Wood-nesting aculeates have high demands with respect to their breeding substrate and they choose carefully where to construct their nests. We (humans) have great opportunities to create suitable nesting substrates for this group of organisms in young forests through tree retention. By retaining dead trees and cutting high stumps in sun-exposed locations, we can create dead wood that has characteristics which favor wood-nesting aculeates.
Dead wood structures originating from retention measures, i.e. retained dead trees and high stumps, are used to a much higher degree as nesting substrate by solitary wood-nesting bees and wasps than stumps and logging waste. Many species rejected stumps and lying tops and branches. A suitable nesting hole for most wood-nesting bees or wasps should be just large enough for their body to fit into and situated in hard and dry wood.
Most holes in dead wood do not possess these qualities, resulting in a low occupancy by aculeates: less than 2% in our study.
If a wood-nesting bee species has the ability to nest in holes in stumps, the nesting resource is virtually unlimited and population size is solely explained by its food resource in young boreal forest stands. If a wood-nesting bee species rejects stumps, lying tops and branches as wood-nesting substrate, the population size is best explained by both its food- and nesting resource. It seems reasonable that a strong dependence on a specific food resource could be associated with limited demands on nest characteristics.
This suggests that one needs to consider specialization with respect to both nesting and pollen resources to make a sound assessment of a bee species’
habitat requirements. Forest roadsides may provide a stable food resource for solitary bees in managed forested landscapes. Flower density along
forest roadsides explained the abundance of the generalist bee H. annulatus to a greater extent than the flower density in young forests.
The number of potential nesting holes in the dead wood increases with time during the years following final felling as the decay stage reaches the threshold for more wood boring species, resulting in an accumulation of holes. Thus the availability of nesting holes for solitary aculeates increases over time. Still, sun-exposed, hard and dry wood is favored by the wood-nesting aculeate fauna. Thus, wood-boring species that use newly dead wood are likely to be of great importance to facilitate aculeate nesting in recently cut forest stands.
Edge zones between young forests and older forest stands are favorable bumblebee habitats in managed boreal forest landscapes. The reason for this is probably that edge zones provide suitable nesting sites for bumblebees because they 1) possess a favorable microclimate, 2) facilitate mate-finding and 3) facilitate navigation between the nest and the foraging site.
A high stump acting as a nesting substrate for many wood-nesting aculeates is characterized by being sun-exposed and having a high diversity of hole types, i.e. has been used by many species of wood-boring beetles. Aculeate species that are primary wood-burrowers do not favor high stumps with many pre-existing holes but prefer those with appropriate decay of white-rotting fungi.
The findings presented in thesis contribute to our understanding of the habitat requirements of forest-dwelling bees and wasps. However there are still large gaps in our knowledge. More research is needed, specifically on the following topics
• The importance of forest roadside flora as a food resource for a variety of bee species.
• Forest edges – why are they favored by bumblebees? What is their relative importance for foraging, orientation, nesting and mating?
• Food resources for solitary wasps – how important are food resources for species that collect insects and spiders for their brood? What environments offer high densities of preferred prey?
6 Acknowledgements
I would like to thank all my supervisors; Jan Weslien, Olof Widenfalk, Åke Lindelöw and Lena Gustafsson for valuable comments on this thesis. Thanks to Jan ten Hoopen and the staff at Faunistica for your support in the field. Thank you Mikael Andersson Franko for guiding me calmly through statistical analyses in R. Thanks to Bergvik Skog AB for providing me with study sites and stand data in ArcMap. Thank you Line Djupström for support with rarefaction analyses in estimateS and Märtha Wallgren for solving the problem with the word template in a critical moment. This work was funded by the Swedish research council FORMAS (grant no. 215-2009-569) and Skogforsk.
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