Cumulative Effects of Disturbances on Soil Nutrients: Predominance of Antagonistic Short-Term Responses to the Salvage Logging of Insect-Killed Stands

Nutrient cycling generally recovers rapidly following disturbance in forest ecosystems. Concerns have been expressed that the resilience of this function may be altered by enhanced disturbance frequency, and especially by the use of salvage logging. A sudden hemlock looper (Lambdina fiscellaria) outbreak in a boreal forest leading to tree mortality in discrete patches allowed us to evaluate the impact of disturbance type (logging vs insect defoliation) as well as the cumulative effects of both disturbances (that is, salvage logging of defoliated sites) on soil nutrients, providing a rare opportunity to test for interactions between these two disturbances. We assessed, within 2–3 years following treatments, whether both individual disturbances had distinct effects on soils properties and whether their cumulative effects were either additive, synergistic or antagonistic. Defoliation generally increased the soil nutrient concentrations, especially of soluble nitrogen forms, when compared to the undisturbed controls. The spatial heterogeneity of soil nutrient concentrations was also much greater in defoliated stands than in the other treatments. Logging alone had an effect that was less variable and generally of lower amplitude than defoliation. Contrary to what was expected, salvage logging led to soil properties that were more similar to those of undisturbed controls, indicating that antagonistic interactions occurred. Our results showed that, under the conditions of this study, salvage logging could reduce the potential for soil nutrient leaching and lead to more homogenous soil conditions. These results bring new light on the effect of cumulative disturbances and could be useful to guide post-disturbance forest management decisions.