Alpine ectomycorrhizal plants as potential inoculum reservoirs for treeline conifers in southern British Columbia

Globally, many montane tree ranges are shifting elevation upward with climate change. Forest encroachment into alpine ecosystems may lead to alpine habitat loss, reduced biodiversity, and carbon cycle alterations. Alpine habitats may also become refuges for endangered subalpine tree species. Critical for successful upward tree migrations is seedling establishment. While many factors can influence this process, the presence or absence of root symbionts, which can facilitate seedling establishment, has received little attention. Many alpine plants form symbioses with ectomycorrhizal fungi (EMF) and tend to associate with a diverse array of generalist EMF. Hence, these plants may serve as sources of fungal inoculum for establishing trees. Here, I test this idea across an alpine treeline ecotone in southeastern British Columbia. I designed a study to (i) identify and quantify the EMF taxa shared between the roots of an alpine shrub, Dryas octopetala, and established seedlings of the federally-listed whitebark pine (Pinus albicaulis), and (ii) evaluate how the distribution of pine-compatible inoculum in soil relates to the proximity and cover of alpine host plants and distance from treeline. I used 3rd generation sequencing to characterize the root and soil-associated fungal communities. Results revealed high overlap between the EMF communities associated with alpine EM hosts and P. albicaulis. However, pine-compatible EMF richness in soils only varied with distance to the nearest tree and sample read depth, while neither proximity to alpine hosts, tree density, nor treeline showed any effects. EMF inoculum compatible with P. albicaulis appears to be widely available above treeline. This study provides new insights into the potential roles that alpine ectomycorrhizal plants play in facilitating climate-induced tree migrations.