Responses of forest ecosystems in Europe to decreasing nitrogen deposition

Elevated nitrogen (N) deposition can cause substantial changes in forest ecosystems. In Europe, N deposition increased for several decades until it peaked around 1990. Since then, moderate decreases in N deposition have been observed especially in Central and Western Europe where deposition levels are highest. We reviewed observational studies on the responses of forest ecosystems in Europe to the decrease in N deposition. We focused on five indicators, i.e. soil acidification and eutrophication, understory vegetation, tree nutrition (foliage concentrations), tree vitality and tree growth. We found a limited decrease in soil solution nitrate concentrations and a potential reaction in foliar N concentrations but no broad-scale responses in understory vegetation, tree growth, or vitality. Results from experimental studies supported the observation of a greater sensitivity of soil solution and foliar element concentrations to changes in N supply. Despite the decline in N deposition in the last 30 years, deposition rates are still high compared to pre-industrial levels in most parts of Europe and future decreases are expected to be small. We hypothesize that the past and future decline in N deposition alone is unlikely to reverse the effects of N saturation in Europe’s forests. Instead, the reaction of soil N pools to climate change and forest management might be decisive for forest ecosystem N cycling in the future.