Individual study: Dead birch Betula trees left in clear-cuts benefit saproxylic Coleoptera adapted to natural disturbances in boreal forest in in Heinola, southern Finland, and Juva, eastern Finland
Kaila L., Martikainen P. & Punttila P. (1997) Dead trees left in clear-cuts benefit saproxylic Coleoptera adapted to natural disturbances in boreal forest. Biodiversity and Conservation, 6, 1-18
Intensive forest management has reduced the amount of decaying wood in boreal forests and, as a consequence, large numbers of saproxylic insects have declined. Natural disturbances, such as forest fires, storm damages, and flooding caused by beavers Castor spp., create open areas with a considerable amount of dying and dead wood. In old taiga, such patches can be locally quite frequent and support a number of specialized saproxylic species. Dead trees purposely left in otherwise clear-cut areas where timer has been harvested, are similar to some naturally disturbed areas and provide microhabitats for saproxylic insects. To assess the significance of such trees for saproxylic beetles researchers in this present study investigated the difference between beetle-species assemblages on dead standing birch Betula trunks located in mature forest versus trunks left in clear-cut areas after logging in southern and eastern Finland.
Study areas: The study was undertaken at two localities, in Heinola (61°10'N, 26°08±16'E) southern Finland, and in Juva, eastern Finland (61°42±45'N, 28°00'E). Assemblages of saproxylic Coleoptera on dead, standing birch trunks left behind in eight recently clear-cut areas were compared with assemblages in standing birch trunks in seven closed mature forests. Secondly, sampling was undertaken following a BACI design (before-after-with-control-impact).
Heinola: Sampling sites were mixed forests of 70-80 years age, tree-species composition dominated by birch (40 %) and Scots pine Pinus sylvestris (60%). Tree canopy cover was estimated at 85 %.The amount of decaying wood, especially birch, was relatively high. Sampling was undertaken from 6 May to 4 October 1993 in three clear-cut areas, 200 m ±1 km from each other. In these clear-cuts, some dead and dying trees had not been removed. Control samples were taken from two plots in adjacent forest at distances of up to 300 m from the corresponding clear-cut patch.
Juva: Sampling was undertaken from 22 May to 2 October 1993 in five clear-cut patches (0.5±8 km from each other) and in five forests in the vicinity of each clear-cut. The age of the forests was 70-80 years, with variable tree composition of birch 7±28 %, Scots pine 0±79 %, Norway spruce Picea abies 5±93 %. Thus, in general, the forests were more conifer-dominated than those in Heinola. Tree canopy cover averaged 70%.
The BACI-experiment included one of the Heinola clearcuts which was sampled before clear-cutting in 1990 (sampling period 22 May to 28 September) and also in 1993 (sampling period as above). To control variation between different years, a non-managed forest patch was sampled in 1990 and 1993 with the same sampling periods as in the treatment.
Sampling method: Samples were collected using window traps attached to standing trunks at a height of between 0.5 and 2.5 m under the perennial polypore fungus Fomes fomentarius. Previous studies have shown that the polypore fungus species affects the beetle assemblage caught from a tree trunk, and that F.fomentarius hosts a large variety of beetles. In order to minimize variation caused by different species of fungi, the study was limited to rotted birch trunks predominantly decayed by this very common polypore species. No sporocarps of other polypore species were visible on the trunks during the study period.
Beetles included in the analyses: The species included in the study were saproxylics known to be associated with deciduous trees. Data from the two sites were pooled for analyses.
In total, 10,248 individuals of 129 beetle species were recorded. The most abundant were Triplax russica (a shiny fungus beetle), Dorcatoma robusta (a deathwatch beetle), Lordithon lunulatus (a rove beetle), Anisotoma glabra and A.humeralis (round fungus beetles), which together comprised 57% of the total sample. The most frequently caught species were D.robusta (in 69 of the 74 traps), A.glabra (68), A.humeralis (68), Bolitophagus reticulates (67), T.russica(66) and L.lunulatus (63).
Median numbers of beetle species or individuals caught did not differ between closed forests and clear-cuts, but individual species occurred unevenly among these two habitat types. Distinct beetle assemblages were associated with these habitats. Several species associated with open forest habitat, e.g. burned forests or storm-damage areas, including species regarded as threatened in Finland, were found almost exclusively, in clear-cuts. Likewise, a number of species occurring frequently in closed forests were not found in clear-cuts.
Conclusions: The authors conclude that dead birch trunks left in the clear-cut areas may host generalist saproxylic beetle species, but also importantly, many specialized to warm, sun-exposed environments which may not be able to survive in closed forests. Therefore, the simple forestry management technique of leaving standing dead trees is useful in preserving saproxylic beetles, and probably many other saproxylic species.
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