Importance of Fungi in Forest Ecosystems
Fungi perform a number of essential functions in forest ecosystems and are an important forest resource. The following list is certainly not exhaustive, but includes functions for consideration when making forest management decisions.
- Mycorrhizal associates - Mycorrhizal fungi form mutualistic symbioses with host plant roots, increasing plant water and nutrient uptake in exchange for carbon (for reviews see Allen 1991, O’Dell et al. 1993, and Smith and Read, 1997).
- Pathogens - By killing trees, pathogenic fungi can reduce or eliminate plant species, cause gaps in the forest canopy that may increase plant species diversity (Holah et al. 1993), and add to accumulation of dead wood. As a consequence of altering plant diversity, pathogenic fungi in turn alter the fungal community (Christensen 1989). Other pathogenic fungi such as heart rot fungi, while not true tree killers, have an influence on nutrient cycling and wildlife habitat (Hennon 1995).
- Decomposers – Wood and litter decay fungi recycle carbon, minerals, and nutrients for use by other organisms, and contribute to the soil matrix physical properties. Fungal fruiting bodies are a major agent of nitrogen, phosphorus and potassium export from logs, particularly in the early stages of decomposition (Harmon et al. 1994).
- Wildlife food sources - Fungi provide an important food source for many species, including microbes, arthropods, nematodes, and mammals (Fogel and Trappe 1978, Maser et al. 1978, Ingham and Molina 1991).
- Edibles and medicinals – The harvest of edible and medicinal fungi, including chanterelles, morels, matsutake, boletes, truffles, ganoderma (reishi) and others is a growing industry, particularly in the Pacific Northwest (Molina et al. 1993, Schlosser and Blatner 1995, Amaranthus and Pilz 1996, Pilz et al. 1998). In addition, recreational mushroom collecting has become increasingly popular in the past several decades.
Presently no guidelines are in place for dead wood management to provide for the maintenance of fungal biodiversity. The following summary provides information regarding the relationship of dead wood to forest fungi, and direction for management within the context of the DecAID model.
IMPORTANCE OF DEAD WOOD TO FUNGIDown wood can be grouped in decay classes, the most commonly used being the 5-class system (Spies and Cline 1988, Maser et al. 1979). In this system, the five decay classes range continuously from I (recently down wood with intact bark and twigs) to V (soft and powdery texture). Although little is known about fungus-habitat relations, it is apparent that many fungi are associated with down wood. Following is a brief review of current knowledge.
In both old-growth and young stands of Douglas-fir in the West Cascades of Oregon, Smith et al. (2000) observed an increase in the occurrence of the ectomycorrhizal fungus, Piloderma fallax, with an increase in percent cover of down wood in decay class V. Amaranthus et al. (1994) found that down wood presence increased the probability of truffle and false truffle occurrence, particularly within one meter of down wood. Down wood acts as a moisture-retaining substrate, allowing root tips to support active ectomycorrhizae (Harvey et al. 1976, Harvey et al. 1978, Amaranthus et al. 1989, Harmon and Sexton 1995). These fallen tree “reservoirs” may provide refugia for seedlings and mycorrhizal fungi, particularly in more arid forests and at times of seasonal dryness. As stands mature, the availability of down wood may be crucial for establishment of fungi as well as plant seedlings (Kropp 1982).
In the case of decay fungi and pathogens, down wood is a direct food source. Studies in both Scandinavia and North America indicate the presence of large down wood promotes high species diversity of wood-decay fungi (Kruys, et al. 1999, Crites and Dale 1998, Ohlson et al. 1997, Høiland and Bendiksen 1996, Bader et al. 1995, Wästerlund and Ingelög 1981). Høiland and Bendiksen (1996) found that rare wood-inhabiting fungal species occurred primarily on long (average length = 11 meters) and well-decayed (average decay Class III) down wood. When surface area is taken into consideration, fine woody debris appears to be equally important to species diversity (Kruys and Jonsson 1999).
CONSIDERATIONS FOR FOREST MANAGEMENTIf maintenance of fungal biodiversity is a goal, then management options that provide for the needs of multiple species are appropriate. This would include a diversity in size and decay class of down wood. If protection of rare species is a management goal, it is desirable to have knowledge about the requirements for those particular species and how that habitat can be achieved and maintained. Although we know little about down wood requirements for most individual species, knowledge of ecological function may guide decision making in this area. For example, requirements for mycorrhizal fungi include not only down wood, but presence of living host plants of the appropriate species and age. For wood-decay fungi, the size and decay class of the down wood may be the primary factors. If enhancement of mushroom production then the ecological factors controlling fruiting can also become important in guiding the management decision.
How much wood is necessary?Currently no data exist for quantities or coverage of down wood necessary to support viability of fungi in the forest ecosystems of the Pacific Northwest included in the DecAID model. In the dry forests of western Montana, Harvey et al. (1981) estimated that 25-37 tones/hectare of down wood are needed to support ectomycorrhizal activity necessary for a developing ecosystem. Unfortunately there is no easy method to estimate similar values for Oregon and Washington; however drier habitat types presumably will require greater amounts of down wood.
Proximity to inoculum source and time to re-establishRemnant stands of late seral forest adjacent to managed areas may serve as species refugia (Clarkson and Mills 1994). In addition, resistant propagules of some fungi may remain in soils or down wood after disturbance (Baar et al. 1999). For other fungi, dissemination by spores into the disturbed area is the primary method of re-establishment. Regardless of the method of re-establishment, it may take a great deal of time before fungal species can be detected. For example, Danielson (1984) indicates that chanterelles did not occur in a 6-yr-old jack pine stand, regenerating after wildfire. Pilz et al. (1998) found chanterelles in 20-year-old coastal hemlock stands in Washington. After disturbance, colonization of down wood by many ectomycorrhizal fungi may be limited in the early stages of stand development. As stands mature, the availability of down wood is crucial for the establishment of fungi as well as plant seedlings (Kropp 1982). In younger stands, following disturbance or harvest, the presence of large, well-decayed down wood legacy may provide habitat for ectomycorrhizal fungi (Smith, et al 2000). Since many species associate with down wood in advanced stages of decay, the decay rate may become an important factor.
Size and distributionFungi live as aggregations (mycelia) of microscopic strands (hyphae) in soils, wood, litter or living plants. Mycelial colonies can range in size from microscopic to many acres and can persist for years (Smith et al. 1992, De la Bastide et al. 1994, Dahlberg and Stenlid 1995). Thus, the range in size of fungal individuals may influence the scale of down wood needed to provide habitat. In natural settings fungi are patchily distributed in the forest, in part due to the patchy distribution of substrate (living host plants, down wood). Therefore, the distribution of down wood is also a consideration. Scattered islands of down wood, including many sizes ranging from twigs to large pieces, may provide better fungal habitat than one size of down wood homogeneously covering the forest floor.
Timing of surveysIdeally, an inventory of fungal species present before activities occur will provide baseline information on community structure. Once a management decision is implemented, surveys to assess effectiveness are appropriate. However, inventories and surveys can be complicated by the fact that fungi are often difficult to detect. For the most part, fungi can be seen only when reproductive structures are produced in the form of cups, truffles, conks, and mushrooms. Timing of mushroom formation (and hence organism detection) is species specific, occurring when nutritional and environmental conditions (temperature, light, pH, moisture) are appropriate during particular seasons of the year (Hunt and Trappe 1987, Luoma 1991). Year-to-year variability in mushroom production also complicates surveys. Over a 3-year period, Luoma (1991) documented a similar number of truffle species occurring each year; however, the proportions of those species differed greatly from year to year. O’Dell et al. (1996) suggest monthly surveys in both spring and autumn for a minimum of five years in order to maximize detection.
ACKNOWLEDGEMENTSThanks to Jane E. Smith, Randy Molina (PNW Research Station, Corvallis, OR), Thomas O’Dell (Grand Staircase-Escalante National Monument, Kanab, UT), Efrén Cázares (Oregon State University), and the DecAID Science Team for critical review of this manuscript.
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