Humans and nature competing for trees

Coarse woody debris: Humans and nature competing for trees

Hagan, John M

Dead wood is usually the last thing foresters and forest products companies want to see in their forests. However, before humans discovered so many practical uses of wood, dead and dying trees were basic to forest development. Not surprisingly, many plants and animals evolved dependencies on dead wood. Today, with maintaining biodiversity a primary goal of forest management, foresters are confronted with seemingly contradictory goals: prevent or minimize agents that damage trees, but also maintain biodiversity, including the species that need dead wood.

Natural phenomena that kill trees are the nemeses of foresters. Fire, disease, and windthrow are all common agents of tree mortality. Standing dead trees are of no value in the production of forest products, and foresters have long worked to minimize agents of tree mortality In a sense, humans compete with nature for wood.

Dead wood in a forest is commonly called coarse woody debris. It can take the form of standing snags, fallen logs, or broken branches or tops. Before humans discovered so many practical uses of wood, trees died of whatever cause and became coarse woody debris, often simply accumulating, decomposing, and slowly dissolving back into the forest floor.

The ecological life of a tree extends far beyond its own biological life in time, and sometimes in space. Depending on the size of a tree, its composition, and the climate, dead trees can persist in a forest from a few years in the tropics to centuries in temperate forests. It should not be surprising that many organisms have evolved a dependency on such a major physical component of forests. Less obvious may be the role decomposing wood can play in supporting living trees. Arguably, coarse woody debris has become a premier conservation concern for managed forest ecosystems.

The notion of sustainable forestry has captured the interest and attention of both the public and the private forestry sectors in many countries with large forest product industries. In its simplest form, sustainable forestry means maintaining the full array of benefits people derive from forests, now and in the future. But what happens when increasing one benefit might come at the expense of another benefit? For example, forest managers are typically expected to protect forests from wildfire, pests, diseases, and other damaging agents to maintain and improve long-term forest health and productivity. Thus the production of fiber and the production of ecologically important dead wood seem to be in conflict.

Managing a forest for dead wood represents one example of why the notion of sustainable forestry can be confusing at times. Which is more important: protecting a forest from damaging agents, such as those that generate coarse woody debris, or maintaining wildlife habitat in the form of coarse woody debris? The goals seem contradictory, yet foresters are charged with meeting both goals (and many others) simultaneously. Our purpose here is to provide a basic understanding of the ecological importance of coarse woody debris, to offer some suggestions on ways to generate coarse woody debris with relatively little effort, and to call on forest ecologists and foresters to work together to sort out the sometimes confusing and seemingly ambiguous tenets of sustainable forestry.

The Role of Debris

The extent of species’ dependence on dead wood is impressive. The most conspicuous dependents are birds that require cavities for nesting and roosting. Some species, most notably woodpeckers, are primary excavators of cavities in trees, and without cavities for nesting these species cannot persist in the forest. Most primary cavity excavators use dead or dying trees. Many more species are secondary cavity nesters and depend on woodpeckers or some other force to create a cavity for them. Twenty percent to 40 percent of the birds in a forest community can be dependent on cavities (Hunter 1990). Not surprisingly, population density of cavity-nesting species decreases with snag removal. In the Sierra Nevada forests of the western United States, abundance of cavity-nesting birds decreased by 77 percent after snags were removed (Raphael and White 1984). In Finland the elimination of large snags from the forest reduced the density of cavity nesters by 44 percent (Haapanen 1965).

Eventually, snags fall over and become logs on the forest floor. Although no longer useful for roosting and nesting woodpeckers, whole new communities of organisms take over. Logs provide a moist microclimate for amphibians and refuges for many forest-dwelling small mammals and, if the log is large enough, for large mammals, such as bears. Ruffed grouse use logs for their drumming courtship displays to attract mates. The American marten and other members of the weasel family Mustelidae use tunnels under suspended logs during winter for traveling and resting.

Of wildlife species in the northeastern United States, 28 birds, 18 mammals, 23 reptiles and amphibians, and hundreds of invertebrates and fungi use dead wood, either standing or fallen (Degraaf and Rudis 1986; Keddy and Drummond 1996). Of the 378 vertebrate species of the Blue Mountains of Oregon and Washington, 45 percent make some use of fallen logs (Maser et al. 1979). Clearly, coarse woody debris is important to meeting the biodiversity goal of sustainable forestry. In the northeastern United States and Canada the spruce budworm can damage or kill regionalscale tracts of forest. This budworm-the nemesis of foresters and wood production-also may be the most influential producer of coarse woody debris in the system. Although no one is concerned at present about conservation of the spruce budworm, there is concern for a host of invertebrate species that cannot exist without dead wood. Most invertebrates, such as bark beetles, wood borers, termites, ants, wasps, bees, and microorganisms not only require dead wood but are also important to the decomposition of wood and the reintroduction of nutrients into the soil (Harmon et al. 1986; Nilsson and Baranowski 1997).

Many species of ectomycorrhizal fungi have formed mutualistic relationships with live trees and serve to assist tree roots in the uptake of nutrients. Nutrient-rich, moist downed logs are important substrates for colonization of new fungal units and serve as “stepping stones” of fungi dispersal through the forest. Fungi represent an enormous taxonomic component of forests. In the Pacific Northwest, 527 species of fungi are closely associated with old-growth forests (FEMAT 1993). Almost 2,000 species of ectomycorrhizal fungi are associated with Douglas-fir forests (Bernstein 1977). Loss of coarse woody debris from Swedish forests is believed to account for the large proportion of threatened fungi species in that country (Rydin et al. 1997).

Woody debris also plays a role in nutrient cycling in the forest. Dead wood typically has a slow nutrient turnover rate and tends to store and even accumulate nutrients as it decomposes. Over time, however, nutrients are slowly released back into the soil, and dead wood provides a stable, longterm source of nutrients.

Coarse woody debris can promote soil and slope stability by reducing the rate of runoff and erosion. Logs that fall into streams can have a strong effect on stream morphology. They can alter water direction and rate of flow, cause the creation of deep pools, and provide shade and protection for fish and other aquatic organisms. Downstream logs can accumulate into piles of driftwood that cause deposition of sediment loads and create new semiterrestrial habitats, such as alder thickets (Maser and Sedell 1994).

The size of coarse woody debris and state of decay are important to biodiversity. For example, the largest of North America’s woodpeckers, the pileated woodpecker, nests in snags 14 inches dbh or greater. The downy woodpecker will nest in snags only 6 inches dbh. Logs and snags can be host for several different communities of plants and insects over time as the wood gradually decays. Different organisms are adapted to use dead wood at different stages of decay.

Development of Debris

How does coarse woody debris develop over time in a forest? For evenaged stands, the volume of coarse woody debris is very high immediately after disturbance events like windstorms, fire, or disease. Even-aged management, including clearcutting, where tops and branches are left at the stump, can mimic to some degree the volumes of coarse woody debris produced by natural stand-replacing disturbances. As woody debris generated by the disturbance event decays, and as the new stand is initiated, coarse woody debris volumes drop dramatically because there is no new source for coarse woody debris input (Bormann and Likens 1979). Coarse woody debris volume is often at a minimum when the forest is entering its steepest growth phase, and gradually begins to increase again as the stand enters the stem exclusion stage.

During later stand development when the mean annual growth rate begins to slow, coarse woody debris volumes reach a peak as stem competition and other mortality agents lead to the death of some larger trees. Thus, over time from the initial disturbance event to maturity, coarse woody debris volume follows a Ushaped pattern.

When the rate of coarse woody debris production in a stand becomes too great, foresters typically refer to it as overmature. In an old-growth forest the rate of tree growth is roughly counterbalanced by death (i.e., coarse woody debris production equals tree growth, or a “steady-state” condition), resulting in no net growth in the stand.

In some forest types the rate of production of coarse woody debris may fall off in the old-growth stage. The timing of the peak in coarse woody debris production depends entirely on the forest type, the life histories of the tree species, and the kind and frequency of natural disturbance characteristic of the region. In the beech, birch, and maple forests of the Northeast, coarse woody debris volumes peak at about 100 years of age, then drop and stabilize for the long-term, or until another major disturbance (Tritton 1980). In Douglas-fir forests of the Pacific Northwest, coarse woody debris volumes peak 400 to 500 years after a disturbance and then drop to a lower, somewhat stable level for hundreds of years (Spies et al. 1988). However, a decline in coarse woody debris production was not found in old-growth hardwood stands of the southern Appalachians (Harte and Swank 1997) or in spruce-fir forests in Newfoundland (Sturtevant et al. 1997). Tyrell and Crow (1994) found ever-increasing volumes of coarse woody debris in old-growth hemlockhardwood forests in northern Wisconsin and Michigan. Thus, an “overshoot” in the volume of coarse woody debris prior to the old-growth stage is not universal.

The temporal development of coarse woody debris is tightly linked to the natural disturbance regimes of a region. At some point all stands encounter a stand-replacing event, such as a catastrophic fire, hurricane, or insect infestation. Coarse woody debris volumes increase dramatically after such events. However, stands sometimes persist for very long periods, even centuries, in a steady-state condition where growth equals death. The challenge to foresters is to understand the natural dynamics of coarse woody debris production for the forest types in which they work, and to integrate that knowledge into timber harvesting and management.

Guidelines for Debris

How is understanding the ecological importance of coarse woody debris and its temporal development supposed to help the forester do his or her job? The first step involves thinking about coarse woody debris in the context of forest productivity. What agents produce coarse woody debris in this forest? When is it produced? How do forestry practices alter coarse woody debris volumes and patterns of development?

Creating coarse woody debris artificially can be fairly straightforward. It is more problematic to determine whether there is an ecologically serious shortage of coarse woody debris that requires management action. Because ecologists have not figured out how much coarse woody debris is required to support all coarse woody debris-dependent species for an area, or how much coarse woody debris may be needed to maintain nutrient balance, most best management practices (BMP) assume that some is better than none, and that more is almost always better.

Two specific examples of BMPs for coarse woody debris are provided here. The state of New Hampshire recently produced a manual of recommended voluntary forest management practices (NH Forest Sustainability Standards Work Team 1997). In stands under uneven-aged management, the manual recommends retaining a minimum of six secure (stable) snags per acre, with at least one exceeding 18 inches dbh and three exceeding 12 inches dbh. If such trees do not exist, live trees with defects that can become snags and cavity trees should be left. In stands under even-aged management, the manual recommends leaving a patch of uncut trees for every 10 acres harvested. The uncut patches should constitute at least 5 percent of the landscape. The manual suggests that any tree with existing cavities be retained, regardless of the management regime.

The New Hampshire manual recommends that loggers avoid damaging downed logs, especially those more than 18 inches in diameter. Cull material, especially hollow logs, should be left in the woods. Large cull material bucked at the landing should be returned to the woods.

Another example of managing for coarse woody debris comes from British Columbia. In contrast to the New Hampshire recommendations, which are voluntary, these criteria are regulated by the 1994 Forest Practices Code of British Columbia Act. The British Columbia guidelines involve retention of wildlife tree patches, although isolated, individual wildlife trees are also considered desirable. Wildlife trees are defined as “any standing live or dead tree with special characteristics that provide valuable habitat for conservation or enhancement of wildlife” (BC Ministry of Forests 1995).

The amount of forest in “wildlife tree patches” depends on whether landscapelevel objectives for biodiversity have been set by the landowner. If landscape biodiversity objectives have been set, then 7 percent to 15 percent of the land unit must be in wildlife tree patches. If landscape biodiversity objectives have not been set, 10 percent to 18 percent of the landscape must be designated as wildlife tree patches. The specific percentage further depends on the proportion of the landscape unit that is harvestable or operable, and the proportion of the landscape not previously managed under the wildlife tree patch guidelines (BC Ministry of Forests 1995).

The British Columbia Forest Practices Code states that maintaining downed coarse woody debris after harvesting is a critical element of managing for biodiversity, but that this goal “conflicts with existing utilization standards.” Nevertheless, the code recommends delimbing and topping trees at the stump, leaving nonmerchantable material on site (at or near the stump), and avoiding piling and burning slash.

There are similarities and differences in the British Columbia and New Hampshire guidelines for coarse woody debris. The British Columbia guidelines seem to be directed more toward even-aged forestry and have resulted in relatively complicated landscape-level calculations for meeting coarse woody debris goals. This probably reflects the larger spatial scale and different type of forestry operations in British Columbia compared with New Hampshire. The New Hampshire guidelines provide specific goals for the number and size of snags per unit area, which are more easily accomplished in a selection cutting regime or on a small woodlot.

Both sets of guidelines recognize that it is important to manage coarse woody debris and biodiversity at the landscape scale. It would be impossible to maintain all of a region’s biodiversity on every acre, or even on every 100 acres of forest. Thus, small woodlot owners, if interested in biodiversity, must consider which subset of biodiversity they are most interested in enhancing. Many large forest landowners (including members of the American Forest and Paper Association) have accepted the challenge of maintaining biodiversity. For large tracts of timberland, plantations and intensive shortrotation stands that have low levels of coarse woody debris can be a part of a larger landscape unit that successfully maintains biodiversity.

What ecologists do not yet know is what proportion of the forested landscape can be under intensive management before some species, especially debris-dependent species, become extirpated from the area. It is likely this proportion will vary with forest type and stand dynamics. Forests that go many hundreds of years without standreplacing disturbances-western hemlock or coast redwood, for examplemay have more species dependent on old-growth and coarse woody debris than forests that undergo more frequent natural disturbances. However, some frequent disturbance agents, such as wind, ice storms, or insect outbreaks (e.g., the spruce budworm cycle), also can generate large amounts of coarse woody debris, so dead wood is not always associated only with old forests.

But how much coarse woody debris is enough? How should it be distributed in the stand, or across the landscape? If forest ecologists don’t know how much coarse woody debris is needed to maintain biodiversity, how are foresters supposed to know? One useful strategy for tackling the issue at the present time is for foresters and ecologists to begin working together. They should ask the following questions:

What is the natural range of values for coarse woody debris in our forest types?

How do our managed stands compare with natural regimes in coarse woody debris volume?

Are our silvicultural methods diminishing coarse woody debris over the long term, and if so, can we devise operational strategies to generate coarse woody debris within acceptable cost constraints?

Knowledge will grow over time, and management strategies must remain flexible and responsive to new information. In the end, foresters and ecologists working together offer the best hope for understanding the ecology of coarse woody debris and for meeting the ambitious biodiversity goals of sustainable forestry.

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NILSSON, S.G., and R BARANOWSKI. 1997. Habitat predictability and the occurrence of wood beetles in oldgrowth beech forests. Ecography 20:491-98. RAPHAEL, M.G., and M. WHITE. 1984. Use of snags by cavity-nesting birds in the Sierra Nevada. Wildlife Monographs 86:1 GG.

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SPIES, T.A., J.E FRANKLIN, and TB. THOMAS. 1988. Coarse woody debris in Douglas-fir forests of western Oregon and Washington. Ecology 69:1,689-702. STURTEVANT, B.R., J.A. BISSONETTE, J.N. LONG, and D.W ROBERTS. 1997. Coarse woody debris as a function of age, stand structure, and disturbance in boreal Newfoundland. Ecological Applications7:702-12. TRITTON, L.M. 1980. Dead wood in the northern hard

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John M. Hagan (e-mail: jmhagan@ ime. net) is senior scientist and Stacie L. Grove is project manager and GIS specialist, Manomet Center for Conservation Sciences, 14 Maine Street, Suite 404, Brunswick, ME 04011.

Copyright Society of American Foresters Jan 1999

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