Role of mycorrhizal networks and tree proximity in ectomycorrhizal colonization of planted seedlings

The importance of mycorrhizal network (MN)-mediated colonization under field conditions between trees and seedlings was investigated. We also determined the combined influences of inoculum source and distance from trees on the ectomycorrhizal (EM) community of seedlings. On six sites, we established trenched plots around 24 residual Pseudotsuga menziesii var. glauca trees and then planted seedlings at four distances (0.5, 1.0, 2.5, and 5.0 m) from the tree into four mesh treatments that served to restrict inoculum access (i.e., planted into mesh bags with 0.5, 35, 250 μm pores or directly into soil). Ectomycorrhizal communities were identified after two growing seasons using morphological and molecular techniques. Mesh treatments had no effect on seedling mycorrhizal colonization, richness, or diversity, suggesting that MN-mediated colonization, was not an essential mechanism by which EM communities were perpetuated to seedlings. Instead, wind-borne and soil inoculum played an important role in seedling colonization. The potential for MNs to form in these forests was not dismissed, however, because trees and seedlings shared 83 % of the abundant EM. Seedlings furthest from trees had a simpler EM community composition and reduced EM richness and diversity compared to seedlings in closer proximity.