Landscape- and age-based factors affecting fungal colonization of conifer seedling roots at the alpine tree line

Survival of seedlings is a demographic bottleneck for tree populations in alpine tree-line ecotones (ATE) and may play a pivotal role in determining the elevation of timberlines and tree lines. We evaluated (1) fungal colonization of root surfaces of natural seedlings of Picea engelmannii Parry ex Engelm. and Abies lasiocarpa (Hook.) Nutt. in an ATE, (2) the relative importance of soil versus microclimate variation to differences in colonization in the ATE, determined using a reciprocal soil transplant experiment, and (3) ecophysiological responses of seedlings to natural and experimental variation in colonization. Many root systems lacked ectomycorrhizae but had extensive quantities of white mycelium attached to root surfaces. This mycelium was composed of a group of species referred to as the "alpine white complex" (AWC), and included Mortierella parvispora Linneman, Trichoderma viride Pers.:Fr., and a basidiomycete. Nearly all seedlings that survived beyond the first year of growth were colonized by AWC, whereas <10% of emergent seedlings had AWC where they commonly do not survive, in microsites distant from trees. No relationships of AWC and microclimate, photosynthesis, or water relations of seedlings were detected, and low colonization levels likely resulted from scarcity of AWC in ATE soils. Determining if and how AWC or emergent conifer seedlings benefit from their symbiosis in ATEs will require further inquiry.