Effects of a spruce budworm outbreak on element export below the rooting zone: a case study for a balsam fir forest

Spruce budworm outbreaks are among the major natural disturbances affecting the dynamics and functioning of Canadian boreal forests. However, the element losses potentially associated with spruce budworm outbreaks have not been quantified. We evaluated the influence of spruce budworm outbreaks on nutrient export from boreal forest soils by comparing nutrient leaching losses during a spruce budworm outbreaks episode (1981-1984) to an unperturbed period (1999-2003) in a calibrated catchment located in a balsam fir forest. Nutrient soil leaching losses were significantly higher during the spruce budworm outbreaks (1981 1984) for N-NO3 (30.1 fold), K (8.3 fold), N-NH4 (6.2 fold), Mg (2.7 fold) and SO4 (2.2 fold), as compared to an unperturbed period (1999-2003). When the recurrence of spruce budworm outbreaks (33 years) and a plausible average length of such events (5 years) are taken into consideration, it is estimated that in the long term, 5.6 more NO3, 1.5 more K and 1.2 more NH4 are leached from the soil profile during outbreaks. The important leaching losses during spruce budworm outbreaks, when added to the losses due to tree harvesting and fire (and acid deposition for K), may have considerable effects on soil fertility and ecosystem sustainability.