Jouni Kilpeläinen

The effect of red wood ant (Formica rufa group) mounds on root biomass, density, and nutrient concentrations in boreal managed forests

Red wood ants (Formica rufa group, RWAs) are common insects in boreal forests in Fennoscandia, and they build large, long-lived mounds as their nests. RWA mounds are enriched with carbon and nutrients, but little information is available about how they affect root distribution and the nutrient uptake of trees. In this study, we investigated the biomass, biomass density, nutrient concentrations, and amounts of fine (<2 mm) and coarse (>2 mm) roots in RWA mounds, and compared them with those of surrounding forest soil in mixed coniferous stands of different age classes in Finland. Neither fine nor coarse root biomasses differed significantly between the aboveground parts of the mounds and the organic layer of the soil. Root biomass density was lower in mounds than in the organic layer. However, fine root biomass and biomass density were higher in the belowground parts of mounds than in the surrounding mineral soil. Macroelement (N, Ca, K, P, S, Mg) and Zn and Cu concentrations in roots in the mounds were significantly higher than those in the organic layer. Root biomass and biomass density did not differ between stands of different age classes. The results of this study indicate that RWA mounds increase heterogeneity in root distribution in forest ecosystems, and also increase the availability of nutrients for plants that extend their roots inside RWA mounds.

Distribution of mound-building ant species (Formica spp., Hymenoptera) in Finland : preliminary results of a national survey

In the first years data of the 10th Finnish National Forest Inventory (2005–2008) 533 active and 178 abandoned ant mounds were found yielding 4.2 mounds ha-1. We found 11 species: Formica rufa, F. polyctena, F. aquilonia, F. lugubris, F. pratensis, F. exsecta, F. fennica, F. pressilabris, F. forsslundi, F. suecica and F. uralensis. Five species occurred throughout the country, and four were restricted to the south. We found species-specific associations with either mineral soils or mires, with forest site type and with tree-canopy openness. Low fertility decreased the occurrence of polygynous species with large worker force, and sun-exposition was favourable for species with smaller colonies presumably because only large worker force enables metabolic thermoregulation of nests. Forest fragmentation and increased amount of edge habitats favourable for colony founding have presumably increased nest density whereas drainage of mires has reduced the amount of habitat of three species since the 1950s.

Distribution of ant species and mounds (Formica) in different-aged managed spruce stands in eastern Finland

Mound‐building ants (Formica spp.), as key species, have large impacts on organisms and ecosystem functions in boreal Eurasian forests. The density, sizes and locations of ant mounds determine the magnitude and the spatial distribution of ant activities in forest ecosystems. Clear‐cutting can destroy wood ant colonies, and the species, abundance, dimensions and locations of ant mounds may change as forest stand structure changes with stand age. We compared ant species composition, ant mound numbers and dimensions, and the spatial distribution of mounds in Norway spruce [Picea abies (L.) Karst.] stands of different age (5, 30, 60 and 100 years) in eastern Finland. The mound density of Formica aquilonia Yarr. was greater in the two oldest stand age classes, while most mounds of Formica rufa L., Formica polyctena Först., Formica lugubris Zett., Formica exsecta Nyl. and Formica pressilabris Nyl. were found in the two youngest age classes. The mean volume, the volume per area and height/diameter ratio of F. aquilonia mounds increased with stand age. In the oldest stand age class, mounds were slightly smaller in well‐lit locations than in shade and near stand edges than further from the edges indicating that new mounds are established in well‐lit locations. Similarly, the longest slopes of the mounds faced south, indicating the importance of exposure to the sun. F. aquilonia mounds were concentrated near stand edges, and the spatial distribution of the mounds was aggregated in some stands. At the ecosystem level, the aggregation of ant mounds near stand edges may increase the edge productivity, as mounds concentrate resources to the edges and release nutrients after abandonment.

Organic mound-building ants : their impact on soil properties in temperate and boreal forests

Ants are important components of most soil invertebrate communities, and can affect the flow of energy, nutrients and water through many terrestrial ecosystems. The vast majority of ant species build nests in the mineral soil, but a small group of ants in temperate and boreal forests of Eurasia and North America build large parts of their nests above‐ground using organic materials collected from the surrounding soil. Many studies have shown that ants nesting in mineral soil can affect water infiltration rates, soil organic matter (OM) content, and nutrient cycling, but much less is known on how mound‐building ants influence soil physical and chemical properties. In this paper we summarize what is known on the soil impacts of organic mound‐building ants in temperate and boreal forests, and how these ants could be affected by ecosystem disturbance and future climate change. Much of this information comes from studies on Formica rufa group ants in Europe, which showed that CO2 emissions and concentrations of C, N, and P are usually higher in ant mounds than in the surrounding forest soil. However, ant mounds are a minor component of total soil C and nutrient pools, but they do increase spatial heterogeneity of soil water and available nutrients. Mound‐building ants can also impact tree growth, which could change the quantity and quality of OM added to soil. Forest management, fire, and projected climate change, especially in boreal forests, could affect mound‐building ant population dynamics, and indirectly, soil properties.

Stand type is more important than red wood ant abundance for the structure of ground-dwelling arthropod assemblages in managed boreal forests

1 The relationships between red wood ants (Formica rufa group) and other ground‐dwelling arthropods were studied in young managed forests stands in Eastern Finland. The main objectives were: (i) to test the influence of stand type (dominant tree species; age: sapling versus pole stage) and numbers of red wood ants on the occurrence of other ground‐dwelling arthropods and (ii) to study the occurrence of red wood ants versus other arthropods on a distance gradient from ant mounds. We used pitfall traps set in 5–14‐year‐old sapling stands and 30–45‐year‐old pole‐stage stands of Scots pine (Pinus sylvestris L.) and birch (Betula spp.) forests. 2 Pitfall trap catches of red wood ants did not vary significantly between the forest stand types, although some groups of other arthropods showed clear responses to stand type (e.g. catches of other Formicinae and Gnaphosidae were higher in sapling stands than in pole‐stage stands). The number of red wood ants clearly explained less of the variation in assemblages of other ground‐dwelling arthropods than the forest stand type. 3 Red wood ant numbers decreased significantly with distance from the mounds, but the other ground‐dwelling arthropods were insensitive to this gradient or even showed a preference for proximity to ant mounds and high ant activity. 4 The results obtained in the present study suggest that wood ants do not have strong effects on several other ground‐dwelling arthropod groups in young managed forests other than in the immediate vicinity of their mounds.

Sources of variation in the incidence of ant–aphid mutualism in boreal forests

1 The mutualism between wood ants of the Formica rufa group and aphids living in the canopy of trees is a widespread phenomenon in boreal forests, and it can affect tree growth. However, not all trees in the forest are involved in this interaction. 2 To assess the incidence of host trees involved in this ant–aphid mutualism and its spatial distribution in boreal forests, we inventoried sample plots with a radius of 10–15 m around wood ant mounds in 12 forest stands of two age classes (5–12‐year‐old sapling stands and 30–45‐year‐old pole stands) and two dominant tree species (Scots pine and silver birch) in Eastern Finland from 2007 to 2009. 3 The proportion of trees visited by ants out of all trees on the individual study plots were in the range 4–62%, and 1.5–39% of the trees on the plots were consistently visited by ants during all 3 years. The percentage of host trees increased with the ant mound base area on the plots. Trees visited by ants were larger and closer to the mound than trees not visited by ants. Within the group of visited trees, more ants were found on bigger trees and on trees close to the ant mounds. 4 Extrapolated from plot to stand level, we estimated that 0.5–6.6% of the trees were host trees in at least one of the three study years, and that only 0.01–2.3% of all the trees were consistently visited by ants during all 3 years. It is concluded that ant–aphid mutualism is a minor occurrence at the stand level.

A review of the study of Oinonen (1956) on ants on rocks and their contribution to forest regeneration in Southern Finland

Abstract The classical and still valid study of Oinonen (1956) on the species distribution and nesting habits of ants on rocks and their contribution to forest regeneration and tree growth on rocks was reviewed. Oinonen searched ant colonies in 103 sample plots situated on rocks in southern Finland. The plots were classified into three categories according to their vegetation succession stage. The locations of origin, ages, heights and height increment of seedlings were determined. The results indicated that the number of ant species decreased from sites with >50% lichen cover to sites with >50% moss cover and to sites with <50% lichen cover. Lasius flavus was the most common species (5.14 nests per 100 m2), followed by Lasius niger (0.72), Myrmica lobicornis (0.57), Formica fusca (0.38) and Myrmica scabrinodis (0.36). Ant nests were favourable sites for tree establishment, growth and survival. The height growth of 3–14-year-old seedlings averaged 39% higher in ant nests than elsewhere. Ants form favourable microsites for seedlings by collecting organic material and fine mineral soil in their mounds, and thus significantly contribute to forest regeneration on rocks with thin soil cover. The role of ants in postfire forest regeneration on mineral soils was also discussed.

Does severe soil drought have after-effects on arbuscular and ectomycorrhizal root colonisation and plant nutrition?

Background and aimsArbuscular (AM) and ectomycorrhizas (EM) predominate in different soils. We hypothesise that also climatic factors affect directly their relative success and AM fungi (AMF) generally tolerate drought better than EM fungi (EMF). Here we tested the colonisation rates of soil-borne AMF and EMF after soil drought treatments.MethodsForest and meadow soils were mixed to include AMF and EMF propagules. There were three soil treatments: soil moisture corresponding to 60% (control) or 10% in mass of maximum water retention, or air-dry. The bait plants were EM-forming silver birch (Betula pendula), dual-mycorrhiza forming grey alder (Alnus incana) and AM-forming white clover (Trifolium repens). After re-watering, bait plants were sown and grown in a growth chamber in favourable conditions. This way, host-plant responses to drought were eliminated.ResultsPrevious soil drought decreased EM colonisation in alder. The spore formation by AMF in clover was negatively affected after the most severe soil drought treatment, suggesting slightly retarded AM formation. Soluble soil potassium concentrations were lower in drought-treated soils and this was reflected in birch foliar concentrations.ConclusionsThe results provide some support to the hypothesised better drought tolerance of AMF than EMF propagules but further studies in milder drought and in other plant-fungus combinations are needed.