4 reasons why vanishing deadwood is a great catastrophe

Deadwood amounts have dramatically declined all over the world. Here I present four reasons why deadwood is so important:

1. Deadwood remains in the forest for a long time
When wood decays, it transforms into carbon dioxide, water and minerals. These are exactly the materials that a living tree binds during photosynthesis. The complete degradation of a tree takes 50 to 100 years in northern regions. Deadwood therefore remains a part of the forest ecosystem for a long time, thus enabling the survival of species depending on deadwood as a substrate.

2. Deadwood is nutrition for fungi and invertebrates

Fungi are the main decomposers of deadwood, but bacteria and invertebrates also take part in the decaying processes. These organisms have special digestive compounds, enzymes, to cut the wooden structure into more easily digestible forms. This works in the same way as the enzymes in our own stomachs that cut the food we eat into more usable shape. Fungi can be divided into three main decomposer groups: white, brown and soft rot. White-rot fungi, e.g. Phellinus nigrolimitatus, lives mainly on deciduous wood, whereas brown-rot fungi, such as Coniophora olivacea, are mostly in charge of decomposing conifers. Beetles (Coleoptera), ants (Formicidae) and termites (Isoptera) are examples of invertebrates that use deadwood as a form of nutrition, but e.g. pin lichens (Calicioid) can also more or less decompose wood.

Pin lichens (Calicioids) grow on deadwood surface. © Mia Vehkaoja

Pin lichens (Calicioids) grow on deadwood surface. © Mia Vehkaoja

3. Deadwood is home for animal offspring
Deadwood is home for thousands of species. For some species deadwood can be an incubation place and a safe nest for newborn offspring. Several beetles and termites lay their eggs inside deadwood, where the hatching larvae are safe in their own chambers. As for Nematocera, Brachycera and Aculeata, the deadwood-decomposing fungi functions as a rearing place for larvae. In addition to invertebrates, birds, bats and flying squirrels (Pteromys volans) also use the holes in deadwood as nesting places. Furthermore woodpeckers (Picidae) as cavity nesters are a good indicator for deadwood abundance.

Several beetle species lay their eggs inside deadwood. © Mia Vehkaoja

Several beetle species lay their eggs inside deadwood. © Mia Vehkaoja

4. The disappearance of deadwood creates local extinctions at the very least
Nowadays deadwood is a dying natural resource. Forestry has decreased the amount of deadwood in Finnish forests by over 90%, concurrently causing the local extinctions of several species. Species that depend on deadwood throughout their entire lives are at greatest risk. Such species include the fungi Phellinus igniarius and the three-toed woodpecker (Picoides tridactylus).

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The needy beetle of the boreal forest

Animals, plants, and fungi can be divided into two basic types depending on their level of specialization. Generalists (e.g. rats) are species that do well in several different environments; e.g. they typically eat several other species, grow in varying conditions, and can move around more efficiently. Specialists (e.g. the koala) on the other hand are pickier; they require certain specific foods for energy, specific levels of e.g. sunlight or rain to grow, and might not produce young ones in great numbers. Specialists are more likely to fall into trouble as environmental conditions change, while generalists are capable of withstanding fluctuations. Characteristics increasing this risk include low population density, a small geographic range, slow growth and reproduction, and a high trophic level.

Habitat loss drives species extinctions globally. It occurs on several levels, and can lead to habitats becoming resource-poor or low in quality, and unconnected and fragmented to an extent where species cannot spread to new areas. To be able to save threatened species it is necessary to understand the traits that are specific to them. Red-listing a species means that the species is internationally endangered. Most red-listed forest species seem to have traits connected to specialization, and they typically occur more frequently in forests that offer large amounts of resources and are natural, closed and well connected. Therefore conservation efforts for such species are most successful when targeted in well-connected areas that are of high quality. For example, a tenfold increase in red-listed species is predicted for areas with little fragmentation compared to highly fragmented forests. Species with specialized habitat requirements that also naturally occur in low numbers are typically the most vulnerable for fragmentation.

Fragmented landscape in Finland © Sari Holopainen

Fragmented landscape in Finland © Sari Holopainen

On the other hand, generalist species can do particularly well in fragmented areas. One reason for this is that they can take over resources that were previously used by specialists once these species disappear through fragmentation. Generalists tend to be inferior competitors, so once stronger specialist competitors disappear generalists can thrive. Generalists therefore often benefit from small-scale conservation efforts, e.g. retention trees on felling sites, while specialists require protection schemes that target larger areas and entire habitats. Generalists can also spread to new areas, becoming detrimental invasive species.

Sometimes being choosy about what to eat and where to live goes to the extreme. The hero of our story, Agathidium pulchellum, is a small beetle known to occur only in Norway spruce-dominated old-growth forests that also house European aspen. It is one of the rarest beetle species in Europe, and its life stages are still poorly known. For example, we do not understand why it needs aspen trees to complete its life cycle. It feeds on slime molds, which are a separate life form just like animals, plants, and fungi. Slime molds have several life stages, e.g. fruiting bodies and plasmodia, some of which occur on wood and bark, and some in less-visible places such as inside tree wood or on dead leaves fallen to the ground, also knows as leaf litter.

A. pulchellum is so specialized that it only feeds on one species of slime mold, scientifically named Trichia decipiens. This slime mold is associated with large-diameter mid-decay-stage logs of spruce, aspen and birch, and the beetle has been found on slime molds on each of these three tree species. However, T. decipiens is a fairly common species in boreal and temperate forests, and while it has mostly only been found in conserved or old-growth forests, it can also occur in managed forests if dead wood levels are high enough. A. pulchellum on the other hand requires nearly three times the amount of dead wood as the slime mold, so it is unlikely that it can be found in managed forests. It also appears that the slime mold and dead wood are not the only requirement of the beetle, but much still remains unknown about its habitat demands and life cycle.

However, both slime mold and beetle can utilize small-diameter dead wood in certain conditions. Beaver-influenced sites may prove to be favorable due to their increased levels of moisture and mid-decay-stage and deciduous dead wood. The habitat requirements, i.e. aged spruce-dominated forests with aspens, of the beetle are also similar to those of the flying squirrel, which has become both a symbol of conservation efforts and a curse word for private forest owners in Finland. Targeting conservation for both species in suitable areas would be very beneficial. The good news is we may unknowingly already be protecting several A. pulchellum populations in areas conserved for their flying squirrels. As the beetle is classified as endangered and the EU has placed it on a list of species requiring special attention and specific areas of conservation, it would be good to assess areas already conserved for the flying squirrels. This may help prevent tension from building up among private forest owners, and may also help the general public realize the importance of these areas once it can be shown that two endangered species use practically the same habitat.

Beaver-created dead wood © Mia Vehkaoja

Beaver-created dead wood © Mia Vehkaoja