The structure of an ectomycorrhizal community was assessed on a 100-m2 plot in a 100-year-old, oligotrophic Norway spruce, Picea abies (L.) Karst., forest in southern Sweden. During the 6-year study (1986–1992) sporocarps were identified and their biomass determined. Late in the fall of 1993, we identified mycorrhizas and estimated their abundance. Forty-eight epigeous, ectomycorrhizal taxa were identified based on the examination of sporocarps. Hygrophorus olivaceoalbus (Fr.:Fr.) Fr. and six species of Cortinarius, i.e., C. acutus (Pers.:Fr.) Fr., C. brunneus (Pers.:Fr.) Fr., C. evernius (Fr.:Fr.) Fr., C. obtusus (Fr.) Fr., C. paleaceus Fr., and C. strobilaceus Moser, were found every year. For the period as a whole, they accounted for 32% of the annual sporocarp biomass. Twenty-one species were observed during 1 year only. Cenococcum geophilum Fr. and Piloderma croceum Erikss. & Hjortst. accounted for 18 and 19%, respectively, of the mycorrhizal abundance of the mycorrhizal root tips examined. Using polymerase chain reaction (PCR) based molecular methods, we were able to distinguish 25 taxa forming mycorrhiza from soil cores covering a total of 22.5 cm2 of the forest floor. Twelve of these taxa were identified using a sporocarp or mycelial culture based reference data base containing 25 of the sporocarp-producing species. These 12 species accounted for an average of 74% of the sporocarp biomass. In contrast, their share of the estimated mycorrhizal abundance and biomass was about 30%. At least half of the abundance of the belowground ectomycorrhizal community was accounted for by species that did not produce conspicuous epigeous sporocarps. Ascomycetes accounted for about 20% of the mycorrhizal abundance. Calculations showed that on a per hectare basis there was 8.8 kg of fungal biomass in the form of sporocarps (average annual cumulative production), an estimated 250–400 kg as mycorrhiza (standing crop) and 440 kg in the form of sclerotia of Cenococcum geophilum (standing crop). Key words: ectomycorrhizal community structure, ITS–RFLP, Picea abies.

Development of direct and indirect methods for measuring root turnover and the status of knowledge on fine root turnover in forest ecosystems are discussed. While soil and ingrowth cores give estimates of standing root biomass and relative growth, respectively, minirhizotrons provide estimates of median root longevity (turnover time) i.e., the time by which 50% of the roots are dead. Advanced minirhizotron and carbon tracer studies combined with demographic statistical methods and new models hold the promise of improving our fundamental understanding of the factors controlling root turnover. Using minirhizotron data, fine root turnover (y )1 ) can be estimated in two ways: as the ratio of annual root length production to average live root length observed and as the inverse of median root longevity. Fine root production and mortality can be estimated by combining data from minirhizotrons and soil cores, provided that these data are based on roots of the same diameter class (e.g., <1 mm in diameter) and changes in the same time steps. Fluxes of carbon and nutrients via fine root mortality can then be estimated by multiplying the amount of carbon and nutrients in fine root biomass by fine root turnover. It is suggested that the minirhizotron method is suitable for estimating median fine root longevity. In comparison to the minirhizotron method, the radio carbon technique favor larger fine roots that are less dynamics. We need to reconcile and improve both methods to develop a more complete understanding of root turnover.

SUMMARYThe effect of excess nitrogen alone, and in combination with phosphorus and magnesium starvation, on the production of extramatrical mycelium was studied in Scots pine seedlings ectomycorrhizal with Laccaria bicolor (R. Mre.) Orton, Hebeloma crustuliniforme (Bull, ex St-Amans) Quel. and Suillus bovinus (L. ex Fr.) O. Kuntze. Seedlings were grown in a semi-hydroponic cultivation system and the ergosterol assay was used to estimate fungal biomass. The mycelial biomass increased rapidly when N was kept low (10-20 mg 1 ') and in balance with other nutrients, but no extramatrical mycelium was produced when the N concentration was raised to 200 mg 1 '. The growth of the extramatrical mycelium resumed when the excess N treatment was terminated and the seedlings were returned to a low nutrient regime, Laccaria showing more complete resumption of growth than Suillus, while Hebeloma had a low production of extramatrical mycelium in all treatments. Compared with the external mycelium, the amount of fungal tissue on the mycorrhizal roots (mantle and Hartig net) was much less affected by the high N treatment. P starvation increased the production of extramatrical mycelium tenfold, with almost no difference between high and low N nutrient regimes, while Mg starvation had no effect on the fungal biomass. The high N treatment lowered P and particularly Mg concentration of the needles, regardless of the mycorrhizal status of the plant.It is suggested that nitrogen deposition from the atmosphere may damage the function of mycorrhiza even before root tip studies reveal any decline in the symbiotic state. On the other hand, moderate nitrogen fertilization of forest land unaffected by nitrogen pollution is likely to have only passing effects on mycorrhizal development.

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A technique for determining the ergosterol content in mycorrhizal pine roots using HPLC was developed. Pure cultures of different mycorrhizal fungi contained very similar and constant amounts of ergosterol, and calculations of fungal biomass in mycorrhizal roots based on ergosterol readings agreed well with results using other methods. The extraction and sample purification were simple and reliable. Consequently, the technique is considered practical wherever accurate estimates of the intensity of mycorrhizal infection are required.