Investigating the Fire-Mediated Belowground Drivers of Stress Resilience in Eastern US Forests

Future environmental stressors in eastern US forests are numerous and include the increasing intensity and frequency of drought and wildfire. As a result, drought-tolerant, shade-intolerant, and fire-adapted tree species like oaks (Quercus spp.) – which were historically abundant and maintained frequent, low-intensity fire – will likely be favored. However, these species are presently struggling to regenerate. More than a century of widespread fire suppression has led to mesophication: a positive feedback whereby forest densification promotes darker, wetter, and cooler microsites that favor fire-sensitive trees like maples (Acer spp.) that further promote such conditions and outcompete fire-adapted oaks. Fire and canopy disturbance can reverse this process, but success in regenerating ecologically and economically important oaks has been mixed. We propose that failures to reverse mesophication are the result of overlooking belowground drivers. Although aboveground drivers like fire-fuel feedback and decreased canopy openness have been explicitly implicated, fire suppression can also induce changes in abiotic and biotic soil properties – like increases in soil moisture and nutrients, and shifts in organic matter composition and microbial community structure – which may alter forest regeneration. Subsequently, plant-soil feedback may further amplify these changes and preliminary research indicates fire can affect relationships between ectomycorrhizal (EM) fungi, nitrogen-fixing bacteria, and trees – allowing persistence of EM trees despite scarce nutrients. Many EM trees like oaks are fire-adapted, and the absence of fire may restrict seedling access to the benefits derived from EM fungi and other soil microbes. In addition, fire suppression may be driving observed shifts towards the dominance of arbuscular mycorrhizal (AM), fire-sensitive tree competitors like maples through creating high-nutrient soils that promote AM over EM symbioses. However, little research has evaluated these hypotheses. We will pair observational and experimental greenhouse studies with cutting-edge analyses at EMSL and JGI to reveal: 1) how fire-related shifts in belowground biotic and abiotic factors affect the relative growth, survival, and nutrient content of fire-adapted seedlings (i.e., oaks) and fire-sensitive seedlings (i.e., maples), and 2) how these seedlings subsequently shift biotic and abiotic soil properties to drive or reverse the mesophication process. Our proposed research will address the Grand Challenge of characterizing biogeochemical exchanges driven by plant-microbe interactions and evaluating their process-level impacts, sensitivity to fire as a forest disturbance, and shifting resource availability under changing environmental regimes.