgements
The study was funded by two now defunct arms of the UK Government: the Central Electricity Generating Board (1977–82) and the Department of Trade and Industry (1999–2003). They had no influence on the study design, the collection, analysis and interpretation of data, the report writing or the decision to submit this paper for publication. Data were collected and analysed by Sue Cumberland, Tim Hill, Ele Miles and Mike Russell with contributions from many others. The National Trust and Wotton
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2020, Geoderma RegionalCitation Excerpt :Though the acid deposition especially in case of sulphur compounds was reduced significantly at the end of the 20th century (Kopáček and Veselý, 2005; Schöpp et al., 2001), huge amounts of sulphur are accumulated in the acidified forest soils and are still being released, which makes the ecosystem recovery process very slow (Akselsson et al., 2013; Marx et al., 2017; Oulehle et al., 2017; Watmough et al., 2016). Skeffington et al. (2016) predict using a model that the recovery will last for as long as approximately 250 years. Berger et al. (2016) found that though the forest soils slowly recover, the positive changes are not reflected by an improved tree nutrition.
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Present Address: School of Geography and the Environment, University of Oxford, South Parks Road, Oxford OX1 3QY, UK.