The birth and death of ectomycorrhizal mats: Response of ectomycorrhizal fungal communities to a reciprocal soil transplant experiment

Background/Question/Methods

Ectomycorrhizal fungi (EmF) form symbioses with trees and profoundly influence the ecology of coniferous forests in the Pacific Northwest, USA. Certain EmF form specialized profusions of hyphae, known as fungal mats, which are visible to the naked eye, alter forest soil biogeochemistry, substantially contribute to forest soil microbial biomass/total respiration, and support unique soil microbial communities. Two ubiquitous mat forming genera, Piloderma and Ramaria, tend to stratify in organic and upper mineral soil, respectively. The development (birth) and decline (death) dynamics of these mats are unknown. Investigating the disturbance susceptibility and establishment capability of these mat forming EmF and their associates will provide understanding of a microbial group vital to nutrient cycling and carbon storage in Pacific Northwest forest ecosystems. 

A reciprocal soil core transplant experiment was established at seven old-growth Douglas-fir sites at the HJ Andrews LTER, Oregon. At each site Piloderma, Ramaria, and non-mat areas underwent birth (non-mat into mat), death (mat into non-mat) and no manipulation. After 51 months, treatments were harvested and EmF species presence/absence determined; EmF root tips were sorted by morphotype, PCR amplified, and unique RFLP patterns sequenced to determine fungal species affinities.  

Results/Conclusions

A total of 111 EmF species in 32 genera were observed, capturing ~70% of expected diversity, with mineral soil more diverse than the organic horizon in all treatments. In both horizons natural EmF mats exhibited the highest diversity, with birth treatments intermediate between non-mat and mat soils. Preliminary community analyses indicate Piloderma and Ramaria mats support unique EmF communities compared to each other and non-mat soil. Even with the high heterogeneity expected in EmF studies, after 51 months Piloderma birth communities more closely resemble natural Piloderma mats than they do the non-mats from which they originated. Ramaria birth community response is more variable, but generally similar to that of Piloderma. These results suggest that EmF communities in mats are susceptible to disturbance, have long establishment periods (> 51 months), and play a crucial role in supporting EmF diversity.