Progressing understanding of episodic stream acidification in upland plantation conifer forested subcatchments in Ireland

Episodic stream acidification associated with managed conifer forests in upland acid-sensitive subcatchments, sometimes referred to simply as ‘the forest effect’, is among the most scientifically complex water quality challenges facing Irish foresters and water quality managers today. The lack of adequate understanding of this evolving phenomenon has arguably stifled the emergence of a universally accepted set of mitigating management measures. This study addresses four general areas of the topic in which a further progression of understanding may advance efforts to reach a consensus on how and where to introduce or implement policy modifications. First, data from two previous studies are combined and supplemented with new data from this study and all are reanalysed to verify the existence of the forest effect in nearly all types of Ireland’s acid-sensitive subcatchments with substantial managed conifer forest cover. Second, the various mechanisms, or drivers, by which the streams are acidified, are explored in further detail. In contrast with the findings of studies two decades ago, no evidence of significant acidification effects from pollutant deposition from the atmosphere in the East of Ireland was found in more recent data. Third, the factors external to the forest potentially contributing to acidification are tested for associations with increasing stream acidity. In some cases, associations between stream acidity and subcatchment and meteorological variables were found to be the opposite of those found in studies of other northern European subcatchments. Finally, the question is answered in the affirmative as to whether such potentially causal variables, including per cent of subcatchment forested, could be used in regression analysis to allow geology and soil-specific forest cover thresholds critical to proper management of water quality to be predicted. Conclusions, some unprecedented in Ireland, are made about the extent of evidence for the forest effect and its associated drivers and the explanatory powers of potentially acidifying variables. Recommendations are offered consistent with continuing efforts to develop models to predict forest effect acidification at the subcatchment level.