Low root biomass and occurrence of ectomycorrhizal exploration types in inhabited wood ant ( Formica polyctena ) nests in a temperate spruce forest

Trees growing in nutrient-limited temperate forest soils can gain nutrients by root proliferation into nutrient-rich hotspots and/or by forming mycorrhizal symbioses. In this study we investigated the effects of nutrient-rich hotspots (inhabited wood ant nests) on Norway spruce root biomass and occurrence of ectomycorrhizal (EcM) exploration types. Substrates were collected from the mineral soil layer in a temperate middle-European spruce forest (Nationalpark Harz, Germany) from four micro-regions within each of the five wood ant nests sampled, i.e. 1) centre of the belowground part of a nest, 2) nest's rim, 3) nest's run-off zone (ca. 20 cm from nest's rim), and 4) from the surrounding forest soil (>10 m from nest's rim). Root biomass, EcM exploration types, moisture and nutrient contents were determined in all substrates. Although naturally enriched, wood ant nests had neither root biomass nor occurrence of EcM exploration types higher compared to the surrounding forest soil. The probable main reasons were high phosphorus content and low moisture maintained inside inhabited nests, although the effect was not significant. Apparently, other substrate properties not determined in our study also affect the occurrence of exploration types in wood ant nest substrates. Inhabited wood ant nests thus seem unfavourable for root proliferation and their subsequent EcM colonization. However, roots can gain nutrients from the run-off zone around nests where low moisture is not maintained and nutrient contents are higher due to leaching from the nest substrate.