Ecosystem Processes Related to Wood Decay
10 February 2003
Wood decay elements – snags, down wood, root wads, tree stumps, litter, duff, broomed or diseased branches, and partially dead trees -- provide for more than just wildlife habitat. They also provide resources and substrates for many other organisms that perform vital ecological roles of transformation and cycling of nutrients, decomposition, respiration, and other biologically-mediated transformations (Edmonds et al. 1989). In turn, such roles affect ecosystems far beyond the confines of the wood decay elements per se, and can greatly contribute to overall ecosystem health, soil productivity, and growth of desired tree species (Harmon et al. 1986, Tinker and Knight 2000, Franklin et al. 2000).
Little research has quantified these relations in forests of Washington and Oregon. A few studies have been conducted in other regions and biomes (e.g., Clark et al. 2002 in tropical forests). Thus, we have not yet been able to develop quantitative guidelines for the type, amount, and distribution of wood decay elements needed to maintain specific levels of productivity, tree growth, and other ecosystem processes. However, it is clear that such processes associated with wood decay elements are nonetheless a natural and vital part of native forests and ecosystem processes, as reviewed here.
Down Wood as Reservoirs for Moisture and MycorrhizaeDown wood has a high pore volume and thus can serve as moisture reservoirs and provide persistent microsites that aid in forest recovery after prolonged drought or fire (Amaranthus et al. 1989). For example, in one study in southwest Oregon, down logs provided considerable rooting and mycorrhizal activity, and mean moisture content (157%) was 25 times greater than mean soil moisture (6%) (Amaranthus et al. 1989).
In forests of western North America, decomposing wood occurs in the organic humus horizon of soils (McFee and Stone 1966) and, indeed, throughout the entire soil horizon (Harvey 1993, Harvey et al. 1976b).
Down wood is also a major source of mychorrizal fungi (Amaranthus et al. 1996). Decaying wood retains moisture and serves as important reservoirs of such fungal activity during dry summer months (Harvey 1993, Harvey et al. 1976a). For example, commonly found in down wood are sporocarps of Douglas-fir tuberculate ectomycorrhizae, formed by Rhizopogon vinicolor. R. vinicolor is more routinely found on seedlings grown in clearcut soils, where it aids the host tree during drought by blocking entrance by pathogens or aphids (Zak 1971). A coarse woody debris-dependent ectomycorrhizal fungus is Philoderma fallax (Smith et al. 2000).
To a limited extent, ectomycorrhizae in down wood can break down lignins and convert nutrients including P, K, Ca, Mg, Mn, and Na into forms usable by insects, mollusks, and mammals (Maser et al. 1979). Although some ectomycorrhizal fungi have this lignin-degrading capacity, it is probably not much compared to decomposer fungi (Smith and Read 1997), which is also found in decomposing down wood, including that of Douglas-fir (Crawford et al. 1990).
In general, mycorrhizae provide moisture, phosphate, and nitrogen from the soil to a substantial degree to coniferous plants, and serve as important mediators in soil nutrient cycles (Fogel and Hunt 1983). In this symbiotic relation, conifer trees in turn provide carbohydrates for the mycorrhizae. This is a relationship critical for tree productivity particularly for conifers in relatively infertile soils. Amounts of mycorrhizae are closely correlated with conifer tree growth and tend to be concentrated in the organic horizons of the soil. For example, in one study, during peak growth (June-July), 95% of the mycorrhizal mass in a midslope stand of Northern Rocky Mountain subalpine fir forests occurred in the organic horizon of the soil. This underscores the important role that decaying wood and the organic soil horizon play on affording fungi and influencing tree production (Harvey 1993).
Nutrients and Microbes in Decaying Wood
Decaying wood also is a major contributor
to humus and soil organic matter that, in turn, help maintain or improve soil
structure, productivity, and nutrients (Rose
et al. 2001, Grier 1978, VanCleve and
Noonan 1975). The available nitrogen in forest soils is largely found in organic
matter and woody material within the soil (Means et al. 1992). Woody material
in the soil creates acidic soil conditions which are favorable for soil microbial
activity that help fix nitrogen.
The amount and distribution of nutrients in different woody tissues vary among regions and forest types (Rose et al. 2001). In forests, most of the nutrients used are found in leaf litter, small twigs, and small roots, rather than those bound up in larger woody structures (boles, branches, large twigs, and large roots).
However, after large-scale disturbance such as fires and blowdown, the nutrient pool in woody structures becomes available as an important source to the regenerating forest during secondary succession. Down wood and other wood decay elements likely play key roles in nutrient release (mineralization), particularly as mediated through the biological activities of fungal sporocarps, mycorrhizae and roots, leaching, fragmentation, and insects (Harmon et al. 1986, Hyvonen et al. 2000; see summary in Rose et al. 2001).
That is, when a tree trunk decomposes, free-living N-fixing bacteria invade and pull available nitrogen into that site from the outside. So the fresh down log does not have very much nitrogen in it, but older decomposing logs serve to pull in nitrogen, making it available then for conifer tree roots to transport it out.
Residual (dead, decaying) tree roots can also add to soil organic matter and can play positive roles in soil ecology. Removing soil organic matter by removing or reducing natural levels of wood decay elements, including old tree roots, stumps, and down wood, results in lowering soil cation exchange capacity, reducing soil moisture retention, and increasing soil compaction (Amaranthus and Steinfeld 1997, Li and Crawford, in press, Page-Dumroese et al. 1998).
Nitrogen can get into forest soils through two microbial processes: (1) symbiotic processes of N-fixation through nodulated plants with bacteria, in the roots, that use energy supplied by such plants as alfalfa, and (2) nonsymbiotic processes of N-fixation, through free-living N-fixing bacteria that use energy from the organic matter in the soil. Forest management essentially depends on this latter process, although the former process occurs uncommonly in forest soils as well. The free-living, N-fixing soil bacteria occur within decayed logs on the top of the soil. Such logs are thought of as a nitrogen sponge or nitrogen pump (Harvey 1993).
Free-living, N-fixing soil bacteria are more common in wood within the soil in dry sites than in wet sites. Again, this highlights the important role of down and decaying wood. The bacteria concentrate in the organic soil horizon, where nitrogen is stored and fixed. That is, N-storage and –fixation both occur in soil woody material. N-fixation is highly afforded by alder and some from ceanothus. Other N-fixing nodule plants in Douglas-fir and grand fir forests include Shepardia, Astragalus, Lupinus, and Trifolium.
Standing snags, too, play roles in providing forests with nutrients. A decomposing snag, like down wood, serves as a nitrogen sponge. Once fallen, it begins its life as soil wood and provides the ecological services thereof.
Down Wood as Nurse Logs
Large down wood (“coarse woody
debris”) often serves as nurse logs for many tree and shrub species. In
the Pacific Northwest, species often found growing on down wood include Picea
sitchensis, Tsuga heterophylla, Alnus rubra, Pseudotsuga menzeisii, and
(Harmon et al. 1986), as well as many shrub and herb species. Nurse logs can
provide highly space-efficient growing substrates for trees; for example, Graham
and Cromack (1982) reported that 94-98% of the tree seedlings growing on coarse
woody debris in a P. sitchensis-T. heterophylla
forest occurred on only 6-11% of the forest floor. Decomposing nurse logs provide
a superior seed bed for some plants because the logs concentrate nutrients,
store water, accelerate soil development and organic matter input, reduce erosion,
and lower competition from mosses and herbs (Rose et al. 2001). Coarse
wood can also help stabilize slopes and stave off surface erosion.
Down wood, including nurse logs, can facilitate seedling establishment in other ways, as well. Gray and Spies (1997) found that the shade from woody debris facilitated seedling establishment in canopy gaps within forest stands. Additionally, they found that western hemlock seedling establishment under forest canopies was greater on retained decayed wood than on forest floor or mineral soil.
Wildlife and Insects Associated with Wood
Decay and Down Wood
Many forest-dwelling mammals associated
with wood decay elements (Bowman et al. 2000, Aubry et al. 2003, Butts and McComb
2000) eat mycorrhizal fungi and disperse the spores through their feces (Maser
and Maser 1988, Maser et al. 1978). The feces often contain N2-fixing microbes
(Li et al. 1986a, 1986b), which in turn play vital roles for tree establishment
and the maintenance of ecosystem productivity (Li and Crawford, in press).
Many instances of wildlife and insect use of wood decay and down wood can be found in the literature (Fischer and McClelland 1983). As examples: Bull and Blumton (1999) reported that fuels reduction following timber harvest in lodgepole pine forests resulted in a decline in numbers of red squirrels, snowshoe hares, and red-backed voles, but an increase in chipmunks. Tallmon and Mills (1994) reported use of logs by California red-backed voles in a forest patch. Tinnin and Forbes (1999) reported red squirrel nests in witches’ brooms in Douglas-fir trees. Bull et al. (2000) reported on black bear dens in hollow trees and logs in northeastern Oregon. Aubry et al. (1988) reported on use of down wood by plethodontid salamanders in Douglas-fir forests in Washington. Vonhof and Barclay (1997) found western long-eared bats using tree stumps. Rabe et al. (1998) found bats using Ponderosa pine snags as breeding roosts in northern Arizona. A number of papers report use of standing and down wood-decay elements by invertebrates (e.g., Koenigs et al. 2002), including use of residual snags in clearcuts (Kaila et al. 1997) and hollow trees (Ranius 2000). Many other examples can be found in the DecAID Advisor.
Fire and Decaying Wood
Fire can affect the amount and distribution
of wood decay elements (Everett et al. 1999) and their associated ecological
roles and microbial constituents (Harvey 1994, Hansen et al. 1991, Harvey et
al. 1976a) with various influences on soil productivity and subsequent growth
of conifer trees (Zabowski et al. 2000). Intense, hot fires can do a lot of
damage to the soil ecosystem by excessively removing decaying wood from the
forest floor. In forests of the inland west U.S., Harvey (1993) found that severe
and extreme burns resulted in loss of major amounts of mineralizable nitrogen
and organic matter that provided nutrient-cycling roles, whereas slight burns
had little effect.
Wildfire can greatly increase the net amount of down wood in a stand, whereas timber-harvesting may increase or decrease down wood, depending on post-harvest and site preparation activities, and if unmerchantable woody material is left on site, piled and burned, or otherwise removed, and depending on time since last fire, the type and intensity of fire, and other factors. Foster et al. (1998) reported that ecological results and subsequent patterns of forest development following various kinds of major, infrequent disturbance events – fire, hurricanes, tornadoes, volcanic eruptions, and floods – varied greatly depending on the specific disturbance, the abiotic environment (especially topography), and the composition and structure of the vegetation at the time of the disturbance. Franklin et al. (2000) similarly found great differences in kinds and amounts of legacy wood (large, remnant trees, snags, and down wood) resulting from even-age silvicultural disturbances (especially clearcutting) and natural disturbances, such as windthrow, wildfire, and volcanic eruptions.
In one study in lodgepole pine forests of Wyoming, Tinker and Knight (2000) found that with repeated timber harvests, dead wood remaining as slash and stumps may decline and that forest floor and surface soil characteristics may be beyond the historic range of variability of naturally-developing stands. In another study, burning of logging residue (“slash”) after clear-cutting aided 2nd-year survival and height growth of seedlings planted in a high-elevation subalpine fir and lodgepole pine forest in north-central Washington (Lopushinsky et al. 1992). However, longer-term effects of removing wood decay elements from subsequent growing forests were not included in this study, and productivity (seedling growth and survival, as distinguished from initial seedling establishment) may later decline (Minore 1986). Dead wood, and to a lesser extent humus, are habitat for mycorrhizae that provide for early forest regeneration in moist, moderate, and dry conditions alike, but especially so in dry conditions. When dead wood and soil organic matter are reduced or removed such as by site preparation and slash burning, plantations might still become established but subsequent tree growth, health, and survival may be poor (Harvey 1993).
Of course, human safety can be a major concern with wildfire or prescribed fires, and such concerns may override the need to retain wood decay elements in fire-prone forests near human habitations (Winter et al. 2002). Balancing forest restoration with safety concerns is no trivial matter (Fule et al. 2001) and is beyond the scope of this discussion.
Case-hardening or external charring of down logs from surface fires does not significantly reduce the microbial and mycorrhizal functions of the wood, and in fact is habitat for a number of fungi species that specifically tolerate such charred surfaces. However, charring and hardening might adversely affect the value of a down wood and the soil organic horizon as habitat for some invertebrates and wildlife (Wikars and Schimmel 2001, Simon et al. 2002).
Standing live trees and snags have little direct effect on soil temperature during forest fires. Rather, it is the down wood, especially the large coarse wood on the forest floor, that affects soil temperature during burns (Harvey 1993).
Managing Forests for Benefits of Wood Decay: How Much, and How?Decaying wood is a natural part of forest ecosystems. If depleted, it may take a long time to get wood back into a forest soil. Woody material that is completely buried in some soils of the inland west U.S. have been carbon dated to about 500 years old, and some might be on the order of 1000+ years old, especially in stable soils on flat slopes (Harvey 1993). So coarse down wood that enters the soil generally tends to stay there. As well, forest soils tend to develop in place, unlike agricultural soils. All this means that restoring natural levels of coarse wood incorporated into soil horizons may be an immensely long-term process.
Within soil horizons, some species of wood are more persistent than others, especially pines, larches, and Douglas-fir, which decompose largely to a “brown rotted wood” condition. These have very high persistence times within soils, as they have high lignin content that resists decomposition. This means that their beneficial function as reservoirs as moisture, mycorrhizae, microbes, and nutrients can last for decades and centuries.
Harvey (1993) has initially recommended, in forests of the inland west U.S., providing about 30% of organic volume content in soils to maintain peak mycorrhizae amounts in the organic soil horizon. This translates to about 22-34 metric tons/ha (10-15 short tons/acre) of surface down wood, which should be relatively large woody residue, scattered across areas with minimal soil disturbance. Follow-up research on this recommendation (e.g., Graham 1981, Graham and Cromack 1982, Harvey et al. 1989) generally supported this recommendation but also found a variation among forest types throughout the southwest and west U.S., with western hemlock/Clintonia forests having much higher levels, and grand fir/Acer forests having a lot less.
In some forests, providing more than 22-34 metric tons/ha (10-15 short tons/acre) of coarse down wood may impart a fire hazard. In forests of the inland west U.S., one rule of thumb is that about 135 metric tons/ha (60 short tons/acre) is a fire hazard (Harvey 1993). So this still gives a broad scope for managing variable amounts of soil organic matter and the coarse woody debris that creates it, in inland west forests.
Harvey (1993) also recommended providing coarse, large down wood as sources of soil wood for future nitrogen and nutrient sources, and leaf litter, small twigs, and roots as more immediate sources of nitrogen. But it is only as large chunks does decaying wood provide its most beneficial, long-term (“time-released”) ecological services. Large decaying wood provides an acidic, high-phenolic, lignin matrix that best serves conifers and certain soil microbes (but not herbaceous plants and other microbes). Coarse wood in the soil is a very unique and critical element for forest productivity.
Chipping of fuel wood and distributing the chips on site does not seem to be an ecologically viable way of reducing excess fuels. In one experiment in a high-elevation forest in Wyoming, it was found that rainfall leached large amounts of toxic, water-soluble phenolics from the chips, and as a result all tree seedlings dies on the site (Harvey 1993). This also caused blocked soil structure following winter freeze.
Instead, Harvey (1993) recommended potentially using chips to create “artificial logs” (e.g., the artificial “Aqua Log” produced by Big Creek Stream Care Products) to cover <25% of the area, creating piles large enough to provide deposits of large coarse wood similar to natural levels. Whether such an approach is economical has not been studied.
For Further Reading
For more information on ecosystem processes
related to wood decay, we direct the reader to reviews by Harmon et al. (1986),
Rose et al. (2001), and
Maser and Trappe (1984). The ecological roles of wood legacies left in forest
stands after timber harvesting were discussed by Franklin et al. (2000)
and Foster et al. (1998). Managing forests for wood decay elements was also
discussed by Harvey (1994) and Harvey et al. (1994); also see Hollenstein et
al. (2001). Also, we have not discussed here the role of wood decay in riparian
and aquatic systems, although these are roles vital to maintaining productivity
and diversity of those systems as well (e.g., Keim et al. 2000, Sedell and Maser
AcknowledgmentsMany thanks to David Perry, Sue Livingston, and numerous other peer reviewers of the DecAID Advisor for their helpful suggestions and comments.
DisclaimerAny mention of private organizations and commercial products is for illustrative purposes only and not intended to suggest endorsement by USDA Forest Service.
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