The present study aims at providing standard values for the exploration type (ET)-specific quantification of extramatrical mycelium (EMM) of ectomycorrhizal fungi applicable to ecological field studies. These values were established from mycelial systems of ectomycorrhizae (ECM) synthesized in rhizotrons with near-to-natural peat substrate. Based on image analysis, the "Specific Potential Mycelial Space Occupation" (sPMSO), i.e. the ET-specific complete area that is covered by the EMM systems (mm 2 cm −1 ECM −1 ), and the "Specific Actual Mycelial Space Occupation" (sAMSO), i.e. the projection area of mycelial systems (mm 2 cm −1 ECM −1 ), were analyzed as an extension of a previously described approach. The "Specific Extramatrical Mycelial Length" (sEML) [m cm −1 ECM −1 ] and the "Specific Extramatrical Mycelial Biomass" (sEMB) (μg cm −1 ECM −1 ) were calculated for each of the ET via the proportion of hyphal projected area, hyphal length and biomass, the latter two being derived from previous measurements on Piloderma croceum, a "Medium-Distance" (MD)-ET. Both sPMSO and sAMSO were highest for the "Long-Distance" (LD)-ET, whereas those of the "Short-Distance" (SD)-ET and MD-ET were similar, although showing high variation. In contrast, mycelial density per occupied area of the MD-ET was twice as high as that of the LD-ET. Proportional to the sAMSO, the EMM length and biomass differed considerably between the three ET with values of the MD-ET being 1.9 times higher than those of SD-ET, and those of the LD-ET being 15 times higher than those of the SD-ET. These standards in relation to ECM length may ease quantification of mycelial space occupation and biomass in a relatively simple way. Thereby, the ET-specific contribution of EMM can be distinguishedalso of non-cultivable species-and up-scaling to large-scale estimation of cost/benefit relations is possible.

Elevated tropospheric CO(2) concentrations may increase plant carbon fixation. In ectomycorrhizal trees, a considerable portion of the synthesized carbohydrates can be used to support the mutualistic fungal root partner which in turn can benefit the tree by increased nutrient supply. In this study, Norway spruce seedlings were inoculated with either Piloderma croceum (medium distance "fringe" exploration type) or Tomentellopsis submollis (medium distance "smooth" exploration type). We studied the impact of either species regarding fungal biomass production, seedling biomass, nutrient status and nutrient use efficiency in rhizotrons under ambient and twice-ambient CO(2) concentrations. A subset was amended with ammonium nitrate to prevent nitrogen imbalances expected under growth promotion by elevated CO(2). The two fungal species exhibited considerably different influences on growth, biomass allocation as well as nutrient uptake of spruce seedlings. P. croceum increased nutrient supply and promoted plant growth more strongly than T. submollis despite considerably higher carbon costs. In contrast, seedlings with T. submollis showed higher nutrient use efficiency, i.e. produced plant biomass per received unit of nutrient, particularly for P, K and Mg, thereby promoting shoot growth and reducing the root/shoot ratio. Under the given low soil nutrient availability, P. croceum proved to be a more favourable fungal partner for seedling development than T. submollis. Additionally, plant internal allocation of nutrients was differently influenced by the two ECM fungal species, particularly evident for P in shoots and for Ca in roots. Despite slightly increased ECM length and biomass production, neither of the two species had increased its capacity of nutrient uptake in proportion to the rise of CO(2). This lead to imbalances in nutritional status with reduced nutrient concentrations, particularly in seedlings with P. croceum. The beneficial effect of P. croceum thus diminished, although the nutrient status of its host plants was still above that of plants with T. submollis. We conclude that the imbalances of nutrient status in response to elevated CO(2) at early stages of plant development are likely to prove particularly severe at nutrient-poor soils as the increased growth of ECM cannot cover the enhanced nutrient demand. Hyphal length and biomass per unit of ectomycorrhizal length as determined for the first time for P. croceum amounted to 6.9 m cm(-1) and 6.0 μg cm(-1), respectively, across all treatments.

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Species within the genera Tomentella are among the most important ECM in forests. However, our knowledge about their functional characteristics is still rather limited. The ectomycorrhizae of Tomentella badia on Picea abies are described here in detail and compared to the non-identified ECM Piceirhiza obscura. A pseudoparenchymatous mantle formed by epidermoid cells is covered by heaps of epidermoid cells. This mantle type is regarded as a new one and designated as mantle type R. Many cells filled with dark blue contents and/or blue granules, together with clampless hyphae, are distinct characters of these ectomycorrhizae. Molecular-phylogenetic analysis of the ITS region was used for identification.

From the mycelial coverage per occupied space (cf. Table 2), and an average hyphal diameter of 2.2 μm (based on measurements of the MDf-ET P. croceum; Weigt et al. 2010), the average distance between two hyphae of P. croceum amounted to 0.045±0.035 (SD) mm.The online version of the original article can be found at http://dx.doi.