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

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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. © 2011 The Authors. Agricultural and Forest Entomology © 2011 The Royal Entomological Society.

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Agricultural and Forest Entomology