Canopy Interaction with Precipitation and Sulphur Deposition in Two Boreal Forests of Quebec, Canada

The interaction of atmospheric sulphur (S) was investigated within the canopies of two boreal forests in Québec, Canada. The net canopy exchange approach, i.e. the difference between S-SO(4) in throughfall and precipitation, suggests high proportion of dry deposition in winter (up to 53%) as compared to summer (1-9%). However, a 3.5‰ decrease in δ(18)O-SO(4) throughfall in summer compared to incident precipitation points towards a much larger proportion of dry deposition during the warm season. We suggest that a significant fraction of dry deposition (about 1.2 kg ha(-1) yr(-1), representing 30-40% of annual wet S deposition) which contributed to the decreased δ(18)O-SO(4) in throughfall was taken up by the canopy. Overall, these results showed that, contrary to what is commonly considered, S interchanges in the canopy could be important in boreal forests with low absolute atmospheric S depositions.