Grubbing by wild boars (Sus scrofa L.) and its impact on hardwood forest soil carbon dioxide emissions in Switzerland

Interest in soil C storage and release has increased in recent years. In addition to factors such as climate/land-use change, vertebrate animals can have a considerable impact on soil CO₂ emissions. To date, most research has considered herbivores, while the impact of omnivorous animals has rarely been investigated. Our goal was to determine how European wild boars (Sus scrofa L.), large omnivores that consume soil-inhabiting animals and belowground plant parts by grubbing in the soil, affect soil C dynamics. We measured soil respiration (CO₂), temperature, and moisture on paired grubbed and non-grubbed plots in six hardwood forest stands for a 3-year period and sampled fine root and microbial biomass at the beginning and after 2 years of the study. We also measured the percentage of freshly disturbed forest soil within the larger surroundings of each stand and used this information together with hunting statistics and forest cover data to model the total amount of CO₂ released from Swiss forest soils due to grubbing during 1 year. Soil CO₂ emissions were significantly higher on grubbed compared to non-grubbed plots during the study. On average 23.1% more CO₂ was released from these plots, which we associated with potential alterations in CO₂ diffusion rates, incorporation of litter into the mineral soil and higher fine root/microbial biomass. Thus, wild boars considerably increased the small-scale heterogeneity of soil properties. Roughly 1% of Switzerland’s surface area is similar to our sites (boar density/forest cover). Given the range of forest soil disturbance of 27–54% at our sites, the geographic information system model predicted that boar grubbing would lead to the release of an additional 49,731.10–98,454.74 t CO₂ year⁻¹. These values are relatively small compared to total soil emissions estimated for Swiss hardwood forests and suggest that boars will have little effect on large-scale emissions unless their numbers increase and their range expands dramatically.