Beavers restore the dead wood of boreal forests

Dead wood is a necessary element for numerous species living in the boreal zone. It functions as a food resource, nesting space or growth substrate for several mammals, fungi, insects, and birds. Dead wood is produced through two main mechanisms: senescence and disturbances e.g. forest fires or wind damage. A controlled forest has less ageing trees and disturbances, and currently up to 90% of Fennoscandian forests have been influenced by forest management. The recent drop in dead wood levels due to intensive forest management across the globe has concurrently led to dead wood-dependent (= saproxylic) species becoming rare as well, which weakens food webs and ecosystem functionality. Managed forests may only contain a few cubic meters of dead wood per hectare, while dead wood levels in old-growth forests and forests influenced by disturbances can rise up to hundreds of cubic meters per hectare.

Beaver, the ecosystem engineer © Sari Holopainen

Beaver, the ecosystem engineer © Sari Holopainen

Strong disturbances are less frequent in moist lowland areas of the boreal zone, where dead wood is mainly created as single trees die due to competition and ageing. However, beavers act as wetland ecosystem engineers, raising floodwaters through the damming of water systems. These floodwaters kill surrounding shore forests due to oxygen deprivation, thus creating significant amounts of dead wood into the habitats. In certain cases the flooding may kill entire forest stands. Beavers can therefore be considered the main natural disturbance factor of lowland forests.

Beavers require wood for food and as a building material for their nests and dams. Foraging for woody materials causes the resource to run out within a few years, forcing the beavers to move location. The process of flooding and dead wood creation begins again in a new area, thus producing a continuation of dead wood hotspots into the landscape. Eventually after several years the beavers can return to a previously inhabited location, which will be then be repeatedly subjected to their engineering. These hotspots may be very important to dead wood -dependent species, especially as they uphold a network and continuous supply of different-aged dead wood.

Calculating dead wood levels at a beaver flood - spot the researchers! ©Mia Vehkaoja

Calculating dead wood levels at a beaver flood – spot the researchers! ©Mia Vehkaoja

Despite an overall decrease in dead wood levels, certain types of dead wood have become rarer in the boreal forest than others. Currently the rarest forms are standing dead trees (snags) and deciduous dead wood. Both have declined more rapidly than other types due to forest management actions and attitudes. Beavers create a broad range of dead wood types (e.g. downed wood, stumps and coniferous dead wood), but they particularly aid in the production of snags and deciduous dead wood. This is good news for many saproxylic species, as these organisms are often strongly specialized, utilizing very specific dead wood types.

The dead wood produced by beaver-induced flooding is also very moist, which may affect the wood-decay fungi species that begin colonizing the dead wood. For example, sac fungi are more tolerant of wet conditions, and may therefore outcompete Basidiomycetes at beaver sites. This in turn will lead to differing invertebrate communities that utilize sac fungi instead of Basidiomycetes. Very different dead wood –dependent species assemblages may therefore be formed at beaver sites compared to fire areas of clear-cuts. The interactions of these species are currently poorly understood.

The beaver offers a possibility for all-inclusive ecosystem conservation compared to the conservation of single species. The species could be used to produce dead wood and restore the shore forests of wetlands.

Our research group has recently published an article concerning the impacts beavers have on boreal dead wood. The article can be accessed from http://www.sciencedirect.com/science/article/pii/S0378112715005757

Calculating dead wood levels at a beaver flood - spot the researchers! ©Mia Vehkaoja

Calculating dead wood levels at a beaver flood – spot the researchers! ©Mia Vehkaoja

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A whole new universe in dead wood

Before I took part in the course Biodiversity in dead wood (organized by the University of Helsinki), I thought that there is dead wood only aboveground. So it was an elevating experience to realize that most of the dead wood is underground. Boreal forests have more underground dead wood than for example broadleaved forests. Underground dead wood is very common in Finland as there are lots of pine-dominant bogs. There pines sink underground before they are decayed, and lots of underground dead wood is produced.

Standing dead pine, possible becoming a kelo. © Stella Thompson

Standing dead pine, possible becoming a kelo. © Stella Thompson

One other special characteristic for Finland forests is kelo, which is a dead standing pine. Usually the kelo has lost its pine bark. Kelo trees have unique biodiversity, which is completely different than for example in dead birch. It is quite typical that the saproxylic species are specific to certain tree species. This feature enhances the saproxylic diversity in forests. Saproxylic species are defined as any species that depends upon decaying woody material.

The carboniferous was a starting point for saproxylic evolution. Almost immediately when the first trees arrived on land, the first decomposing fungi evolved. After fungi the first decomposing invertebrates developed. The rapid specification of the saproxylic species occurred in the Triassic and Jurassic ages. The first termites existed about 100 million years ago. The evolution of saproxylic beetles is better known than the evolution of basidiomycetes. This is due to better preserved fossils.

There are approximately 400 000–1 000 000 saproxylic species in the world, and in the Nordic countries the same number is 7589 known species. Finland alone has 4 000–5 000 dead wood species. This is about 20–25 % of all forest species. It could be argued that there are several unknown saproxylic species in the Nordic countries. Solely in Finland, almost every year new saproxylic species are found. Boreal forests have almost as much dead wood as tropical forests. This correspondence between the two forest types results from the different decaying rates.  The decaying rate of tropical forests is much faster.

Basidiomycetes decaying goat willow (Saprix caprea). © Stella Thompson

Basidiomycetes decaying goat willow (Saprix caprea). © Stella Thompson

The decaying rates also differ in different parts of boreal forests. In the fast-decaying parts 90% of organic matter can be decomposed in 50 years, whereas the same process will take 100 to 200 years in the slow-decaying parts. At an early successional stage the decaying process is usually slow and accelerates towards the pristine forest stage. The main hazard to saproxylic species is forestry. For instance in Finland forestry has reduced the amount of dead wood from 60–120 m3 per hectare to 2–10 m3 per hectare. This reduction is severe. The new trend in forestry, at least in the Nordic countries, is bio-fuel in the form of intensive residue harvesting. This means that even the branches, crowns, and stumps are collected from the logging area. This leads to an even more decreased amount of dead wood in forests. It can be quite easily calculated that without dead wood about 25% of Finland’s forest species are lost. Why is nobodytaking strong action to prevent this from happening? Researchers and conservationists should come together, and force decision-makers to see the dramatic downside of residue harvesting.

Dead wood on top of lingon- and blueberries in boreal forest. © Mia Vehkaoja

Dead wood on top of lingon- and blueberries in boreal forest. © Mia Vehkaoja

 

Check out also this amazing article about Ancient Forest Found Thawing Beneath Melting Glacier in Alaska