The influence of forestry on acidification and recovery: Insights from long-term hydrochemical and invertebrate data

The profile of acidification has declined markedly in recent years, despite significant impacts remaining across large areas of Europe and North America. While evidence of chemical recovery has been reported, the apparent magnitude of improvements depend on the indicator(s) used, the quality of data and length of time series. Evidence of biological recovery is more elusive and questions remain as to the historical and contemporary effects of forestry on acid impacts and recovery. This study uses long term data (1976-2009) of unusually high quality and consistency to assess evidence of chemical and biological recovery in eight streams with contrasting landuse. Streams ranged from highly acidic (median annual pH 4.1) to circum-neutral (pH 7.1). Significant reductions in non-marine sulphate (NM-SO4) concentrations closely followed reported trends in deposition. Reductions in NMSO4 delivered significant increases in pH and ANC, and reductions in toxic inorganic labile aluminium (L-Al). Significant non-linear increases in dissolved organic carbon (DOC) were also observed, potentially offsetting some of the improvements in acid status associated with reductions in NM-SO4. Streams draining large areas of mature or 2nd phase forestry were characterised by greater NM-SO4 and L-Al concentrations, and lower pH and ANC than sites with a more modest forestry influence or moorland vegetation. Recovery (reductions in NM-SO4 and L-Al concentrations and increases in pH and ANC) at sites with a strong forestry influence was greater than observed for the moorland catchment. Nonetheless, relative inter-site differences persisted, indicating the continued influence of forestry on hydochemical conditions under contemporary conditions and the legacy of forestry effects from previous decades. Invertebrate assemblages showed strong relationships with acidification gradients. Constrained ordination indicated that both spatial differences and temporal trends in community structure were strongly associated with pH (and other correlated variables such as ANC) and L-AL. Despite their geographic proximity, there was complete turnover of species between some of the streams, reflecting differences in acid status. Non-linear temporal trends consistent with ecological recovery from acidification were detected in all streams, regardless of their absolute chemical status. Important questions remain as to the most appropriate way to assess biological recovery given the complicated chemical and biological interactions, the often non-linear nature of change, the potential for chemical and biological hysteresis associated with acidification and recovery, and ongoing debates over recovery endpoints. Consequently, the definition of appropriate recovery endpoints required for the management of landuse and freshwaters remains a significant challenge. Crown Copyright (C) 2012 Published by Elsevier Ltd. All rights reserved.