Biomass and community structure of sporocarps formed by hypogeous ectomycorrhizal fungi within selected forest habitats of the H.J. Andrews Experimental Forest, Oregon

This study characterizes the production of hypogeous sporocarps(broadly referred to as truffles) by ectomycorrhizal fungi within Douglas-firdominated forests that are considered typical of those found on the west slopesof the central Cascade mountains in Oregon. Three aspects of sporocarpproduction are addressed: 1) the distribution of total biomass and biomass ofeach species by season and habitat, 2) analysis of sporocarp biomass from theperspective of community structure, and 3) correlation of biomass productionwith sporocarp number and selected forest floor parameters.
Sporocarps with an equivalent dry standing biomass of 1.3 kg/ha wereharvested from ten Douglas-fir stands in and near the H. J. AndrewsExperimental forest. The maximum single stand sample biomass wasequivalent to 9.9 kg/ha. Forty-seven species of hypogeous fungi were recordedduring the study (although some collections are of uncertain taxonomic affinityand some taxa are of uncertain status). Fourteen species account for 93% ofthe total biomass. Five species account for 73% of the biomass (Elaphomycesgranulatus, Gautieria monticola, Hysterangium coriaceum, Leucogasterrubescens, and Rhizopogon parksh).
Individual species showed strong differential trends in seasonalproduction of sporocarp biomass, with spring and summer production beinggreater than fall in contrast to fall fruiting epigeous species. Many speciesshowed differences in sporocarp production by habitat. Sporocarp productionwas evaluated in five Douglas-fir habitats, (wet old-growth, mesic old-growth,dry old-growth, mesic mature, and mesic young). The mesic mature foresthabitat had 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 thedistribution of sample values indicates that samples of small total areaoverestimate 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) whenlocalized concentrations of biomass are included in samples.
Vegetation studies have shown that, for vascular plants, similar speciescombinations recur under similar habitat conditions. Also, species abundanceand composition change more or less continuously over the landscape. Thisstudy found communities of hypogeous ectomycorrhizal fungi to be co-extensive with associated vascular plant communities and sensitive to subtlevariations in habitats spanning wet-to-dry and young-to-old gradients.
A profound dichotomy in seasonal fruiting pattern between spring and fallprecludes the use of single season sampling to reveal fungal communitystructure. Furthermore, yearly variation in weather patterns causes variation insporocarp biomass production that tends to obscure community structureresponses to environmental gradients. When fungal data collected over anumber of years from a stand are integrated, subsequent classification andordination closely reflect the vascular plant classification and subtle responsesto a moisture gradient. A fungal community guild structure was delineated thatreflected the subtle variation in the studied habitats. The guild of hypogeousectomycorrhizal fungi has Rhizopogon parksii as the subterranean dominantcounterpart to Pseudotsuga menziesii with Gautieria monticola nearly as widespread and abundant. Changes in fungal community structure along the standage gradient are noted, but the limited amount of replication in the present studymakes this interpretation tentative.
Within three old-growth stands ranging from wet to dry, sporocarpbiomass and numbers of hypogeous sporocarps were assessed in relation toeach other, coarse woody debris, forest floor litter, and other selected forestfloor parameters by use of regression models. Significant regressions betweenforest floor parameters are also examined.
Transformation of data values improved the normality of the distributionsfor most parameters. The regression of sporocarp biomass and number ofsporocarps is significant, however, it is not a strong relationship and the use ofnumbers of sporocarps as a substitute for biomass is not recommended. Thecorrelation between sporocarp biomass and forest floor depth was significant inthe mesic old-growth stand only. Significant regression relationships betweenthe parameters are highly individualistic within each stand. Over all stands, aslight tendency for forest floor depth to increase with coarse woody debris coverwas noted.
Regression analysis may have been hampered by the old-growth statusof all stands. Perhaps, due to centuries of development without catastrophicdisturbance, within—stand variation in the chosen parameters has been reducedto the point that trends in the relations between selected parameters are difficultto detect. In designing future research, it would be advisable to include standsvarying considerably in old-growth characteristics and to assess the degree towhich "carry over" of characteristics into second growth stands affects sporocarpproduction.