Proportions of saproxylic beetle populations that utilise clear-cut stumps in a boreal landscape - Biodiversity implications for stump harvest

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 24,499 ha 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.