A group of ectomycorrhizal fungi that form dense fungal mats in the litter layer and A horizon of forest soils can make up to 50% of the mass of mineral soils (Ingham et al., 1991). Because of the density of fungal material in these mats, they have proven to be excellent systems for measuring mycorrhizal function in forest soils (Griffiths et al., 1990). Studies comparing the chemistry and biology of mat-colonized soils and adjacent soils without obvious mat colonization have shown that these fungi are capable of increasing nutrient availability to trees by weathering mineral soil and decomposing soil organic material (Griffiths et al., 1990; Griffiths et al., 1991b; Griffiths and Caldwell, 1992; Aquilera and Griffiths, 1993; Griffiths et al., 1994). In addition, these mats have the ability to enhance Douglas-fir seedling survival under low-light conditions (Griffiths et al., 1991a).
Because of the commercial importance of yew bark for the production of the anticancer drug taxol and an increased interest in vegetation associated with old-growth Douglas-fir forests, there has been renewed interest in determining which factors influence the establishment and survival of Pacific yew. We have informally observed that Pacific yew in Douglas-fir old-growth forests is usually associated with ectomycorrhizal mats. This observation is all the more curious since T. brevifolia usually forms symbiotic relationships with vesicular arbuscular mycorrhizal (VAM) fungi but not with ectomycorrhizal fungi (Trappe, pers. comm.). The same is generally true of other Taxus species found in Canada and Europe (Prat, 1934; Bakshi, 1960), however there is one report of ectomycorrhizal colonization of Canadian yew roots (Boullard and Ferchau, 1962). Establishment of Taxus plants from seed is thought to be plagued by low seed germination rates. High seedling losses are also due to fungal root disease caused by Cylindrocarpon radicicola Manka et al. (1968) and other fungal pathogens. Since ectomycorrhizal fungi are known to protect trees from root pathogens (Marx, 1970) we hypothesized that these ectomycorrhizal mats relate to T. brevifolia seedling establishment and survival in coniferous forests of the Pacific Northwest. Because of the implications of this relationship, we initiated a study to determine if T. brevifolia was always associated with ectomycorrhizal mats and to determine if these mats were more likely to be associated with seedlings or larger trees.
Randomized plots 10-20 meters in diameter were examined for the presence of ectomycorrhizal mats at the base of all yew, maple, and understory hemlock trees within each plot. For each tree plot examined, there was a control area of equal size that was selected randomly from the base of each tree. The litter layer was removed and the mineral soil was raked to a depth of 10 cm in an area 25 cm from the base of the tree; comparable control areas were also raked to the same depth.
The presence of fungi having visual characteristics of mat-forming ectomycorrhizal fungi of the genera Hysterangium spp., Gautieria spp., and Piloderma bicolor (Peck) Jülih, was determined by visual inspection. Gautieria and Piloderma were the easiest to identify since there are few other fungal mats in Douglas-fir forests with which they can be confused. Gautieria mats are usually restricted to the top portion of the mineral soil and are typically very dry, almost powdery in consistency and associated with soils that have a bleached appearance (Griffiths et al. 1991a). Piloderma mats are typically found in the litter layer and at the interface between litter and mineral soil and have relatively coarse, bright yellow rhizomorphs. Hysterangium mats are typified by relatively coarse white to cream rhizomorphs normally found in the litter and/or top of the mineral soil (Griffiths et al., 1991a) and are the most difficult to field identify by mat morphology. If there were any doubt if the mats were mycorrhizal, the investigator would visually inspect roots running through the mat to determine if they were mycorrhizal. Tree size, mat thickness, presence of large quantities of wood, type of mat and occurrence of mats in control areas were also noted. The direction and distance of a control plot relative to each individual tree plot was determined by a throw of a die. All control plots were from 1 to 6 meters from the corresponding tree plot. No control plots were within 1 meter from the closest tree. If a large rock or log occupied more than 50% of the control plot, another one was chosen.
All 12 study sites were located in Oregon. Sites 4-9 were all located in the H.J. Andrews Experimental Forest (HJA) within the central Cascade Mountains, sites 10-13 were located 20 km north and west of HJA, and sites 14 + 15 were located in southern Oregon approximately 200 km south of HJA (Table 1). With one exception, all sites at the HJA were in old-growth stands dominated by Douglas-fir in which there had been no prior harvesting. Site 6 had been heavily thinned but the remaining trees were all old-growth. Site 10 was a stand of second-growth Douglas-fir that had been thinned to a 70% crown cover. Sites 11 and 12 were both old-growth stands that had minor harvesting, leaving the sites with typical old-growth structure. Site 13 was clear-cut but not burned 3 years before this study. The yew at this site had typically resprouted from older trees. The site had been replanted in Douglas-fir 1 year before this study. Sites 14 and 15 were paired plots in which mat distributions in old-growth and harvested stands were compared in the much drier climate of southern Oregon. Site 15 had been harvested 5 years before this study but it was left as a shelderwood to facilitate stand reestablishment. The concentration of remaining old-growth trees is 7 trees per hectare. The 12 study sites ranged from moist to dry, and from flat to relatively steep with aspects ranging from north to west. The elevation ranged from a low of 610 meters at site 10 to 1430 meters at site 15. All sites were selected for the relatively high abundance of yew, yet all sites also had varying abundances of maple and hemlock.
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Manka K, M Gierczak, and Z Prusinkiewicz 1968 Zamierianie siewek cisa (Taxus baccata L.) w Wiezchiesie na tle zesprofitycznych grzybow srondowiska glebowego. PTPN Prace Komisji Nauk Roln. I Komiskji Nauk Lesnych 25,177-195.
