Abstract
Ectomycorrhizal fungi (ECMF) are an important biotic factor in the survival of conifer seedlings under stressful conditions and therefore have the potential to facilitate conifer establishment into alpine and tundra habitats. In order to assess patterns of ectomycorrhizal availability and community structure above treeline, we conducted soil bioassays in which Picea mariana (black spruce) seedlings were grown in field-collected soils under controlled conditions. Soils were collected from distinct alpine habitats, each dominated by a different ectomycorrhizal host shrub: Betula glandulosa, Arctostaphylos alpina or Salix herbacaea. Within each habitat, half of the soils collected contained roots of ectomycorrhizal shrubs (host+) and the other half were free of host plants (host−). Forest and glacial moraine soils were also included for comparison. Fungi forming ectomycorrhizae during the bioassays were identified by DNA sequencing. Our results indicate that ECMF capable of colonizing black spruce are widespread above the current tree line in Eastern Labrador and that the level of available inoculum has a significant influence on the growth of seedlings under controlled conditions. Many of the host− soils possessed appreciable levels of ectomycorrhizal inoculum, likely in the form of spore banks. Inoculum levels in these soils may be influenced by spore production from neighboring soils where ectomycorrhizal shrubs are present. Under predicted temperature increases, ectomycorrhizal inoculum in soils with host shrubs as well as in nearby soils without host shrubs have the potential to facilitate conifer establishment above the present tree line.
Highlights
The elevational and latitudinal limits of the boreal forest are expected to expand with increasing temperature and longer growing seasons, resulting in the encroachment of seedlings into habitats currently supporting tundra vegetation [1,2]
Seedling establishment is facilitated by a number of direct and indirect biotic factors [3,4,5], including the availability of ectomycorrhizal fungi (ECMF) [6], which colonize the roots of woody plants in a mutually beneficial symbiosis
We found that Ectomycorrhizal fungi (ECMF) were readily available to black spruce above treeline and that important differences in colonization levels, richness, diversity and species composition occur both among habitats and among soils within habitats
Summary
The elevational and latitudinal limits of the boreal forest (treeline) are expected to expand with increasing temperature and longer growing seasons, resulting in the encroachment of seedlings into habitats currently supporting tundra vegetation [1,2]. ECMF are especially important for seedling establishment in arctic and alpine soils, where cold temperatures can make water inaccessible through frost drought and slow microbial mineralization rates result in nutrients being bound to accumulated organic matter [12,13,14,15]). An important inoculum source for newly established seedlings is the dense network of fungal mycelia extending from and connecting to the roots of previously established ectomycorrhizal plants. These ‘‘common mycorrhizal networks’’ facilitate resource sharing between established plants and can be important for seedling recruitment [16]. Seedlings establishing in close proximity to mature ectomycorrhizal host trees often exhibit better growth and survival, as well as greater ECMF colonization and species richness than seedlings establishing at greater distances from ECM trees [17,18,19]
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