Into Darkness: The Effects of Mycorrhizal-associated Trees on the Forest-floor Food Web

Belowground soil systems are among the world’s most species-rich communities. However, the belowground food web is poorly understood because of high species diversity, cryptic feeding relationships, and covarying environmental factors. Scientists have sought out ecological traits to explain, predict and ultimately simplify belowground processes. My dissertation utilizes fungal and root (mycorrhizal) tree associates as a predictive trait to evaluate how fluctuations in fungi alter the belowground food web. Using trees to estimate fungal abundances, I investigate how a change in basal resource alters soil fauna’s structure and composition. First, I test the predictive capability of a mycorrhizal tree gradient for estimating fungal abundances (the resource base) in three forested sites. The mycorrhizal tree gradient is defined as trees associated with arbuscular mycorrhizae (AM) gradually increasing to trees associated with ectomycorrhizae (ECM), here on known as the ECM Tree Index. Using amplicon sequencing, I confirmed the ECM Tree Index hosted different but predictable fungal communities along the gradient. Next, I investigated how the mycorrhizal tree index impacts faunal community structure. I found the ECM Tree Index plays a significant role in predicting most fungivore and predator abundances when paired with litter depth. Fungivore (oribatid mites) and predator (mesostigmatid mites) abundance increased by 2X as plots shifted from AM-dominated systems to ECM-dominated systems. Additionally, predator abundances were tightly correlated with fungivores, suggesting substantial bottom-up control for this predatory group. Finally, I utilized a long-term manipulative and natural field experiment examining AM and ECM forest types’ interaction with nitrogen deposition on community structure. Nitrogen weakly affected fungivores and primarily altered predator abundance. Predators exhibited a tight correlation with fungivores regardless of N concentration; however, they appear to shift their prey with forest type + N. The combined results of this dissertation and previous research suggest belowground systems are heavily bottom-up limited. Furthermore, N’s effect on faunal groups may indirectly change predator-prey interactions depending on the resource base.