Abstract
Atmospheric deposition to forests has been monitored within the International Cooperative Programme on Assessment and Monitoring of Air Pollution Effects on Forests (ICP Forests) with sampling and analyses of bulk precipitation and throughfall at several hundred forested plots for more than 15 years. The current deposition of inorganic nitrogen (nitrate and ammonium) and sulphate is highest in central Europe as well as in some southern regions. We compared linear regression and Mann–Kendall trend analysis techniques often used to detect temporal trends in atmospheric deposition. The choice of method influenced the number of significant trends. Detection of trends was more powerful using monthly data compared to annual data. The slope of a trend needed to exceed a certain minimum in order to be detected despite the short-term variability of deposition. This variability could to a large extent be explained by meteorological processes, and the minimum slope of detectable trends was thus similar across sites and many ions. The overall decreasing trends for inorganic nitrogen and sulphate in the decade to 2010 were about 2% and 6%, respectively. Time series of about 10 and 6 years were required to detect significant trends in inorganic nitrogen and sulphate on a single plot. The strongest decreasing trends were observed in western central Europe in regions with relatively high deposition fluxes, whereas stable or slightly increasing deposition during the last 5 years was found east of the Alpine region as well as in northern Europe. Past reductions in anthropogenic emissions of both acidifying and eutrophying compounds can be confirmed due to the availability of long-term data series but further reductions are required to reduce deposition to European forests to levels below which significant harmful effects do not occur according to present knowledge.
Highlights
Forest ecosystems have been exposed to increased atmospheric deposition of sulphur (S) in the form of sulphate (SO42À) and inorganic nitrogen (N) since the 1950s, resulting from anthropogenic emissions of sulphur dioxide (SO2), nitrogen oxides (NOx) and ammonia (NH3)
The methods used in the various countries (France: Ulrich and Lanier, 1993; Norway: Kvaalen et al, 2002; Moffat et al, 2002; Italy: Mosello et al, 2002; Switzerland: Thimonier et al, 2005; Finland: Lindroos et al, 2006; Denmark: Gundersen et al, 2009; Czech Republic: Bohacova et al, 2010; Latvia: Lazdin s et al, 2010; United Kingdom: Vanguelova et al, 2010; Swedish Throughfall Monitoring Network (SWETHRO): Pihl Karlsson et al, 2011; Belgium: Verstraeten et al, 2012) follow the ICP Forests manual
The laboratory results are checked for internal consistency based on the conductivity, the ion balance, the concentration of total N and the sodium to chloride (Na/Cl) ratio, and analyses are repeated if suspicious values occur (Mosello et al, 2005, 2008; ICP Forests, 2010)
Summary
Forest ecosystems have been exposed to increased atmospheric deposition of sulphur (S) in the form of sulphate (SO42À) and inorganic nitrogen (N) since the 1950s, resulting from anthropogenic emissions of sulphur dioxide (SO2), nitrogen oxides (NOx) and ammonia (NH3). Deposition of these compounds is a major driver for various changes in forest ecosystems. Eutrophication effects include loss of nutrients by leaching, elevated nitrate (NO3À) levels in percolation and runoff water (Dise et al, 2009), nutrient imbalances in trees, and altered susceptibility to pests and diseases (Flückiger and Braun, 1999). Deposition assessments in long time series are required (i) to monitor the success of these measures in reducing deposition and (ii) to investigate the impact of deposition on the long-term stability of forest and its ecosystem services at selected intensively monitored sites (Paoletti et al, 2010)
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