Chapter 13 - Responses of forest ecosystems in Europe to decreasing nitrogen deposition

Abstract

Average nitrogen (N) deposition across Europe has declined since the 1990s. This resulted in decreased N inputs to forest ecosystems especially in Central and Western Europe where deposition levels are highest. While the impact of elevated atmospheric N deposition on forests has been receiving much attention for decades, ecosystem responses to the decline in N inputs received less attention. Here, we review observational studies reporting on trends in a number of indicators related to: Soil acidification and eutrophication, understory vegetation, tree nutrition (foliar element concentrations) as well as tree vitality and growth in response to decreasing N deposition across Europe. The reported trends were generally heterogeneous across Europe. Several studies found a limited decrease in soil solution nitrate concentrations, which is likely an ecosystem response to decreasing N deposition. There was no large-scale response in understory vegetation, tree growth, or vitality. Negative trends in foliar N concentrations are likely caused to a substantial degree by an increase in foliar mass (dilution effect). Experimental studies support the observation of a more distinct reaction of soil solution to changes in N supply compared to tree growth, tree vitality and understory vegetation. According to the most likely scenarios, further decrease of N deposition will be limited. We hypothesize that this expected reduction will not cause major responses of the parameters analyzed in this chapter. Instead, future changes might be more strongly controlled by the effects of climate change and forest management on the N pools accumulated within forest soils.