Modelling Interacting Effects of Invasive Earthworms and Wildfire on Forest Floor Carbon Storage in the Boreal Forest.

In forest ecosystems, earthworms and wildfire are two ecological agents that cause carbon (C) stored in the forest floor to be transferred to the atmosphere as greenhouse gases, either through heterotrophic respiration (earthworms) or through periodical combustion (wildfire).  For centuries, wildfire has been an important ecological driver in the boreal forests of Canada where most fire emissions to the atmosphere originate from the forest floor.  In contrast, earthworms are recent invaders, having been introduced to the Canadian boreal during the 20^th century.  We examined stand-level effects of earthworms and wildfire on forest floor C by adapting an earthworm-C simulation model for the boreal and using it in combination with a forest C accounting model.  We combined results from the stand-level simulation with a spatial model of earthworm spread to calculate the total predicted change in C storage at the landscape-level following earthworm invasions in northeastern Alberta.  Within 35-40 years after initiation of the invasion forest floor C stocks were reduced by 49.7 to 94.3% depending on earthworm species.  Because earthworm activities reduced the amount of forest floor C available for burning, emissions from wildfire were lower in the presence of earthworms.  Spatial modelling of earthworm effects within a 6 million hectare area projected that forest floor C stocks decreased 50,875 Mg C by 2006, and 2,706,355 Mg C by 2056, compared with the same area if earthworms were not present.  Loss of forest floor C averaged over the 50 year simulation was 10 g m² yr^-1; similar in magnitude to estimates for C loss in the Canadian boreal due to wildfire or harvesting.  These results indicate effects of non-native earthworms on the forest floor should be included in predictions of forest ecosystem C budgets to ensure accurate attribution of emissions to heterotrophic respiration or combustion.