Assessing the dual-mycorrhizal status of a widespread tree species as a model for studies on stand biogeochemistry

Viewing plant species by their mycorrhizal type has explained a range of ecosystem processes. However, mycorrhizal type is confounded with plant phylogeny and the environments in which mycorrhizal partners occur. To circumvent these confounding effects, "dual-mycorrhizal" plant species may be potential models for testing the influence of mycorrhizal type on stand biogeochemistry. To assess their use as models, duality in mycorrhizas within a single host species must be confirmed and factors underlying their variation understood. We surveyed roots, soils, and leaves of mature aspen (Populus tremuloides) across 27 stands in western Canada spanning two biomes: boreal forest and parklands. Aspen roots were mostly ectomycorrhizal with sporadic and rare occurrences of arbuscular mycorrhizas. We further tested whether a climate moisture index predicted abundance of ectomycorrhizal roots (number of ectomycorrhizal root tips m^-1 root length) surveyed at two depths (0-20 cm and 20-40 cm) and found that ectomycorrhizal root abundance in subsoils (20-40 cm) was positively related to the index. We subsequently examined the relationships between ectomycorrhizal root abundance, leaf traits, and slow and fast pools of soil organic carbon and nitrogen. The ratio of leaf lignin:N, but not its components, increased along with ectomycorrhizal root abundance in subsoils. Soil carbon and nitrogen pools were independent of ectomycorrhizal root abundance. Our results suggest that (1) categorizing aspen as dual-mycorrhizal may overstate the functional importance of arbuscular mycorrhizas in this species and life stage, (2) water availability influences ectomycorrhizal root abundance, and (3) ectomycorrhizal root abundance coincides with leaf quality.