Tom's Blog

Friday, April 15, 2016

Coarse woody debris (CWD): role in oak savanna/woodland function and biodiversity

There was a time when dead wood lying on the forest floor was looked at as undesirable. This probably derived from those historic times when wood was the major source of heat for rural homes and wood of any size or shape was used as kindling or fire wood.

Those days are long past. What is now called coarse woody debris (CWD) is known to play major roles in ecosystem function. At one time “dead wood” was the bane of foresters but now it is considered an environmental boon. The US Forest Service, since the 1970s, has recognized the importance of CWD in forests and has published the research described in several conferences. 

The British were probably the first to recognize the importance of CWD. Here is an early (1966) quote from Animal Population Ecologist Charles Elton concerning the state of British woods: “When one walks through the rather dull and tidy woodlands…that result from modern forestry practices, it is difficult to believe that dying and dead wood provides one of the two or three greatest resources for animal species in a natural forest, and that if fallen timber and slightly decayed trees are removed the whole system is gravely impoverished of perhaps more than a fifth of its fauna.”

 According to Elton, and confirmed from numerous later studies, large numbers of species of animals live in wood or under bark. Fungi, especially lignin digesters such as basidiomycetes, use wood as their principal carbon and energy source. Birds, small mammals, and other critters thrive on CWD. Research has shown that biodiversity is many-fold higher with CWD on the ground than without it.

Why encourage CWD?
  • Important component of a forest ecosystem
  • Major habitat of arthropods (many species)
    • Mites
    • Bark beetles
    • Wood borers
    • Carpenter ants
    • wasps
  • Provide nesting, roosting, feeding, and shelter sites for birds
  • Haven for
    • Small mammals
    • Reptiles
    • Amphibians
  • Reduce erosion
  • Important for soil development
  • Store nutrients and water
  • Provide source of energy and nutrient flow in the system
  • Serve as seedbeds for forbs and grasses
  • Mosses (bryophytes) especially grow on CWD
  • Provide habitat for decomposers, especially fungi
    • Fungi are a very diverse greoup of organisms that are essential for other organisms that depend on CWD and ecosystem functioning
  • CWD is an important indicator of a “natural” forest
  • Carbon sequestration (significant for climate change studies)
  • Nature’s unsung heroes of recycling
  • Examples of important species in recycling
    • Fungi (most important biochemically)
    • Beetles
    • Flies and maggots
    • Wood lice
    • Slime molds
    • Bacteria
    • Slugs
    • Snails
    • Millipedes
    • Springtails
    • Earthworms

Animals and fungi work together in the decomposition of CWD. Smaller particles decompose more rapidly than larger, so that insects and other animals that physically break down large pieces of wood, greatly increase fungal activity. On the other hand, fungal infection promotes insect attack, creating a feedback loop that greatly increases the rate of CWD decay.

Note that the animals involved in CWD decomposition are different species from those that affect (grow on) living trees. 

Coarse woody debris is much more common in unmanaged than in managed forests. CWD is an indicator of an “old-growth”forest, as well as an indicator of a high-biodiversity environment.

Stumps are a separate category. A stump has an above-ground and a below-ground component. The below-ground portion is very important. As much as 80% of total CWD carbon of a stump can be below ground (hidden).

From the time it dies, a dead tree contributes to many ecological processes
  • As a standing snag
  • As a major piece of fallen woody material lying on and in the soil
  • Fuel for fire
  • Biological decomposition

Which is the most important in recycling of CWD, fire or biological activity? It depends on the species of wood, and the environment, Without fire nutrients may be tied up in dead woody material for a long time.

Rate of decomposition varies greatly among the tree species. Oaks of the white-oak group (white, bur, swamp white) are the most resistant to decay.

Especially in colder climates, dead wood lasts a long time on the ground, slowly being converted into carbon dioxide, minerals, humus, and living critters. The rate of decay will depend upon the species, the size, shape, and position relative to the forest floor, fire, and other factors.

Carbon release from decaying wood is a major component of the global carbon balance and hence of major importance in climate-change studies.

Wood decomposition is an aerobic process although slow because of the presence of lignin in the heartwood. Lignin decomposition takes place primarily by white-rot fungi (Basidiomycetes). Lignin is one of the mostly slowly decomposing components of wood. Because of this, significant amounts of lignin remain when the other principal biochemical components of wood, cellulose and hemicellulose, have been degraded. Residual lignin is a major fraction of the soil humus. 

Snags were once considered safety hazards and down wood was considered unsightly, a haven for pests, and a fire hazard. Snags may still be a safety hazard but the probability of falling at an inappropriate time is very low. Snags are major habitats for birds.

Coarse woody debris at Pleasant Valley Conservancy.
We don’t need quantitative methods to discern that the CWD varies markedly from habitat to habitat at PVC.

Prairies, of course, have almost no CWD, whereas dense woods, such as the north-facing slope, have large amounts. See last December's post.  Closed savannas have more CWD than open savannas. In wetlands the amount of CWD varies. Stream banks, with lots of willows, have lots of CWD, both near the bank and submerged in the water.

Oak savanna (Unit 19E) with scattered CWD.
Note also the dead oaks in the background,destined to become future CWD

This dead oak came down after the fire of 3-29-2016. Note that none of the wood is charred except the base..
The dead base was weakened by the fire, and the snag came down in the next windstorm.

The top end of the burned snag shown in the above photo. Lots of potential CWD here!
The cracks and tarnish indicate that this log has been down for some years.
Note all the plants that are growing in and among this CWD.


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