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

Summary 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 (CO 2 and CH 4 ) 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 CH 4 emissions from stems but did not affect CO 2 fluxes from stems or soils, nor CH 4 fluxes from soils. All stems emitted CO 2 and CH 4 , while soils absorbed CH 4 and emitted CO 2 . Stem CH 4 fluxes strongly correlated with decay severity, but CO 2 fluxes did not. CH 4 was produced in the heartwood, CO 2 in the sapwood, and methanogens were present in all stems. Severe heart rot often caused bark fractures, enhancing CH 4 diffusion to the atmosphere and creating emission hotspots. These findings show that forest health influences carbon cycling. Capturing stem CH 4 hotspots requires direct measurement, and fungal diseases like heart rot may shift forests from CH 4 sinks to sources, with implications for atmospheric greenhouse gas dynamics.