Understorey communities can dominate forest plant diversity and strongly affect forest ecosystem structure and function. Understoreys often respond sensitively but inconsistently to drivers of ecological change, including nitrogen (N) deposition. Nitrogen deposition effects, reflected in the concept of critical loads, vary greatly not only among species and guilds, but also among forest types. Here, we characterize such context dependency as driven by differences in the amounts and forms of deposited N, cumulative deposition, the filtering of N by overstoreys, and available plant species pools. Nitrogen effects on understorey trajectories can also vary due to differences in surrounding landscape conditions; ambient browsing pressure; soils and geology; other environmental factors controlling plant growth; and, historical and current disturbance/management regimes. The number of these factors and their potentially complex interactions complicate our efforts to make simple predictions about how N deposition affects forest understoreys. We review the literature to examine evidence for context dependency in N deposition effects on forest understoreys. We also use data from 1814 European temperate forest plots to test the ability of multi-level models to characterize context-dependent understorey responses across sites that differ in levels of N deposition, community composition, local conditions and management history. This analysis demonstrated that historical management, and plot location on light and pH-fertility gradients, significantly affect how understorey communities respond to N deposition. We conclude that species' and communities' responses to N deposition, and thus the determination of critical loads, vary greatly depending on environmental contexts. This complicates our efforts to predict how N deposition will affect forest understoreys and thus how best to conserve and restore understorey biodiversity. To reduce uncertainty and incorporate context dependency in critical load setting, we should assemble data on underlying environmental conditions, conduct globally distributed field experiments, and analyse a wider range of habitat types.

Department of Biology University of West Florida Pensacola FL 32514 USA

Department of Botany Faculty of Biological Sciences University of Wrocław Kanonia 6 8 PL 50 328 Wrocław Poland

Department of Botany Faculty of Science Palacký University in Olomouc Šlechtitelů 27 CZ 78371 Olomouc Czech Republic

Department of Botany University of Wisconsin Madison Madison WI 53706 USA

Department of Ecology University of Rzeszów ul Rejtana 16C PL 35 959 Rzeszów Poland

Department of Forest Genetics and Plant Physiology Swedish University of Agricultural Sciences Umeå Plant Science Centre 901 83 Umeå Sweden

Department of GIS and Remote Sensing Institute of Botany The Czech Academy of Sciences Zámek 1 CZ 252 43 Průhonice Czech Republic

Department of Plant Systematics Ecology and Theoretical Biology Eötvös Loránd University Pázmány P sétány 1 c H 1117 Budapest Hungary

Department of Silviculture and Forest Ecology of the Temperate Zones University of Göttingen Büsgenweg 1 D 37077 Göttingen Germany

Department of Vegetation Ecology Institute of Botany The Czech Academy of Sciences Lidická 25 27 CZ 60200 Brno Czech Republic

Department of Wildlife Ecology and Conservation University of Florida Gainesville FL 32611 USA

Ecosystem Restoration and Intervention Ecology Research Group School of Biological Sciences The University of Western Australia 35 Stirling Highway Crawley WA 6009 Australia

Faculty of Forestry and Wood Sciences Czech University of Life Sciences Prague Kamýcká 129 CZ 165 00 Prague 6 Suchdol Czech Republic

Faculty of Science and Technology Free University of Bozen Bolzano Piazza Università 5 39100 Bozen Bolzano Italy

Forest and Nature Lab Department of Environment Faculty of Bioscience Engineering Ghent University Geraardsbergsesteenweg 267 9090 Melle Gontrode Belgium

Institute of Ecology and Evolution Friedrich Schiller University Jena Dornburger Str 159 DE 07743 Jena Germany

Museum of Natural History University of Wrocław Sienkiewicza 21 PL 50 335 Wroclaw Poland

National Ecological Observatory Network 1685 38th St Suite 100 Boulder CO 80301 USA

National Forest Centre T G Masaryka 22 960 92 Zvolen Slovakia

Senckenberg Biodiversity and Climate Research Centre Frankfurt Germany

Spittelauer Lände 5 1090 Vienna Austria

Swedish University of Agricultural Sciences Southern Swedish Forest Research Centre 230 53 Alnarp Sweden

Technical University in Zvolen Faculty of Forestry T G Masaryka 24 960 53 Zvolen Slovakia

Vegetation Ecology and Conservation Biology Institute of Ecology FB 2 University of Bremen Leobener Str 5 DE 28359 Bremen Germany

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