Abstract
Many saproxylic insects (insects dependent on deadwood) are threatened because the amounts of suitable substrate, coarse woody debris (CWD), are low in managed forests. Harvesting clear-cut stumps for bioenergy will decrease these amounts further and may therefore contribute to loss of biodiversity. Earlier studies have demonstrated that many saproxylic beetle species utilise clear-cut stumps. However, the importance of stumps compared with other kinds of deadwood remains unknown. In particular, some species adapted to sun-exposed deadwood created by disturbances might, in managed forest landscapes, be dependent on stumps in clear-cuts. In the present study we quantify the proportions of landscape-wide populations of saproxylic beetles that are recruited from clear-cut stumps in a managed boreal forest landscape. The amount of bark on clear-cut stumps and population densities (beetles per m2 bark) were estimated for a 24,499ha landscape in Sweden. Our data were pooled with estimates from a similar study in the same landscape, including all other types of CWD and beetle densities. The clear-cut stumps were estimated to contribute 15.5% of total bark to CWD in the landscape. For spruce, clear-cut stumps contributed 37.9% of the total bark. Of 39 analysed beetle species, 13 were estimated to have more than 10% of their populations in clear-cut stumps, 9 more than 50%, and 5 more than 80%. Clear-Cut stumps are thus an important breeding substrate for many beetle species and consequently, intensive stump harvest risks decreasing biodiversity.
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