Limited impact of soil inocula from arbuscular and ectomycorrhizal-dominated sites on root morphology and growth of four tree seedling species from a temperate deciduous forest

Mycorrhizal fungi are well known to enhance nutrient acquisition through hyphal foraging beyond the root depletion zone. However, plant species vary widely in the degree to which they rely on mycorrhizal fungi for nutrient acquisition. Species which rely more on mycorrhizal colonization are expected to perform poorly in soils with lower density/availability of mycorrhizal partners. Mycorrhizal fungal density/availability is expected to be influenced by the mycorrhizal guild of dominant plant neighbors. However, little is known how the local density/availability of mycorrhizal partners may affect fungal colonization, root morphological traits and plant growth.

We inoculated seedlings of ectomycorrhizal (ECM) and arbuscular mycorrhizal (AM) tree species with soils from ECM-dominated forests or AM-dominated open fields. We evaluated seedling mycorrhizal colonization, root morphology and growth in response to these inocula.

Both AM species (Acer rubrum L. and Ulmus americana L.) had higher AM colonization rates when inoculated with AM-dominated open field soils. However, only Acer grew better and invested less in autonomous root-based foraging, when inoculated with AM-dominated soils. Responses of ECM species to soil inocula were more nuanced. Both species remained unresponsive to inoculation treatment. Populus tremuloides Michx. ECM colonization was higher when inoculated with ECM-dominated soils, while neither root morphology nor mycorrhizal colonization responded to inoculum origin for Quercus rubra L.

Our results show that local variation in mycorrhizal fungal abundance does not necessarily lead to an integrated adjustment of autonomous foraging traits. In addition, we found that the variation in root colonization degree of seedlings from different species was not always accompanied with an increase of seedling performance.