Forests take up gaseous, liquid, and particulate substances that are present in the air. Although considered nutrients on the one hand, nitrogen inputs exceeding the critical load that can be absorbed by an ecosystem act as pollutants. This chapter outlines the effects of N deposition to forest ecosystems and discusses recent progress that has been made to more accurately quantify dry deposition, which at many a forest location in Europe is larger than wet and occult deposition. To quantify the effects of N deposition on tree growth, a good measure for net ecosystem production (NEP) is needed. Eddy covariance (EC) flux measurements are one established way to quantify NEP. While EC flux measurements are costly and remain restricted in their application to a few suitable locations, dendrometer measurements with high temporal resolution show a similar seasonal and annual signal to NEP. Such measurements are becoming increasingly important to quantify ecosystem biomass accumulation, which can be related to N deposition rates. The policy relevance of such activities emerges from the UNECE's Gothenburg protocol to abate acidification, eutrophication and ground-level ozone, but also the quantification of natural sinks under the Kyoto protocol profits from such measurements.

Eugster W & Haeni M. 2013. Chapter 3 — Nutrients or pollutants? N deposition to European forests. Pages 37–56 of: Matyssek R, Clarke N, Cudlin P, Mikkelsen TN, Tuovinen JP, Wieser G & Paoletti E (eds), Climate Change, Air Pollution and Global Challenges — Understanding and Perspectives from Forest Research, Vol. 13. Elsevier.