biotic factors such as pests, or abiotic factors such as storms) (hereafter referred as BADo AP), annual total N deposition and a set of variables known to be directly or indirectly related to N deposition, namely foliar N ratios (N/Ca, N/P, N/K, N/Mg) and soil p H, exchangeable base cations (BCE) and C/N ratio in the mineral top soil (Ferretti et al. BADo AP on foliage were evaluated on an ordinal scale for each tree and averaged for each MTS and plot.

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For most compounds, throughfall deposition is usually larger than open field deposition because the former also includes the dry deposition of compounds collected by the tree canopy. Precipitation amounts were measured in a clearing close to the permanent plot.

However, in some of the plots used in this study, throughfall deposition of N compounds (mainly ammonium) was lower than open field deposition because of the active uptake by the foliage (e.g. Chemical analyses of foliar tissues were available for 65 plots, while soil analyses were available for 56 plots, and plot average of the topmost mineral layers was considered.

the sign of the regression coefficient); the number of symbols is equal to the number of models for which the variable obtained a VIP High levels of inorganic N deposition on forest ecosystems due to anthropogenic activities have been reported for decades in Europe and North America.

Most studies have shown how N deposition affects soil and foliar chemistry, tree growth, carbon sequestration and plant species richness and diversity (e.g. was estimated for beech (most of the beech plots have a suboptimal foliar N status), while a negative one was observed for pine, whose foliar N status was low for half of the plots located in Nordic countries where also N deposition is low.

foliar N ratios), while the importance of each variable within the model can be biased by their mutual correlation.).

The most important predictors in the reference models were latitude (for Norway spruce and Scots pine), longitude (for Norway spruce), tree density (for all species), precipitation (for beech) and summer precipitation (for Scots pine) (Tables ).

In this respect, the data collected at the intensive monitoring plots of the International Cooperative Programme on Assessment and Monitoring of Air Pollution Effects on Forests (ICP Forests) are an important asset for two main reasons (Ferretti and Fischer ): Plots cover a wide range of ecological conditions (including a marked N deposition gradient), and—despite some inconsistencies that may always happen in large-scale programs—data are collected according to comparable methods (Ferretti and Fischer ). ) and related variables (soil and foliar nutrients) in defoliation models.

In these plots, monitoring of tree condition is based on the assessment of “defoliation”, a popular (and incorrect) term adopted to mean the reduction of foliage density on trees as compared to a reference standard. To do this, we evaluated the possible, additive influence of N-related predictors and damage to foliage in explaining the spatial variability of ).

Due to the different geographical distribution of tree species, however, the latitudinal gradient was negative for spruce and pine plots (N deposition decreases from central Europe to the Nordic countries) and positive for beech (N deposition decreases from central Europe to southern Italy and France). As foliar and soil variables were correlated among themselves and with N deposition, they were added separately to the reference model.