- This study characterizes the production of hypogeous sporocarps (broadly referred to as truffles) by ectomycorrhizal fungi within Douglas-fir dominated forests that are considered typical of those found on the west slopes of the central Cascade mountains in Oregon. Three aspects of sporocarp production are addressed: 1) the distribution of total biomass and biomass of each species by season and habitat, 2) analysis of sporocarp biomass from the perspective of community structure, and 3) correlation of biomass production with sporocarp number and selected forest floor parameters.
Sporocarps with an equivalent dry standing biomass of 1.3 kg/ha were harvested from ten Douglas-fir stands in and near the H. J. Andrews Experimental forest. The maximum single stand sample biomass was equivalent to 9.9 kg/ha. Forty-seven species of hypogeous fungi were recorded during the study (although some collections are of uncertain taxonomic affinity and some taxa are of uncertain status). Fourteen species account for 93% of the total biomass. Five species account for 73% of the biomass (Elaphomyces granulatus, Gautieria monticola, Hysterangium coriaceum, Leucogaster rubescens, and Rhizopogon parksii).
Individual species showed strong differential trends in seasonal production of sporocarp biomass, with spring and summer production being greater than fall in contrast to fall fruiting epigeous species. Many species showed differences in sporocarp production by habitat. Sporocarp production was evaluated in five Douglas-fir habitats, (wet old-growth, mesic old-growth, dry old-growth, mesic mature, and mesic young). The mesic mature forest habitat has the highest standing biomass value (2.2 kg/ha) of all the habitats. The dry old-growth forest habitat had the lowest (0.7 kg/ha). Analysis of the distribution of sample values indicates that samples of small total area overestimate biomass because of the strong skewing towards high values. Interspersion of the largest practical number of quadrats is required to reduce overestimation of standing biomass (expressed on a kg/ha basis) when localized concentrations of biomass are included in samples.
Vegetation studies have shown that, for vascular plants, similar species combinations recur under similar habitat conditions. Also, species abundance and composition change more or less continuously over the landscape. This study found communities of hypogeous ectomycorrhizal fungi to be co-extensive with associated vascular plant communities and sensitive to subtle variations in habitats spanning wet-to-dry and young-to-old gradients.
A profound dichotomy in seasonal fruiting pattern between spring and fall precludes the use of single season sampling to reveal fungal community structure. Furthermore, yearly variation in weather patterns causes variation in sporocarp biomass production that tends to obscure community structure responses to environmental gradients. When fungal data collected over a number of years from a stand are integrated, subsequent classification and ordination closely reflect the vascular plant classification and subtle responses to a moisture gradient. A fungal community guild structure was delineated that reflected the subtle variation in the studied habitats. The guild of hypogeous ectomycorrhizal has Rhizopogon parksii as the subterranean dominant counterpart to Pseudotsuga menziesii with Gautieria monticola nearly as wide spread and abundant. Changes in fungal community structure along the stand age gradient are noted, but the limited amount of replication in the present study makes this interpretation tentative.
Within three old-growth stands ranging from wet to dry, sporocarp biomass and numbers of hypogeous sporocarps were assessed in relation to each other, coarse woody debris, forest floor litter, and other selected forest floor parameters by use of regression models. Significant regressions between forest floor parameters are also examined.
Transformation of data values improved the normality of the distributions for most parameters. The regression of sporocarp biomass and number of sporocarps is significant, however, it is not a strong relationship and the use of numbers of sporocarps as a substitute for biomass is not recommended. The correlation between sporocarp biomass and forest floor depth was significant in the mesic old-growth stand only. Significant regression relationships between parameters are highly individualistic within each stand. Over all stands, a slight tendency for forest floor depth to increase with coarse woody debris cover was noted.
Regression analysis may have been hampered by the old-growth status of all stands. Perhaps, due to centuries of development without catastrophic disturbance, within-stand variation in the chosen parameters has been reduced to the point that trends in the relations between selected parameters are difficult to detect. In designing future research, it would be advisable to include stands varying considerably in old-growth characteristics and to assess the degree to which "carry over" of characteristics into second growth stands affects sporocarp production.