10. Wood mould boxes

As oaks and other deciduous trees age, they often develop hollows in their trunks. These holes arise when the conditions are right for wood-living fungi to begin the rotting process inside the trunks. This happens for instance when a thick branch dies or when bark is damaged. Depending on the fungus species, the fungi break down and soften up the wood – either by breaking down the cellulose so that brown rot is created, or by breaking down both the cellulose and the lignin so that white rot is created. Both processes make it easier for pecking birds and insect larvae to create holes. Research shows that for oaks, this process begins when the trees are about 250 years old, but for other tree species in northern Europe, it starts earlier. A large number of specialized insects live in rotting trees, and in fungi’s fruit body and mycelia.

Wood mould – an important habitat for threatened species

The most exclusive environment is the wood compost that forms inside the tree trunk – this is called wood mould. In large trees it can reach volumes of several cubic metres. Wood mould consists normally of a mixture of wood fragments, fungi fragments and remains of old bird nests and insects. It’s primarily a home to beetles, but rare hoverflies, crane flies and pseudoscorpions are also included in the complex network of species that live in wood mould.

According to studies, the oak is about 20% more species-rich than other deciduous broad-leaf trees. Still, the other deciduous broad-leaf trees are home to a huge 70% of the species that live in oaks.

Because old hollow trees are uncommon in most places, today this fauna is rare and threatened. With few suitable trees spread out in the landscape, gaps in both time and space emerge, that is, an uneven age distribution with a lack of middle-aged trees and large distances between the old trees. It often takes a long time to fill these gaps naturally, which means that in the meantime, species risk disappearing.

Who wants to live in a wood mould box?

Studies in Sweden have shown that it is possible to imitate some of the microhabitats that exist in hollow trees: in one area, 70% (52 of 82) of the beetle species that were found in hollows in old oaks were also present in boxes filled with oak chips and dead oak leaves. Another important lesson from these studies is that the higher the humidity, the more two-winged insects such as crane flies and hoverflies colonize the boxes. It also emerged that the species not found in the boxes are highly dependent on brown rots (which are created by e.g. sulphur polypore and beefsteak polypore) and on that type of wood mould.

During the 15 years that wood mould boxes have been studied, the hermit beetle was found in just 7% of the boxes. This can be compared with the taxonomically related marbled rose chafer (Protaetia marmorata), which lives in the same environment, and was found in 37% of the boxes. The most probable reason for this is that the marbled rose chafer is a stronger flyer, and more quickly finds suitable habitats. Other species that have been common in the boxes are clown beetles, rove beetles, hide beetles (Trox scaber), Prionychus ater and whitemarked spider beetles (Ptinus fur), but also a few rarities such as the Moccas beetle (Hypebaeus flavipes) and the spotted hairy fungus beetle (Mycetophagus quadriguttatus) have been found.

Examples of species found in wood mould boxes