Soil depth partitioning is thought to promote the diversity of ectomycorrhizal (EM) fungal communities, but little is known about whether it is controlled by abiotic or biotic factors. In three bioassay experiments, we tested the role of vertical soil heterogeneity in determining the distributions and competitive outcomes of the EM sister species Rhizopogon vinicolor and Rhizopogon vesiculosus. We planted Pseudotsuga menziesii seedlings into soils that were either a homogenized mix of upper and lower depths or vertically stratified combinations mimicking natural field conditions. We found that both species colonized the upper or lower soil depths in the absence of competition, suggesting that their distributions were not limited by abiotic edaphic factors. In competition within homogeneous soils, R. vesiculosus completely excluded colonization by R. vinicolor, but R. vinicolor was able to persist when soils were stratified. The amount of colonization by R. vinicolor in the stratified soils was also significantly correlated with the number of multilocus genotypes present. Taken together, our findings suggest that the differential vertical distributions of R. vinicolor and R. vesiculosus in natural settings are probably attributable to competition rather than edaphic specialization, but that soil heterogeneity may play a key role in promoting EM fungal diversity.
Bibliographical noteFunding Information:
The authors thank J. Walker and R. Bubriski for assistance with experimental set-up and maintenance, L. Lofgren for the production of Fig. 1, and M. A. Selosse and three reviewers for constructive comments on a previous version of this manuscript. Financial support was provided by a Natural Sciences and Engineering Research Council of Canada grant to D.M.D. and a National Science Foundation grant (DEB no. 1020735) to P.G.K.
- Competitive interactions
- Ectomycorrhizal (EM) fungi
- Edaphic specialization
- Multilocus genotyping
- Niche partitioning