Episodic dynamics of microbial communities associated with the birth and death of ectomycorrhizal mats in old-growth Douglas-fir stands

Graduation date: 2009Ectomycorrhizal (ECM) fungi of the genera Piloderma and Ramaria form dense hyphal mats in the organic and upper mineral soil layers, respectively, in coniferous forest floors of the Pacific Northwest. Previous studies have determined that fungal mats change the chemical, physical, and biological properties of the soil within. Little information exists on the dynamics of development and decline of Piloderma and Ramaria mats, and the type of fungal and bacterial communities that are associated with the mats. To address these topics, a reciprocal soil core experiment was installed in Piloderma and Ramaria mats found in old-growth Douglas-fir (Pseudotsuga menziesii) stands in the H.J. Andrews Experimental Forest located in the Oregon Cascade Mountains. Samples from each mat type were severed from the host tree and transplanted in closed PVC cores, preventing new root and mat growth, to adjacent non-mat soil. In contrast, non-mat soil cores were transplanted into mats in open mesh netting, allowing tree root and mat development to occur. Cores were sampled at 10, 16, and 24 months after establishment, and changes in mat development and fungal and bacterial communities were monitored by measuring the number of ECM colonized root tips, fungal mat DNA fragment indicator species, and\ud fungal and bacterial community composition with Terminal Restriction Length Polymorphism (T-RFLP) and Length Heterogeneity PCR (LH-PCR) of ITS and 16S rDNA. DNA fragments representing mat-forming indicator species were detected in the development of Piloderma mats starting at 16 and continuing through 24 months post establishment. Both Piloderma and Ramaria mat fungal communities changed rapidly in cores that had been severed from the host but mat-associated fungal communities had not developed even two years post intimate contact by non-mat soil cores with mats. The richness of fungal and bacterial operational taxonomic units (OTU) in the Piloderma mats increased when they were severed from the host, but decreased when Ramaria fungal mats were severed from their host. Further study is required to determine the length of time required to develop mat-associated microbial communities from non-mat soil, as well as the identity and function of fungi and bacteria responsible for community changes