Induction of saprophytic behavior in the ectomycorrhizal fungus Suillus granulatus by litter addition in a Pinus contorta (Lodgepole pine) stand in Yellowstone

In a previous study, we demonstrated that Suillus granulatus exhibits positive growth responses to added litter. From this, we hypothesized that this positive growth response (increased number of EM root tips) would be accompanied by increased activities of enzymes that enable EM fungi to utilize litter as a nutrient source. We tested this hypothesis by adding sterile litter in replicate treatment/control blocks, and assaying cellulase, laccase and phosphatase activities in the dominant fungal species, S. granulatus. We used healthy, growing EM roots for both treatment and control assays. Activities of all enzymes increased significantly in response to litter addition (P < 0.05 laccase; P < 0.01 cellulase; P < 0.001 phosphatase). Hence, litter accumulation apparently causes functional as well as structural changes to the EM fungal community that would significantly affect carbon cycling in forest ecosystems.