Sustained Three-Year Declines in Forest Soil Respiration are Proportional to Disturbance Severity

Soil respiration (R-s) 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 (R-a) and heterotrophic (R-h) 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 R-s and R-h 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 R-s 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 R-h lead us to conclude that R-a drove the suppression of R-s into the third year following disturbance. These responses were conserved across four landscape ecosystems, suggesting the mechanisms causing R-s to decline following phloem disruption were similar despite large differences in composition and productivity. The 3-year reduction of C losses through R-s 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.

Automated summary

Soil respiration is a major carbon flux in terrestrial ecosystems, and its responses to disturbances are important for maintaining ecosystem carbon balance. We studied soil respiration and disturbance severity in a replicated experiment and found that phloem disruption led to a sustained decline in soil respiration for three years. The decline in soil respiration was driven by autotrophic respiration, and this response was consistent across different landscape ecosystems.