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.