COS 196-4 - Mycorrhizal feedback mechanisms of an old growth forest

Background/Question/Methods

Changing climates are predicted to lead to reassortment of forest trees, that could have consequences for ecosystem function, global carbon cycle, and climate systems. The two mycorrhizal guilds Arbuscular Mycorrhizal (AM) and Ectomycorrhizal (EM) of forest trees may provide positive feedbacks and resistance against changes in forests due to climate change. Recent evidence suggests that biogeochemistry signals created by their own mycorrhizal type and access to their fungal symbionts may contributed to positive feedbacks. We performed a greenhouse in which we decoupled variation in soil biogeochemical conditions from variation in soil microbial communities for three each, AM and EM forest seedlings. We collected soil from twelve plots along an EM gradient in a central old growth forest and separated into two parts, sterilized and live. To test abiotic drivers, pots were filled with autoclaved soil from a single field plot and inoculated with an homogenized "average" microbial community of mixed live soils from the twelve plots. To test biotic drivers, pots were filled with mixed autoclaved soil from each of the twelve sites for a homogenous "average" background soil inoculated with live soils from each plot.

Results/Conclusions

EM seedlings grew more, relative to AM seedlings in abiotic soil derived from plots with a high basal are of EM trees but, this effect was more evident in non-fertilized pots suggesting that EM seedlings extract nutrients in EM conditioned soils. Additionally, EM seedlings outperformed AM seedlings in pots inoculated with microbial communities from EM dominated plots when abiotic soil conditions have been homogenized. The effect of the inocula from EM dominated plots on EM seedlings was dependent on water frequency suggesting that the soil microbes created tolerance of EM plants to dry conditions. The mycorrhizal type positive feedbacks were partially determined by both proposed mechanisms but, specifically in soils with low soil nutrients and water availability. Understanding the potential for positive mycorrhizal feedbacks in changing climates has important consequences for our ability to model and predict carbon and nitrogen dynamics in temperate forests.