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abiotic factors affecting ectomycorrhizal diversity in boreal mixed-woods. -Oikos 102: 497-504.Ectomycorrhizal (ECM) diversity was measured in 12 mixed-wood stands in the Abitibi region of north-western Québec. Stands were of similar age and were situated on similar mineral soil deposits, but supported varying proportions of ECM host trees. Host roots were sampled in a manner that enabled their separation into species on the basis of wood anatomy. Shannon diversity indices for the ECM colonizing each host species were determined on the basis of ECM anatomy. The diversity of overstory trees, understory plants and host roots, as well as overstory tree composition, root density and pertinent abiotic factors were measured and used as independent variables in multiple regressions against ECM diversity. We found a positive relationship between overstory tree diversity and ECM diversity, which appears related to fungal host specificity. Although no direct relationship was seen between ECM diversity and soil factors, levels of exchangeable base cations were related to ECM fungal species composition which correlated with ECM diversity at the scale sampled.

The variations in the numbers of propagules and in the colonization rates of vesicular–arbuscular mycorrhizal fungi in soils of two sugar maple forests, one near Waterloo in southern Ontario, the other near Lacolle in southern Quebec, were investigated. Both forests comprised similar plant communities, and in each case the most numerous mycorrhizal spores were identified as those of Glomus macrocarpum, Glomus geosporum, and Acaulospora foveata. In both forests, root colonization and length of hyphae in soil peaked in spring and again in autumn, and spore densities were highest in autumn and gradually decreased during the rest of the year. Mean percent root colonization at Lacolle was twice as high as at Waterloo, and mean spore densities were almost 10 times higher. In contrast, average total length of hyphae in the soil was approximately 10 times higher at Waterloo than at Lacolle. Sporulation by G. macrocarpum was positively correlated with soil organic matter content, and sporulation by A. foveata was negatively correlated with increasing pH. Root colonization was not correlated with spore densities but was positively correlated with the total length of hyphae in the soil. Relationships among root colonization, production of external hyphae, and spore abundance in natural ecosystems are discussed. Key words: vesicular–arbuscular mycorrhizae, Acer saccharum, environmental influences.

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