Forest health, heart rot disease, and their impact on the source of carbon‐based greenhouse gas fluxes

Forest health is critical for sustaining ecosystem services like carbon sequestration. Heart rot, a widespread disease in upland northern hardwood forests, may affect greenhouse gas (CO₂ and CH₄) fluxes, but its impacts remain poorly measured. Using non-destructive tomography and direct gas flux measurements, we quantified the effects of heart rot on sugar maple (Acer saccharum Marshall) stems and surrounding soils. Heart rot increased CH₄ emissions from stems but did not affect CO₂ fluxes from stems or soils, nor CH₄ fluxes from soils. All stems emitted CO₂ and CH₄, while soils absorbed CH₄ and emitted CO₂. Stem CH₄ fluxes strongly correlated with decay severity, but CO₂ fluxes did not. CH₄ was produced in the heartwood, CO₂ in the sapwood, and methanogens were present in all stems. Severe heart rot often caused bark fractures, enhancing CH₄ diffusion to the atmosphere and creating emission hotspots. These findings show that forest health influences carbon cycling. Capturing stem CH₄ hotspots requires direct measurement, and fungal diseases like heart rot may shift forests from CH₄ sinks to sources, with implications for atmospheric greenhouse gas dynamics.