Oak as retention tree in commercial spruce forests

Retaining trees valuable for biodiversity is a common conservation measure. There are indications that such trees have important benefits for the diversity of many species groups. However, retaining trees may also have negative effects on timber production. The aim of this thesis was to study the benefits of retaining oaks (Quercus robur) in spruce forests (Picea abies) for the diversity of oak associated saproxylic (wood-living) beetle species and to assess the negative effects on wood production of spruce. The first three papers, as presented in this thesis, relates to the biodiversity aspect: the effect of openness on species richness (Paper I); the benefit of retaining oaks in spruce plantations, in relation to pasture oaks (Paper II); and the short-term effects of clearing on species richness and composition (Paper III). In Paper IV, the negative effects of gap formation around oaks on wood production of spruce was assessed. The results showed a significant decrease in species richness of oak beetles with decreasing openness. Moreover, spruce trees on the southern side of the oaks appeared to have a larger impact than trees of than those on the northern side (Paper I). According to the results in Paper II, low shaded oaks in spruce plantations harbored slightly more oak associated species than did pasture oaks, and the species composition differed somewhat, although it seemed to be partially overlapping. The amount of dead wood in the oak crown appeared to one of the major explanatory factor for differences in species richness and composition between forest and pasture. Thus, oaks in plantations can harbor a comparatively high number of species. Regarding the shortterm effects of clearing (Paper III), there was a significant positive effect on species richness of saproxylic oak beetles of larger clearings (under and outside the crown) already the first three years after clearing, compared to non-cleared oaks. Smaller clearings (solely under the oak crown) did not yield the same increase in species richness as the larger clearings. However, both small and large clearings differed in species composition when compared to the non-cleared oaks. Retaining oaks has a negative effect on the growth of spruce (Paper IV). The basal area in the area around retained oaks was on average about 83% of the basal area in satellite plots without retained oaks. Since there was no difference in production level between large and small gaps, this study indicates that with the thinning treatments imposed on the studied stands, the reduction in production will be about the same irrespective of gap size for gaps at least up to about 350 m². In summary, increased openness has apparent positive effects on species richness of saproxylic oak beetles and the response to clearing has immediate positive effects on both species richness and composition of oak associated beetles. Retained oaks in spruce forests may, if cleared of encroaching spruce trees, harbor a species richness comparable to, or even higher, than that of pasture oaks. A certain loss in timber volume from the gaps is expected, however, trees lining the edge of gaps can partly compensate for the loss with the gap. The increase in timber loss is smaller for smaller gap sizes, as compared to large. Retaining several oaks in small gaps should thus be more cost-efficient compared to few oaks in large gaps. However, oaks in large gaps may attract additional species that favor high levels of insolation. One single large gap may thus capture more species than several small gaps together, even if the total gap area is equal. These aspects should be considered when planning for clearing around retained oaks in a spruce forest or plantation.