As I have stated earlier -- there is just too much indiscriminate mountain bike trail building going on inside our forests, with many practices that are very detrimental to the temperate rainforests' health and well-being, especially the removal or damage of critical nurse logs and snags.
The debris found in a forest serves as the "capillary bed" of the forest. The forest ecosystem is able to recycle nutrients by using this fallen debris primarily via with the fungus that grows on the decaying matter. Thus, not only does fungus aid in the decay that occurs in the forest, it also helps the forest to grow. Trees that have fungus associated with them actually grow faster and more vigorously than those that lack much fungus.
Moreover, decay itself is necessary for a forest’s survival because of its critical role in renewing and recycling nutrients. It has been calculated that there is actually more life in a fallen tree, or old growth stump than in a living one. As you can see in the picture, a younger tree is growing out of a decaying old growth stump. This is what is known as a "nurse log". The fallen tree, or stump, provides nutrients that the sapling will depend on during its first decades of life:
(Mtn. View Park nurse stump with large live hemlock tree and massive root structure.)
The mountain biking trail builders are damaging the forest ecosystem by removing or destroying very old nurse logs, such as this one off Bobsled Trail on Fromme (to build an "All-Access Trail" for three and four wheeled mountain bikes!) This is detrimental to a healthy forest ecosystem:
(Bobsled Trail, off-trail damage to nurse logs...)
More destructive mountain bike trail building photos:
(Where one or two trails built may not create a great problem, it is too much NEW trail building, reroutes, bypasses, realignments, and "refittings" of all these mountain biking trails that is depleting the forest of the essential nutrients, and necessary topsoil, it requires to remain vigorous. The health of our forests is threatened by all this building to accommodate the insatiable appetites for this off-road thrillsport in our midst.)
And, soon, in 2014, DNV will also be doing more damaging Wildfire Mitigation work on Fromme Mtn and Mtn View Park areas.After noting that a wetland was filled in at Hyannis Park (Mt.Seymour), via "Wildfire Mitigation" work in 2013, we have to seriously question the wrongmindedness of this kind of work.
What is going on inside the District of North Vancouver that pursues the damaging our forests and wetlands, via Mountain Biking activities and Wildfire "Mitigation" -- rather than to protect our forests and disappearing wetlands?!
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 naturaland vital part of native forests and ecosystem processes, as reviewed here.
Down Wood as Reservoirs for Moisture and Mycorrhizae
Down 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 Tsuga placata (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 removedsuch 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).
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 1994).
Many thanks to David Perry, Sue Livingston, and numerous other peer reviewers of the DecAID Advisor for their helpful suggestions and comments.
Any mention of private organizations and commercial products is for illustrative purposes only and not intended to suggest endorsement by USDA Forest Service.
Amaranthus, M. P., D. S. Parrish, and D. A. Perry. 1989. Decaying logs as moisture reservoirs after drought and wildfire. (pp. 191-194) In: E. Alexander (Ed.). Stewardship of soil, air and water resources. Wathershed 89. R10-MB-77. USDA Forest Service, Region 10, Juneau, Alaska. Amaranthus, M. P., D. Page-Dumroese, A. Harvey, E. Cazares, and L. F. Bednar. 1996.Soil compaction and organic matter affect conifer seedling nonmycorrhizal and ectomycorrhizal root tip abundance and diversity. USDA Forest Service Research Paper PNW-RP-494. USDA Forest Service, Pacific Northwest Research Station, Portland OR. 12 pp.
(See article source link above, for more citations...)