Precipitation and wood type determines stem and soil greenhouse gas fluxes in a subtropical forest.

Research on greenhouse gas (GHG) fluxes has predominantly focused on subtropical soils, with far less attention given to emissions from tree stems. In particular, year-long simultaneous measurements of both soil and tree stem fluxes in these forests are lacking, and data on standing dead trees is exceptionally scarce. We determined the dynamics of standing dead and live tree stems, and soil CH4, N2O and CO2 fluxes in a subtropical forest. We determined GHG fluxes from standing dead and live tree stems with three different tree heights (10 cm, 50 cm and 150 cm) of Cunninghamia lanceolata from January 2023 to December 2024 and subjected to analysis by gas chromatography. Measurements of environmental parameters were conducted in tandem with those of fluxes and xylem sap flow. Live tree stems contributed less to the annual GHG dynamics than standing dead trees. Live and standing dead tree stems generally acted as net annual sources of CH4, N2O, and CO2. Tree stem GHG fluxes decreased with decreasing precipitation. Soil was a sink of CH4, but a net CO2 and N2O source. Isolated emission peaks dominated the temporal dynamics of stem CH4, N2O, and CO2 fluxes and significantly contributed to the net annual fluxes. The CH4, N2O, and CO2 efflux from both live and standing dead tree stems exhibited a similar seasonal trend. The status (live or dead) and height of the trees significantly influenced stem GHG dynamics. During the study, CH4 emissions from tree stems (across different heights and precipitation conditions) offset an estimated 55.61~60.03% of the soil's CH4 sink capacity. Here, we demonstrate for the first time a strong correlation between stem greenhouse gas fluxes and sap flow in subtropical forests. The stem fluxes of CH4 in live and standing dead trees represented a combination of soil-derived and stem-produced methane, whereas CO2 and N2O fluxes were primarily soil-derived.