Soil depth distribution of ecto- and arbuscular mycorrhizal fungi associated with Populus tremuloides within a 3-year-old boreal forest clear-cut

Populus tremuloides Michx. is a tree species that have both arbuscular mycorrhizal (AM) and ectomycorrhizal (EM) associations. The mechanisms underlying the coexistence of AM and EM are not well understood. In this study, we test the hypothesis that the two types of mycorrhizae are separated along a soil depth gradient. Vertical distribution of EM and AM colonization in roots of R tremuloides ramets (within a boreal mixedwood 3-year-old clear-cut) was assessed at three different rooting depths (i.e. 0-5, 5-10 and > 10 cm) and compared to edaphic properties in the adjacent soil. We predicted that EM colonization would be more abundant in the shallow organic soils and AM colonization would be more abundant in deeper mineral soils. Significantly fewer EM root tips were found in the >10 cm soil depth than in the 0-5 and 5-10 cm depths. Vesicle, AM hyphae and total fungal colonization were significantly lower in the top 5 cm of the soil than in the 5-10 and >10 cm depths. Overall EM root tip mantle formation approximately averaged 86%, while overall AM colonization averaged 6%. A significant negative correlation between EM and AM fungal colonization over all the depths was found (Spearman correlation, r = -0.314, P < 0.05). Results of this study support our hypothesis that EM and AM are preferentially partitioned at different soil depths. (C) 2002 Elsevier Science B.V. All rights reserved.