Forests hold about 45% of total terrestrial carbon and can offset fossil fuel emission by as much as 20% (Bonan 2008). Therefore, they play an essential role in sequestering CO2 from the atmosphere into biomass. Extreme forest disturbances alter vegetation structure and physiology, which significantly impacts the ability of the forest to sequester carbon (C) (Gough et al 2007). More recent work has shown that some forests, including those at UMBS, have high C cycling stability following disturbance (Gough et al 2013) but less is known about how C cycling stability changes across disturbances of varying severities and types. Since disturbances are increasing globally every year, there is a need to better understand how disturbances change patterns of forest C cycling as well as identify mechanisms supporting thresholds of C cycling stability. The Forest Resilience and Threshold Experiment (FoRTE) is an experimental disturbance project aiming to explore forest structural and functional mechanism underlying thresholds of C cycling stability following disturbance. The motivation behind this project stems from the results of the FASET experiment implemented in 2008. This experiment led to the surprising result that these northern hardwood forests showed high NPP resistance and resilience following mid-severity disturbance. However, since FASET was a landscape experiment that did not specifically target levels of disturbance severity, we are limited in the scope of conclusions we can draw about mechanisms driving thresholds of stability along a disturbance severity gradient. FoRTE has thus been designed to tightly manipulate disturbance severity, providing the opportunity to ask specific questions about the mechanisms that underlie thresholds of forest C cycling stability. My PhD research focuses on belowground C cycling stability across a disturbance severity gradient. On average, two-thirds of forest carbon is stored in the soil and soil respiration (Rs) can account for upwards of 80% of total ecosystem respiration (Curtis et al 2005). Therefore, quantifying belowground C cycling stability is an important, and oftentimes under represented, component of overall ecosystem stability. To quantify belowground C cycling stability, we are using a multi-dimensional stability framework adopted from Hillebrand et al 2018 that allows us to capture the nuances of C cycling response to disturbance with four distinct but related measures of stability—resistance, resilience, temporal stability and recovery (Figure 1). In the first two seasons following the implementation FoRTE via stem girdling, we found a significant, linear decline bulk Rs resistance with increasing disturbance severity, suggesting Rs is initially highly sensitive to rising disturbance severity (Figure 2). In addition, we found complete resistance in heterotrophic respiration (Rh) across the disturbance severity gradient (no significant effect of disturbance severity on Rh) which suggests that the suppression of carbohydrate flow to the roots leading to decreased root metabolic activity is driving Rs resistance patterns in the first season following disturbance (Figure 2). As the structural and functional impacts of experimental disturbance in FoRTE continues to unfold, I aim to quantify changes in belowground C cycling stability in the second year following the disturbance. Building on my work from the last two summers, I will quantify how patterns of Rs across the disturbance severity gradient have changed since the disturbance. Is Rs showing even higher sensitivity to rising disturbance severity, suggesting a further decline in Rs resistance? Or, is Rs showing signs of a path to recovery? Secondly, I will explore how mechanisms driving patterns in Rs response have changed since the disturbance. Is the decline in root metabolic activity still driving patterns of Rs, or is there evidence that Rh has started to play a role from disturbance-driven decomposition?