Interactions of atmospheric deposition with a mixed hardwood and a coniferous forest canopy at the Lake Clair Watershed (Duchesnay, Quebec)

From 1989 to 1996, ion deposition in precipitation, throughfall, and stemflow were measured under a deciduous and a coniferous stand, located in the Lake Clair Watershed, during the growing and the dormant seasons. During the growing season, throughfall deposition under both stands was significantly depleted in H + and NH 4 + compared with wet deposition, and a significant uptake of NO 3 - was observed under the coniferous canopy. Deposition of Ca 2+ , Mg 2+ , K + , Na + , Cl - , and SO 4 2- was significantly higher in the throughfall than in the wet precipitation. During the growing season, the coniferous stand was more efficient in retaining nitrogen (NH 4 + and NO 3 - ), while H + was more intensively retained in the deciduous stand. Significant interactions between precipitation and forest canopies were also observed during the dormant season: throughfall depositions of Ca 2+ , Mg 2+ , K + , Na + , and Cl - were significantly higher than wet precipitation under both canopies, while throughfall SO 4 2- was significantly enriched only under the coniferous stand. Using a Na + ratio method, foliar leaching was found mostly responsible for the throughfall enrichment on a full-year basis in both stands, with values averaging 61, 73, and 96% of the total throughfall fluxes for Ca 2+ , Mg 2+ , and K + , respectively. Under both stands, net canopy exchange (NCE) of base cations, expressed on a monthly basis, were correlated to water volume and to H + and SO 4 2- deposition. Multiple regression models including wet SO 4 2- deposition and an estimate of dry S deposition, explained up to 88% (Ca 2+ in the coniferous stand) of the variance in base cation NCE under both stands.