Criss-crossing trunks at drunken angles, broken lichen-scarred limbs, softly rotting wood beneath further freshly fallen trunks. These might be the nightmare visions of a deep dark wood, but if you dare to look a little closer, what you will actually discover is treasure. This guest article was written by Genevieve Tompkins, the Rare Invertebrates in the Cairngorms Project Officer, and was originally published on the Cairngorms Connect website.
Deadwood is exactly what it says on the tin: wood which is no longer living. This can come in all manner of forms; standing deadwood, fallen deadwood, tree stumps, rot holes within living trees – the list goes on. The one thing all deadwood has in common is its vital importance for invertebrates, from beetles to flies, spiders to millipedes. Sadly, the importance of deadwood went totally unrecognised for decades, leading to declining populations for many of the species which rely on it. But change is happening, with deadwood finally receiving the appreciation it deserves.
Deadwood creation and plantation restructuring is a critical part of the Cairngorms Connect programme of works, bringing biodiversity back to the forests. Let’s take a look at some of the species which benefit…
Images above: (Left) Golden net-wing beetle. (Right) Pine hoverfly larvae. Credit: Genevieve Tompkins.
Golden net-wing beetle (Dictyoptera aurora)
This gorgeous beetle was once widespread and reasonably common across the Scottish Highlands, the only place it is found in the UK, but it has declined over the past few decades. It is now only known from around a dozen sites. The best time to see this beetle is on a warm evening in early summer, clumsily flying through a forest opening, golden light shining through the bright red elytra (modified front wings).
The young larvae develop in fallen or standing Scots pine deadwood, sometimes even in old stumps, while the adults feed on pollen and nectar. This highlights the second vital part of a healthy forest – flowering plants such as rowan, which also benefit from the increase in space and light provided by plantation restructuring.
Pine Hoverfly (Blera fallax)
Arguably the rarest and most well-known denizen of deadwood, in the UK this Critically Endangered hoverfly is restricted to only a tiny handful of sites in the Cairngorms National Park. Again, it is the larvae which rely on deadwood, specifically rot holes in old “granny” pines. Plantation restructuring is moving us towards bringing granny pines back to the forests, but in the meantime, special hoverfly rot holes are being created in stumps during deadwood creation. This also supports other rare hoverflies, such as the Nationally Scarce Callicera rufa.
Images above: (Left) European bark beetle predator, in deadwood creation area. (Right) Three-banded longhorn beetle. Credit: Genevieve Tompkins.
European bark beetle predator (Glischrochilus quadripunctatus)
It is the adult, as well as the larvae, of this species which lives under the bark of conifers, often in deadwood. This makes them tricky to spot, but if you are lucky you might see one flying through a clearing in hot weather or caught out on the surface of a stump. This beetle is widespread across the UK, but fairly local, and generally dependant on conifer woodlands. As the common name suggests, these beetles and their larvae are predators, chasing down other invertebrates under the bark.
Three-banded Longhorn beetle (Judolia sexmaculata)
Longhorn beetles are another group of species which benefit from a diverse woodland, requiring a combination of deadwood and flowering plants. The eggs are laid into deadwood, particularly Scots Pine, where the larvae go on to bore deep tunnels while feeding on the wood, whereas adults feed on pollen and nectar. The three-banded longhorn is one of the rarer species, found only in Scotland in the UK, so it is a treat to spot one. Other species also benefit from the action of longhorn beetle larvae, with the rare pinewood mason bee (Osmia uncinata) nesting in the holes created by longhorn beetle larvae in deadwood.
It may seem counterintuitive at first, but deadwood is the sign of a healthy and diverse woodland. As well as supporting a myriad of specialist beetles, flies and other invertebrates, these specialists also go on to provide for other species through their ecological interactions – be it providing nest holes, acting as prey and predator or recycling nutrients. The key to biodiversity is habitat diversity, with deadwood playing a crucial role.
Main image: Evidence of beetle activity on trunk of felled pine. Credit: scotlandbigpicture.com.
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