Abstract
Old living oaks (Quercus robur) are known as a very species‐rich habitat for saproxylic beetles, but it is less clear to what extent such veteran trees differ from an even rarer feature: downed trunks of large oaks. In this study, we set out to sample this habitat, using window traps, with two aims: (1) to describe the variation of assemblages among downed trunks of different type and (2) to compare beetles on downed oaks with data from veteran standing trees. The results showed that trunk volume and sun exposure better explained assemblages as well as species numbers on downed trunks than did decay stage. Furthermore, species classified as facultative saproxylic species showed weak or no differentiation among downed trunks. Species with different feeding habits showed no apparent differentiation among downed trunks. Furthermore, species composition on dead, downed oak trunks differed sharply from that of living, veteran oaks. Wood or bark feeders were more common on veterans than downed trunks, but there was no difference for those species feeding on fungi or those feeding on insects and their remains. In conclusion, for a successful conservation of the saproxylic beetle fauna it is important to keep downed oak trunks, and particularly large ones, in forest and pastures as they constitute a saproxylic habitat that differs from that of living trees.
Highlights
Given the large amounts of timber in unlogged forests, it comes as no surprise that a large number of species have more or less specialized on the habitat created by dead and dying timber (Schiegg 2001; Grove 2002)
Among the three characteristics of downed logs, we expected stage of decomposition to be most important as a high score represent a greater variety of stages in the succession of decomposition (Pyle and Brown 1999), allowing for successional niche differentiation
Species composition of obligate saproxylic species was clearly differentiated among downed logs
Summary
Given the large amounts of timber in unlogged forests, it comes as no surprise that a large number of species have more or less specialized on the habitat created by dead and dying timber (Schiegg 2001; Grove 2002). Still, looking at the finer details, it is known that saproxylic assemblages can vary between tree species (Milberg et al 2014) and depending on, for example, factors such as whether the wood decay occurs outside of the tree (trunk and branches) or inside the tree in hollow (Winter and Mo€ller 2008; Jansson et al 2009a; Quinto et al 2014), if the wood is living or dead standing (Jonsell and Weslien 2003), standing or downed (Franc 2007; Andersson et al 2015), and burned or not (Toivanen and Kotiaho 2007) Apart from these attributes, degree of decay, trunk size, level of sun exposure, wood decaying fungi colonization have been pointed out as important variables for saproxylic species assemblages (Hj€alten et al 2007; Brin et al 2011; Bouget et al 2013). A main candidate for such a timber in Europe is veteran oaks (i.e., very large and old oaks) that are uniquely rich in saproxylic species, many of which are considered as oak specialists (preferentially using oak), or even considered as confined to oaks (but see Milberg et al 2014)
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