Date of Award
Spring 2023
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Forestry and Environmental Studies
First Advisor
Bradford, Mark
Abstract
Plant-fungal associations can strongly influence the composition, persistence, and turnover of soil organic matter in forests. The prevailing framework for understanding these relationships is through the relative abundance of tree mycorrhizal functional types, the vast majority of which associate with arbuscular (AM) and/or ectomycorrhizal (EcM) fungi. Differences in functional traits and ecosystem characteristics between AM and EcM trees have generated immense interest in assessing the ecosystem biogeochemical effects of tree mycorrhizal dominance within and among forest biomes to improve understanding of patterns in soil carbon and nitrogen pools and how they might respond to factors of global change. Yet, forests include multiple canopy and sub-canopy vegetation layers, and each of the mycorrhizal types associate with different plant growth forms that can co-occur across forest strata. As such, the focus on tree mycorrhizal dominance has precluded consideration of the effects of understory plant mycorrhizal types and how they might interact with AM versus EcM canopy trees. For instance, ericoid mycorrhizal (ErM) shrubs are also common in forests and have functional traits that distinguish them from AM and EcM trees. Using ErM shrubs as a model plant group, my dissertation explores how functional differences between under- and overstory plant mycorrhizal associations interact to collectively affect soil organic matter dynamics in forests. In my first chapter, I evaluate the abundance, distribution, and functional traits of ErM plants in global forest biomes and build a case for their inclusion in research assessing the effects of plant mycorrhizal functional types on ecosystem biogeochemical processes in forests. Specifically, I review empirical literature on the functional traits of ErM shrubs and how they differ from those of AM and EcM trees to formulate hypotheses for how their effects on carbon and nitrogen dynamics might vary under AM versus EcM tree canopy trees. In my second chapter, I test the hypotheses generated in my first chapter using empirical data collected from 414 plots within a temperate forest with varying levels of ErM plant cover and AM versus EcM tree dominance. Given the relative scarcity of data on ErM shrubs, I also analyzed two regional datasets from the eastern United States to estimate the relative abundance, frequency, and richness of ErM plants in temperate forests in this region. I found that ErM shrubs strongly altered AM versus EcM tree dominance effects on surface soil (top 7 cm) carbon and nitrogen concentrations and that the effects of ErM shrubs were functionally distinct from both AM and EcM canopy trees. Finally, in my third chapter, I use data collected from a different set of plots at the same forest to explore the effects of tree and shrub mycorrhizal associations at a more granular scale within the soil profile. Specifically, I looked at how interactions between understory ErM shrubs and AM versus EcM canopy trees influence microbial processes and soil carbon and nitrogen stocks in functionally distinct soil horizons and organic matter fractions. I designed this study using a quasi-experimental approach (paired subplots with and without ErM shrubs nested within larger plots that varied in AM versus EcM dominance) so that I could better disentangle the direct effects of plant mycorrhizal associations on soil organic matter dynamics from those of covarying abiotic controls, such as soil moisture. Through this analysis, I found that the effects of tree and shrub mycorrhizal associations varied strongly by soil depth, soil organic matter fraction, and through interactions with co-occurring mycorrhizal guilds. In particular, ErM shrubs had more pronounced effects on the surface organic horizon and on particulate organic matter pools, which are more vulnerable to changing environmental conditions. In contrast, tree mycorrhizal associations had stronger effects on mineral-associated organic matter pools in subsurface soil horizons, which, on average, have slower turnover times and hence longer-term persistence. Importantly, this study also showed that discrepancies in forest soil sampling methods (e.g. mineral soil sampling depth and inclusion of the Oa horizon) could potentially explain the idiosyncratic effects of EcM tree dominance on soil organic matter accumulation observed among studies. Collectively, my dissertation research underscores a need for further consideration of the effects of ErM plants and fungi on soil organic matter dynamics in the range of forest biomes in which they occur. More broadly, a focus on mycorrhizal trait variation—in addition to mycorrhizal trait dominance—could help to explain soil carbon and nitrogen cycling and storage in ecosystems where multiple mycorrhizal types co-occur in association with dominant and subdominant vegetation.
Recommended Citation
Ward, Elisabeth B., "The Functional Effects of Tree and Shrub Mycorrhizal Associations on Soil Organic Matter Dynamics in Forests" (2023). Yale Graduate School of Arts and Sciences Dissertations. 881.
https://elischolar.library.yale.edu/gsas_dissertations/881
