Effects of wild boar (Sus scrofa) on soil processes and tree growth

Abstract: Wild boars (Sus scrofa) have increased rapidly in population size and home range since their reintroduction to the Swedish fauna. Through their rooting behaviour, the boars can cause great disturbance to the ecosystem. The consequences of this disturbance have been recorded in many parts of the world but the effects they have on boreal forest ecosystems have been less studied.In this study I examined to what extent wild boars affect (i) the soil microbial community, (ii) the soil respiration, (iii) the soil C and nutrient content, as well as (iv) the growth of trees in a boreal forest in central Sweden. Enclosures that have harboured wild boars for seven years were compared with nearby areas without wild boars in both a pine forest and a spruce forest. Using phospholipid fatty acid (PLFA) analyses, the soil microbial biomass, community, and fungi:bacteria ratio were investigated in the treatments. The soil respiration was measured monthly from June to November. The total soil concentrations of C, N and P were examined as well as the availability of N in the form of NO3-and NH4+. Tree growth was measured by measuring tree ring width and comparisons were made by looking at the relative ring width of tree cores before and after the introduction of wild boars. I found that the soil microbial community did generally not differ between the treatments, except for a higher fungi:bacteria ratio within the enclosures. Soil respiration was consistently lower in the enclosures throughout all months. Soil nutrient levels were similar between the treatments except for a slightly lowertotal P content and NO3-content in the pine forest enclosure. Lastly, the tree growth was severely decreased by the presence of wild boars. In summary my study suggests that anincrease of the wild boar population in boreal forests may have a minor impact on many soil processes, while tree growth can be hampered. However, studies in natural ecosystems are needed to investigate this effect further.