This study was designed to determine if ectomycorrhizal mats are more likely to be present in soils at the base of common understory trees than in soils distant from trees and to determine if there is a relationship between the occurrence of mats and tree size. The understory trees studied included the Pacific yew (Taxus brevifolia Nutt.) western hemlock (Tsuga heterophylla (Raf.) Sarg) and vine maple (Acer circinatum Pursh). Twelve sites were studied representing a range of climatic conditions and management histories. More than 2,900 plots were surveyed over an 18-month period. It was found that the incidence of ectomycorrhizal mats was higher at the base of all three species of trees when compared with control plots. A statistically significant positive correlation was also observed between the incidence of mats and tree size. When comparing saplings with larger trees, the saplings consistently had a lower incidence of mats. These observations are not consistent with the hypothesis that ectomycorrhizal mats are required for T. brevifolia establishment or survival.
Robert P. Griffiths
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.
To document the relationship between yew and two other common understory trees and ectomycorrhizal mats, we conducted a series of surveys at 12 locations in Oregon. In these surveys we looked for the existence of ectomycorrhizal mats at the base of understory trees, comparing the incidence of mats in those locations with randomly selected circular plots with a radius of 25 cm. We also measured the size of the trees to determine if this would provide insight into the mechanism behind this relationship.