Does common mycelial network drive the assembly of forest communities? An immigration experiment in mesocosms

Background/Question/Methods

Most temperate forests contain diverse arrays of tree species, and undergo changes in species composition over time, from trees that form arbuscular mycorrhiza (AM) to those that form ectomycorrhiza (EcM), a phenomenon called community succession. Whether mycorrhizal symbiosis can mediate the succession of forest communities remains unclear in the field. In this study, we explored the potential for common mycelial networks created by aggregations of ECM versus AM trees in experimental mesocosms, to enhance seedling survivorship and growth near mycorrhizal parent trees. Our experimental design was fully factorial with three levels of resident treatments (AM, EcM and Control) crossed by three levels of immigration treatments (AM, EcM and Control) in randomized block design, replicated four times over a total of 36 experimental mesocosms (1.2 m x 1.2m x 0.4m in size) in the botanical garden (Kyoto University). The design was also multilevel in that each mesocosm, as an experimental unit, includes 16 individuals (subunits) of 4 tree species (four individuals per species) arrayed in Latin square.

Results/Conclusions

We found that both AM and EcM tree seedlings with access to an EcM network affected growth, leaf number, and survivorship than seedlings without such access, but the EcM tree seedlings with access to an AM network exhibited a reduction in growth rates and P concentration in leaf chemistry. The matching in mycorrhizal types between parent tree and seedling likely affected the diversity of herbivores, predators and soil-dwelling arthropods in experimental mesocosms. Combination of 454 pyrosequencing analysis of root-tips and interaction network theory allowed quantification of the strengths of positive and negative feedbacks that each fungal species has on seedling establishment. These experimental results suggest that the common mycelial network at the seedling stage may be an important factor mediating successional changes in the structure of temperate forest communities.