Debris the Science and Changing Perceptions of Dead Trees by Alexander Evans and Robert Perschel

An Appreciation of Debris The Science and Changing Perceptions of Dead Trees By Alexander Evans and Robert Perschel oarse woody debris. To most people, two Coarse woody debris (CWD) is wood in the forest that’s no of the three words in that phrase have longer part of a living tree. Rather than being dead, this wood negative connotations. Debris is the worst is an important source of life in the forest. Foresters who value offender, suggesting random junk strewn it for its ecological functions have been searching for new terms about. When the debris is coarse, it’s even for CWD, and you’ll hear it referred to as biomass, dead wood, worse – it’s unrefined, it’s big. coarse woody material, and even retained organic matter. VIRGINIA BARLOW 44 Northern Woodlands / Winter 2008 Harry Dwyer, a forester in Fayette, Maine, likens the lan- as a seed bed for trees and plants, a mulch layer, and a slow- guage predicament to that of another formerly maligned mate- release fertilizer. Decomposing fungi depend on dead wood for rial: garden waste. He said, “You could look at a pile of rotting nutrients and moisture, and in turn many trees rely on mutu- vegetables as garbage, or you could note its value and call it alistic relationships with mycorrhizal fungi. Nitrogen fixation compost.” Digging into a rotten log in his woodlot, Dwyer in CWM by microbes (both anaerobic and microaerophyllic displays some of the dark, wet material in his hand. “It doesn’t bacteria) provides this important element in both terrestrial matter what you call this – coarse woody debris, coarse woody and aquatic ecosystems. material, or wildlife habitat – it still functions as a critical com- In general, northern forests have less dead wood in them ponent of forest health. But the words we use do make a differ- now than they did before Europeans arrived. Old growth for- ence, particularly now that everyone is talking about removing ests tend to have more logs, snags, and other dead wood than biomass from the forest for fuel.” managed or younger forests. While dead wood accumulates in Let’s call it coarse woody material or CWM. forests over time, it doesn’t last forever. On average, snags typi- When trees die on their own, they remain part of the forest cally stand about 10 years, while fallen logs may last twice that community – as standing dead trees (snags), stumps, down logs, long. Timber harvests tend to temporarily increase the amount or fallen branches. This CWM is gradually broken down into of dead wood because branches and the unmerchantable parts smaller and smaller parts by fungi, insects, and microorgan- of felled trees are left on the ground, and just the more valuable isms, until it’s fully incorporated into the forest soil. Scientific parts of a tree are removed. research is gradually revealing the important role dead wood While harvests may produce a large amount of dead wood plays in forest ecosystems, from wildlife habitat to carbon stor- in the short term, they can also result in fewer trees left to age, though anyone who has ever disturbed an old wood pile die and become dead wood in the future. Bill Keeton, a forest knows dead wood is alive with animals and plants. Moving an ecologist at the University of Vermont, advocates for a balance old log might reveal a black rat snake using it for a nest site, between removing the maximum volume of wood products and or searching the dead wood for dinner. Above the forest floor, retaining some biomass to maintain the structural complexity snags are high-demand housing for wildlife. When a pile- of forest. To increase dead wood and other old-growth charac- ated woodpecker leaves a cavity in a dead tree, a whole host of animals – from other birds to squirrels to wasps to northern Inset: While tops left in the woods after a logging job might look long-eared bats – might move in. Logs that fall in streams and messy, they serve an important ecological role. These tops will pro- rivers form critical aquatic habitat by ponding water and storing tect new tree seedlings from browsing deer before eventually rot- sediments. When water cascades over a fallen log, it can dig out ting and becoming fertilizer. Bottom: On the other hand, tree tops a small pool that will shelter young trout. can be chipped and converted into heat and electricity, and it’s That’s not all. Coarse woody material serves as a carbon advantageous environmentally to burn local renewable resources JIM BLOCK sink (as much as six percent of the carbon stored in rather than fossil fuels from afar. The key, as always, is finding a Northeastern forests is in dead wood); it also serves balance between human needs and the overall health of a forest ecosystem. Northern Woodlands / Winter 2008 45 teristics in younger forests, Keeton favors harvest prescriptions crates for shipment over here but one was left behind on the that include retaining all trees greater than 24 inches in diam- dock. While Cotta’s perspective did not survive forestry’s initial eter, releasing crowns to accelerate growth of larger trees, and translation to American soil, it has gradually returned to the girdling (or felling) diseased, dying, or poorly formed trees to profession in this country, aided by luminaries such as Aldo create snags or downed logs. This ecological forestry approach Leopold, Rachel Carson, and E. O. Wilson. What was once con- values dead wood left in the forest along with the timber taken sidered debris is now valued for the carbon it stores, the wildlife out in a harvest. habitat it provides, and the nutrients it holds. At the same time, though, society increasingly has its eyes on European roots of forestry wood that has little economic value but has great value as a fuel Forestry in the United States was originally built on ideas source. Low-value trees, slash, and tops are being heralded as imported from Europe in the early 1900s. When early foresters “woody biomass,” a potentially important part of the solution to like Gifford Pinchot brought the foundation of scientific forestry our dependence on fossil fuels. over from France and Germany, they also brought an emphasis Woody biomass has long been a useful, if under-utilized, on the sustained yield of timber. Timber was the most important byproduct of forest management activities. Foresters and log- forest product in their minds, so it drove most decisions about gers have prided themselves on their ability to put much of a forest management. Consequently, dead wood in the forest came felled tree to use. But interest in removing woody biomass has to be considered “debris.” A dead tree on the ground was consid- recently accelerated because of the rising cost of other fuels, ered a waste of resources or a potential fire hazard. concerns about carbon emissions from fossil fuels, and risks in Another theme of early European forestry, however, included western states of catastrophic wildfires. For example, the U.S. a more holistic perspective. Forestry textbooks like those written Department of Energy has set a goal to increase biofuel use in by Heinrich Cotta in the first part of the nineteenth century, are the U.S. 25 times by 2030. While biomass can be produced agri- distinctly different from the kind of forestry brought to America culturally, much of the biomass to fuel this increase in renew- by Pinchot. Early pioneers like Cotta talked about using nature able energy would come from forests. In fact, a U.S. Department as the guide and practicing forestry that preserves the soil’s fertility. WATTS PAT It’s as if European forestry was packed up in Snags and downed trees create high-demand housing for wildlife, including this northern flicker (right) and this chipmunk (below). DRAKE FLEEGE 46 Northern Woodlands / Winter 2008 of Energy report estimates that U.S. forests could yield 368 mil- lished guidelines that suggest how and how much woody biomass lion dry tons of useable biomass per year. can be responsibly removed from forests. A recent experimental So the forest-policy discussion of woody debris has become harvest on the Superior National Forest, in Minnesota, showed increasingly complex. On the one hand, optimizing the utiliza- that when harvesters followed state biomass guidelines, there tion of wood – a naturally renewable resource – can help replace were adequate snags and coarse woody material left behind. A our reliance on foreign oil and can be a substitute for oil and report on impacts of biomass harvesting from Massachusetts has other fossil fuels, thus helping to combat global climate change. good news about calcium, a particularly important soil nutrient. On the other hand, as the allure of using woody biomass for When only 50 percent of the biomass is removed, the current energy increases, the removal of additional biomass raises ques- level of calcium would be replenished in 71 years. tions about how much wood can be taken from a forest before A good general guideline for northeastern forests comes from the forest suffers negative consequences. the Society for the Protection of New Hampshire Forests’ manual When timber harvests remove whole trees for energy pro- Good Forestry in the Granite State: “Manage for coarse woody duction, the risk of nutrient loss increases because more leaves material by retaining material that currently exists and allow- and needles are removed from the forests. Foliage and small, ing its accumulation where it is currently missing.” The Forest fine branches contain much higher concentrations of nutrients Stewardship Council’s standard for green-certified forests in the than stem wood.

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