The condition of forests in Europe. Executive report 2013

Just over three-quarters (78%) of the atmosphere comprises inert nitrogen which has very limited availability for biological use. All terrestrial ecosystems need reactive nitrogen and historically this has been in short supply. Artificial nitrogenous fertilizers and fossil fuel combustion are both sources of reactive nitrogen and their use has dramatically altered the global nitrogen cycle. 1. ATMOSPHERIC DEPOSITION Nitrogen deposition largely originates from fossil fuel combustion and animal husbandry. Deposition is highest in central Europe. Only a minor decrease in deposition has been measured on intensive monitoring plots over the past decade. 2. LICHENS Lichens are very sensitive indicators of nitrogen deposition. Deposition is high on 75% of the European forest plots and this is reflected in the change in lichen species composition over time. 3. MOSSES Mosses absorb most of their nutrients and water across their surface. As a result, they are directly affected by atmospheric deposition. In a study carried out in Austria, Croatia, Italy, and Slovenia the nitrogen content in mosses significantly increased with increasing nitrogen deposition (see p. 10). 4. FUNGI Fungal species diversity and structure significantly decrease with increasing nitrogen deposition. On a European transect through nine countries, 393 mycorrhizal species were determined. These fungal types live in symbiosis with tree roots and play a major role in nutrient uptake by trees (see p. 12). 5. SOIL SOLUTION An analysis of trends on the Level II plots showed exceedances of critical limits for nitrogen in the subsoil on 50% of the plots. Leaching is mainly dependent on nitrogen deposition with other factors playing a relatively minor role. 6. LITTERFALL Litterfall and its decomposition are critical processes for transferring nutrients from above-ground forest biomass to soils. Tree species composition (and thus foliar litter chemistry) affects nitrogen cycling rates at the scale of entire forest stands; for example, the scale of the nitrogen transfer in litterfall determines the amount of nitrogen available for tree growth. 7. SOIL SOLID PHASE Soils play a key role in nitrogen cycling and storage within ecosystems. They host nitrogen-fixing microorganisms, as well as those that release nitrogen back into the air. Mineralization of organically-bound nitrogen takes place within soils and converts nitrogen to a form available for tree growth. Loss of nitrogen from the soil is mainly through harvesting and leaching. One of the key questions concerns the saturation status of European forest soils. 8. GROUNDWATER QUALITY Rain water containing dissolved nitrogen is pulled downwards through the soil by gravity, this is known as leaching. Leaching may affect the quality of drinking water pumped up from groundwater. In forests on sandy soils in the Netherlands nitrogenleaching has decreased by 55% over the past 20 years, showing the success of emission reduction policies. 9. EFFECTS ON TREES 9A FOLIAGE NUTRIENT BALANCE Soil nitrogen generally stimulates plant growth. However, excess nitrogen can cause other nutrients such as magnesium to become deficient. This can affect forest health and enhance the effects of additional stress factors. Nutrient im balances were detected in leaves and needles on 10% of the nitrogen-saturated plots. 9B CROWN CONDITION (TREE HEALTH) Effects of nitrogen deposition on leaf and needle loss can be detected at a local level for some tree species. These impacts are compounded by the effects of weather, insects and diseases, and soil condition. 9C STEM GROWTH Nitrogen acts as fertilizer for trees. At sites with low nitrogen soil levels, atmospheric nitrogen inputs increase growth. But on nitrogen-saturated plots, even high atmospheric nitrogen inputs have no impact on tree growth. 10. CONCLUSIONS Assessing the input of nitrogen to forests and the direct and indirect responses of forest trees are core activities of the large-scale and intensive monitoring of forest ecosystems in Europe. ICP Forests provides policy makers with key information for forest management, especially under the projected future climatic changes