Wood ants (Formica rufa group) in managed boreal forests: implications for soil properties and tree growth

Mound-building ants, and especially wood ants (Formica rufa group), are wide-spread key species that affect ecosystem functioning in several ways in the boreal forests of Eurasia. Forest clear-cutting harms wood ants, but the colonies gradually recover and multiply as forest succession proceeds. Thus their impact on ecosystem functioning may change with stand age. The objectives of this thesis were to study 1) the distribution of wood ants in boreal forests, 2) the role of wood ant mounds in the distribution of nutrients and roots, and 3) the effect of wood ant-aphid mutualism on the growth of Norway spruce (Picea abies L. Karst.). The 3rd National Forest Inventory data from the 1950s was used to study the distribution of ant mounds along major ecological gradients in Finland. Ecosystem-scale studies were conducted in 5-, 30-, 60and 100-year-old Norway spruce dominated stands in eastern Finland. The highest mound densities were found in spruceand birch-dominated mature forests on medium-fertile sites in southern Finland. Ant mounds were more numerous and larger in older than in younger stands. More mound-building ant species were found in younger stands, while F. aquilonia Yarr. was the only species in the oldest stands. Most ant mounds were situated near stand edges. Carbon (C), nitrogen and phosphorus concentrations were higher in ant mounds than in the surrounding soil. More fine roots were found in the mineral soil under ant mounds than in the surrounding mineral soil. Ant mounds contributed only a small proportion of the total C and nutrient pools and root biomass in forest soil. Ant-aphid mutualism significantly reduced the radial growth of individual spruce trees in the 30-year-old stands only. Overall, the impact on spruce growth was marginal at the stand level. However, wood ants increased the heterogeneity in the spatial distribution of C, nutrients, roots, and tree growth.