Fungal Community Structural and Functional Responses to Disturbances in a North Temperate Forest

Abstract

Globally soils contain three times as much carbon (C) as the Earth’s atmosphere, with an additional 5-10% as much C stored in coarse woody debris as in the atmosphere. Due to the large sizes of these organic C stocks, small shifts in the amounts of soil organic matter, coarse woody debris, and the microbial communities responsible for decomposition could have large effects on global carbon cycle. In this dissertation I combine a series of observational and manipulative experiments to analyze fungal community responses to historical and predicted future disturbances in a forest ecosystem. I explore how fungal communities respond spatially and temporally to clear-cutting and burning, and whether they follow trends in diversity predicted by the intermediate disturbance hypothesis (IDH). In Chapter II, I investigate the short-term effects of clear-cutting and prescribed burning on fungal community composition and function in mineral soils, and how spatial variation in disturbance severity within a 1-ha plot structured these communities. I observed differential effects of clear-cutting or clear-cutting + burning within a 1-ha stand and found that the areas of highest burn disturbance yielded lowest fungal diversity and extracellular enzyme activity (EEA). High burn areas also tended to drive plot level differences in soil physio-chemical properties with increases pH and effective cation exchange capacity (ECEC), while plot level increases in fungal diversity were driven by areas that primarily received the clear-cutting disturbance. My results highlight the importance of spatial variation and scale of sampling when examining both abiotic and biotic responses to disturbances. In Chapter III, I leveraged an existing +100-year cut+burn chronosequence to explore whether soil fungal communities follow decadal patterns in successional trajectories predicted and often observed in plant communities by the IDH. I found that plant community diversity in the chronosequence adhered to patterns predicted by the IDH, however, fungal diversity did not follow similar trajectories. Fungal diversity was lowest in the 61-year old mid-successional plot. The low diversity observed in this plot was primarily attributed competitive exclusion due to a dominance of ectomycorrhizal taxa in the Cortinariaceae that was accompanied by a high abundance of oaks relative to the other experimental plots. When the successional patters were analyzed with just the remaining chronosequence plots diversity was seen to decrease with successional stage. Conversely, I observed a steady increase in microbial abundance into later successional stages reflecting similar patters observed in primary producer productivity. In Chapter IV, I investigated patters of fungal decomposer communities in coarse woody debris of a pioneering tree species in temperate forests, Populus grandidentata, from standing dead trees to the incorporation of woody necromass into soil. I showed that fungal communities are dynamic during and after a decay class continuum, colonizing while trees are standing and continuing to shift throughout coarse woody debris decay. Specifically, I note that nitrogen is an important driver of enzymatic activity in microbial communities and show a correlation with bacterial and fungal species composition and abundance along the continuum of decay. Overall, the work described here offers further support for the importance of disturbances in structuring soil fungal communities. Notably, my work highlights the importance of spatial variability in disturbance severity and long-term legacies on fungal composition, activity, and successional trajectories.