Trends in surface water chemistry in afforested Welsh catchments recovering from acidification, 1991-2012.

A key criterion of the UK Government's policy on sustainable forest management is safeguarding the quality and quantity of water. Forests and forestry management practices can have profound effects on the freshwater environment. Poor forest planning or management can severely damage water resources at great cost to other water users; in contrast good management that restores and maintains the natural functions of woodland can benefit the whole aquatic ecosystem. Forests and forest management practices can affect surface water acidification. Monitoring of water chemistry in ten forest and two moorland acid-sensitive catchments in upland Wales commenced in 1991. The streams were selected to supplement the United Kingdom Upland Waters Monitoring Network (UWMN) with additional examples of afforested catchments. Analysis of 22 years of water chemistry data revealed trends indicative of recovery from acidification. Excess sulphate exhibited a significant coherent decline, accompanied by increases in pH and "charge-balance based" acid neutralising capacity (CB-ANC). Alkalinity and "alkalinity-based" acid neutralising capacity (AB-ANC) exhibited fewer trends, possibily due to the variable responses of the organic - carbonate species to increasing pH in these low alkalinity streams. Whilst total anthropogenic acidity declined, dissolved organic carbon and Nitrate-Nitrogen (NNO3) concentrations have risen, and the contribution of NNO3 to acidification has increased. Between-stream variability was analysed using Principal Component Analysis of the trend slopes. Hierarchical clustering of the changes in stream water chemistry indicated three distinct clusters with no absolute distinction between moorland and forest streams. Redundancy analysis was used to test for significant site-specific variables that explained differences in the trend slopes, with rainfall, crop age, base cation concentration and forest cover being significant explanatory variables.

Comparison of different critical load approaches for assessing streamwater acid-sensitivity to broadleaf woodland expansion.

Due to its potential adverse effects on freshwater acidification, risk assessments of the impacts of forest expansion on surface waters are required. The critical load methodology is the standard way of assessing these risks and the two most widely used models are the Steady-State Water Chemistry (SSWC) and First-order Acidity Balance (FAB) models. In the UK the recommended risk assessment procedure for assessing the impact of forest expansion on freshwater acidification uses the SSWC model, whilst the FAB model is used for guiding emission policy. This study compared the two models for assessing the sensitivity of streamwater to acidification in 14 catchments with different proportions of broadleaf woodland cover in acid-sensitive areas in the UK. Both models predicted the exceedance of streamwater critical loads in the same catchments, but the magnitudes of exceedance varied due to the different treatment of nitrogen processes. The FAB model failed to account for high nitrogen leaching to streamwater, attributed to nitrogen deposition and/or fixation of nitrogen by alder trees in some study catchments, while both models underestimated the influence of high seasalt deposition. Critical load exceedance in most catchments was not sensitive to the use of different acid neutralising capacity thresholds or runoff estimates, probably due to the large difference between critical load values and acidic deposition loadings. However, the assessments were more sensitive to differences in calculation procedure in catchments where nitrogen deposition was similar to the availability of base cations from weathering and/or where critical load exceedance values were <1keqH(+)ha(-1)yr(-1). Critical load exceedance values from both models agreed with assessments of acid-sensitivity based on indicator macroinvertebrates sampled from the study catchments. Thus the methodology currently used in the UK appears to be robust for assessing the risk of broadleaf woodland expansion on surface water acidification and ecological status.

Effects of broadleaf woodland cover on streamwater chemistry and risk assessments of streamwater acidification in acid-sensitive catchments in the UK.

Streamwater was sampled at high flows from 14 catchments with different (0-78%) percentages of broadleaf woodland cover in acid-sensitive areas in the UK to investigate whether woodland cover affects streamwater acidification. Significant positive correlations were found between broadleaf woodland cover and streamwater NO3 and Al concentrations. Streamwater NO3 concentrations exceeded non-marine SO4 in three catchments with broadleaf woodland cover>or=50% indicating that NO3 was the principal excess acidifying ion in the catchments dominated by woodland. Comparison of calculated streamwater critical loads with acid deposition totals showed that 11 of the study catchments were not subject to acidification by acidic deposition. Critical loads were exceeded in three catchments, two of which were due to high NO3 concentrations in drainage from areas with large proportions of broadleaved woodland. The results suggest that the current risk assessment methodology should protect acid-sensitive catchments from potential acidification associated with broadleaf woodland expansion.

An investigation of the impact of afforestation on stream-water chemistry in the Loch Dee catchment, SW Scotland.

The impact of conifer afforestation on stream-water chemistry was investigated in the acidified catchment of Loch Dee, SW Scotland. Long-term trends in stream-water chemistry were evaluated during a period of forest growth from age 6 to 17 years. A significant increase was observed for pH (0.2 units) and a significant decline for aluminium (0.05 mg litre(-1)), sulphate (1.2 mg litre(-1)) and nitrate (0.02 mg litre(-1)) concentrations. The long-term decrease in stream-water acidity was ascribed to the marked reductions in sulphur depositions during the 1970s and early 1980s. There was no evidence that this response had been attenuated by afforestation, the improvements in stream-water chemistry being of a similar magnitude to those recorded in nearby moorland lochs and exceeding that in an adjacent moorland-catchment stream. The lack of a clear forest acidification effect is consistent with deposition-model estimates which show the increased scavenging of occult and dry deposition by the growing forest to be small at this site (