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Soil respiration (Rs) is the largest outward flux of carbon (C) from terrestrial ecosystems, accounting for more than half of total temperate forest C loss. Evaluating the drivers of this globally important flux, as well as identifying autotrophic (Ra) and heterotrophic (Rh) responses, is critical in the era of rapid global change because small changes could result in disproportionally large impacts to ecosystem C balance. We assessed four years of Rs and Rh from the Forest Resilience Threshold Experiment (FoRTE) to better understand how soil C fluxes respond to a disturbance simulating phloem-disrupting insects. This replicated experiment spanning multiple landscape ecosystems contains four disturbance severities of 0, 45, 65 and 85% gross defoliation as well as two disturbance types targeting the upper and lower canopy. We found an immediate and sustained decline in Rs following phloem disruption that persisted for three years and was proportional to severity. Proportional declines in basal soil respiration and fine-root production with increasing disturbance severity and stable Rh lead us to conclude that Ra drove the suppression of Rs into the third year following disturbance. These responses were conserved across four landscape ecosystems, suggesting the mechanisms causing Rs to decline following phloem disruption were similar despite large differences in composition and productivity. The 3-year reduction of C losses through Rs and, contrastingly, sustained C storage through wood production suggests ecosystem C balance may have remained relatively stable in the first few years following disturbance, even at the highest severity. [ABSTRACT FROM AUTHOR]

The pre-disturbance vegetation characteristics that predict carbon (C) cycling responses to disturbance are not well known. To address this gap, we initiated the Forest Resilience Threshold Experiment, a manipulative study in which more than 3600 trees were stem girdled to achieve replicated factorial combinations of four levels (control, 45, 65, and 85% gross defoliation) of disturbance severity and two disturbance types (targeting upper or lower canopy strata). Applying a standardized stability framework in which initial C cycling resistance to disturbance was calculated as the first-year natural log response ratio of disturbance and control treatments, we investigated to what extent pre-disturbance levels of species diversity, aboveground woody biomass, leaf area index, and canopy rugosity—a measure of structural complexity—predict the initial responses of subcanopy light-saturated leaf CO2 assimilation (Asat), aboveground wood NPP (ANPPw), and soil respiration (Rs) to phloem-disrupting disturbance. In the year following stem girdling, we found that above-ground C cycling processes, Asat and ANPPw, were highly resistant to increases in disturbance severity, while Rs resistance declined as severity increased. Disturbance type had no effect on first-year resistance. Pre-disturbance aboveground woody biomass, and canopy rugosity were positive predictors of ANPPw resistance and, conversely, negatively related to Rs resistance. Subcanopy Asat resistance was not related to pre-disturbance vegetation characteristics. Stability of C uptake processes along with Rs declines suggest the net C sink was sustained in the initial months following disturbance. We conclude that biomass and complexity are significant, but not universal, predictors of initial C cycling resistance to disturbance. Moreover, our findings highlight the utility of standardized stability measures when comparing functional responses to disturbance. [ABSTRACT FROM AUTHOR]