Cut high stumps of spruce, birch, aspen and oak as breeding substrates for saproxylic beetles

Abstract

Cut, 4 m high, medium to large-diameter stumps of spruce, birch, aspen and oak were monitored for 7 years as breeding substrates for saproxylic beetles, by trapping emerging insects within seasonally applied net enclosures. A total of 47 038 individuals representing 316 saproxylic beetle species, including 40 red-listed species, were recorded. This figure corresponds to 60% of all (527) saproxylic beetle species that were recorded by various forms of flight intercept traps in the study area during the study period. A minimum of 86 species (including seven red-listed species) were estimated to have produced means ≥1 individual, and 26 (two red-listed) species to have produced means ≥10 individuals, per stump of the main host species. Similarly, 100 species (including 10 red-listed species) were estimated to have produced ≥10 individuals in at least one main host stump. Stumps of different tree species hosted different faunas. Spruce held the highest proportions of species recorded from one tree species only. Red-listed species were significantly more frequent in stumps of aspen than in stumps of other trees. Numbers of species and red-listed species per stump were significantly positively related to diameter and sun-exposure, but numbers per stump mantle area only to exposure. We conclude that many saproxylic beetle species adapted to natural disturbances can breed successfully in high stumps, and that stumps of different tree species address complementary spectra of the species pool. To optimise benefits, high stumps should be localised to sun-exposed logging sites, and current practises focusing on spruce should be changed in favour of cutting more even proportions of common tree species like spruce, birch, aspen and pine.