-In order to quantify the total deposition and canopy uptake of N in spruce, plastic Christmas trees were established next to real spruce trees of approximately similar height in clearings of the Strengbach catchment (Vosges mountains, France). Bulk precipitation and throughfall (TF) composition under the artificial trees, the real spruce trees, and in 3 spruce stands aged 15, 35 and 90 years were monitored during 8.5 months. Fluxes of inorganic N, SO 4 2-and Na + in throughfall below plastic trees were 50% (N) to 30% (Na + ) higher to those in bulk precipitation, whereas the flux of water was 30% lower. Net throughfall (NTF) fluxes of NH 4 + were higher below the plastic trees than below the real trees, whereas the reverse was true for NO 3 -. The ratio between total inorganic nitrogen and Na + (or SO 4 2-) in NTF was higher below plastic trees than below closed spruce stands. Assuming that foliar leaching of Na + (or SO 4 2-) was negligible, the ratios between Na + (or SO 4 2-) fluxes in net throughfall under living and plastic canopies were used as deposition indexes, and dry + occult deposition of N was computed for the real spruce trees in the clearings as well as in the 3 closed stands. Results confirmed a significant canopy uptake of dry + occult deposited N and especially NH 4 + by spruce stands. + (pluviolessivat moins pluie) sous les arbres artificiels était supérieur à celui sous les arbres réels, mais inférieur pour le NO 3 -. Le rapport N minéral/ or SO 4 2-(ou Na + ) dans le pluviolessivat net sous les arbres artificiels était supérieur à celui mesuré sous les peuplements fermés. En admettant qu'absorption et lessivage foliaire de Na + et SO 4 2-sont négligeables, nous avons utilisé le rapport entre le pluviolessivat net de Na + (ou or SO 4 2-) sous arbres (ou peuplements) réels et artificiels comme un index de dépôt occulte, et le rapport N minéral/ Na + (ou or SO 4 2-) dans le pluviolessivat net sous les arbres artificiels comme un caractéristique intrinsèque du dépôt occulte. Ceci nous a permis de calculer le dépôt occulte d'azote minéral et l'absorption foliaire dans les différents peuplements étudiés. Les résultats confirment une absorption foliaire d'azote, et plus particulièrement d'NH 4 + , très significative pour la nutrition des peuplements forestiers.dépôt sec / azote / pluviolessivat / secrétion / Picea abiès Ann. For. Sci. 57 (2000) 113-120 113

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Rain and throughfall chemistry has been monitored for five years (1987)(1988)(1989)(1990)(1991) in three adjacent stands (Norway spruce, Silver fir and Scots pine) situated at an altitude of 1500 m above sea level in the Rhodopes mountains (South-Western Bulgaria). Throughfall collectors have been set up near the stem, below the opening between the crowns, and halfway between the stem and the border of the tree crown. Pollutant concentration in bulk precipitation strongly increased during the period of investigation. Because base cation deposition increased together with sulfate deposition, the increase in proton deposition remained moderate. The increase in throughfall concentration was parallel for different tree species. The time course of mineral concentration in throughfall was approximately similar to that of rainfall but the inter-event variability in concentration was reduced. Net throughfall fluxes of Ca, Mg and K increased during the study period whereas net throughfall fluxes of C1, Na and S remained more stable. Although the wet deposition of protons increased, the proton input in net throughfall decreased or remained stable in relation to the increase in base cation concentration. This suggests a low dry deposition in the study region and conversely an increased net leaching of base cations. However, the seasonal variations in net throughfall as well as the time course of the difference between the concentrations of throughfall samples collected at different distances from the stems indicates that most of the increase in the net throughfall of Ca and Mg was due to dry deposition. The difficulties associated with the use of ions such as Na as deposition tracers for other base cations is emphasized. Fig. 1. Location map.