Coarse woody debris (CWD) orcoarse woody habitat (CWH) refers to fallen deadtrees and the remains of largebranches on the ground inforests[1] and in rivers orwetlands.[2] A dead standing tree, known as asnag, provides many of the same functions as coarse woody debris. The minimum size required for woody debris to be defined as "coarse" varies by author, ranging from 2.5–20 cm (1–8 in) in diameter.[3]
Since the 1970s,forest managers worldwide have considered it best environmental practice to allow dead trees and woody debris to remain in woodlands,recycling nutrients trapped in the wood and providingfood andhabitat for a wide range of organisms, thereby improvingbiodiversity. The amount of coarse woody debris is an important criterion for the evaluation and restoration of temperate deciduous forest.[1] Coarse woody debris is also important in wetlands, particularly indeltas where woody debris accumulates.[2]
Coarse woody debris comes from natural tree mortality,plant pathology,insects,wildfire,logging,windthrows andfloods.[citation needed]
Ancient, orold growth, forest, with its dead trees and woody remains lying where they fell to feed new vegetation, constitutes the ideal woodland in terms of recycling and regeneration. In healthytemperate forests, dead wood comprises up to thirty per cent of all woodybiomass. In recent British studies, woods managed for timber had between a third and a seventh less fallen debris than unmanaged woods that had been left undisturbed for many years, while in recentlycoppiced woods the amount of CWD was almost zero.[citation needed]
In old growthDouglas fir forests of thePacific Northwest of North America, CWD concentrations were found to be from 72 metric tons/hectare (64,000 pounds/acre) in drier sites to 174 t/ha (155,000 lb/acre) in moister sites.[4]Australian native forests have mean CWD concentrations ranging from 19 t/ha (17,000 lb/acre) to 134 t/ha (120,000 lb/acre), depending on forest type.[5]
Coarse woody debris and its subsequentdecompositionrecycles nutrients that are essential for living organisms, such ascarbon,nitrogen,potassium, andphosphorus.Saprotrophicfungi anddetritivores such asbacteria andinsects directly consume dead wood, releasing nutrients by converting them into other forms oforganic matter which may then be consumed by other organisms It has almost no physiologically important nutrients, so must be first enriched for consumption by transport of nutrients from outside.[6][7] Thus CWD is important actor contributing to soil nutrients cycles. CWD, while itself not particularly rich in nitrogen, contributes nitrogen to the ecosystem by acting as a host fornonsymbiotic free-livingnitrogen-fixingbacteria.[8]
Scientific studies show that coarse woody debris can be a significant contributor tobiologicalcarbon sequestration. Trees store atmospheric carbon in their wood usingphotosynthesis. Once the trees die, fungi and other saprotrophs transfer some of that carbon from CWD into thesoil. This sequestration can continue inold-growth forests for hundreds of years.[9][10]
By providing both food andmicrohabitats for manyspecies, coarse woody debris helps to maintain thebiodiversity of forest ecosystems. Up to forty percent of all forest fauna is dependent on CWD. Studies in western North America showed that only five per cent of living trees consisted of living cells by volume, whereas in dead wood it was as high as forty percent by volume, mainly fungi andbacteria.[11] Colonizing organisms that live on the remains ofcambium andsapwood of dead trees aid decomposition and attractpredators that prey on them and so continue the chain of metabolizing the biomass.[citation needed]
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The list of organisms dependent on CWD for habitat or as a food source includesbacteria,fungi,lichens,mosses and otherplants, and in the animal kingdom,invertebrates such astermites,ants,beetles, andsnails,[13]amphibians such assalamanders,[12]reptiles such as theslow-worm, as well asbirds and smallmammals. One third of all woodland birds live in the cavities of dead tree trunks.Woodpeckers,tits,chickadees, andowls all live in dead trees, andgrouse shelter behind woody debris.[citation needed]
Some plants use coarse woody debris as habitat.Mosses andlichens may cover logs, while ferns and trees may regenerate on the top of logs. Large fragments of CWD that provide such habitat forherbs,shrubs, andtrees are callednurse logs.[8] CWD can also protect young plants fromherbivory damage by acting as barriers to browsing animals. The persistence of coarse woody debris can shelter organisms during a largedisturbance to the ecosystem such aswildfire orlogging.[citation needed]
Fallen debris and trees in streams provide shelter forfish, amphibians and mammals by modifying the flow of water and sediment.[14][15] Turtles of many species may also use coarse woody debris for basking.[16]Musk turtles may lay their eggs under logs near wetlands.[17]
Coarse woody debris, particularly on slopes, stabilizes soils by slowing downslope movement of organic matter and mineral soil. Leaves and other debris collect behind CWD, allowing for decomposition to occur.Infiltration of precipitation is improved as well. During dry weather, CWD slowsevaporation of soil moisture and provides damp microhabitats for moisture-sensitive organisms.[8]
Infire-prone forests, coarse woody debris can be a significant fuel during awildfire. High amounts of fuels can lead to increased fire severity and size. CWD may be managed to reduce fuel levels, particularly in forests wherefire exclusion has resulted in the buildup of fuels. Reductions in CWD for fire safety should be balanced with the retention of CWD for habitat and other benefits.[18] CWD of 3 to 8 inches (8 to 20 cm) in diameter is classified as 1000-hour fuel by fire managers, referring to the amount of time needed for the moisture content in the wood to come to equilibrium with the surrounding environment.[19][20]
InGlen Affric,Scotland, theTrees for Life group found the black tinder fungus beetle (Bolitothorus reticulatus) is dependent on a particular fungus (Fomes fomentarius), which itself grows only on deadbirch. Another insect, the pine hoverfly (Blera fallax), requires rottingScots pine in order to reproduce.[11]
In thetemperate deciduous forests of eastern North America, CWD provides habitat ranging from salamanders to ferns. It is an important indicator for evaluating and restoring this type of forest.[1]
In certainsubtropical areas such asAustralia wherebushfire constitutes a major hazard, the amount of CWD left standing or lying is determined by what may be considered safe in the course of reasonable fire prevention. When fires occur, some invertebrates find shelter either within or beneath dead tree logs.[citation needed]
In Canada,bears seek out dead tree logs to tear open and look for and feed onants andbeetles, a fact that has encouraged the authorities to reserve a sufficient amount of coarse woody debris for these purposes. In North America, too, CWD is often used as barriers to preventbrowsingdeer andelk from damaging young trees.[citation needed]
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