Impact of harvesting on carbon source/sink of Alnus sibirica var. hirsuta swamps in Daxing’anling Mountains discontinuous permafrost region of northeastern China

ObjectiveDegradation of permafrost caused by climate warming will increase greenhouse gas emissions from wetlands above it, yet it is still unclear that how harvest disturbance affects greenhouse gas emissions from wetlands on frozen soil.

MethodAnnual carbon emission fluxes (ACE) (CO₂ and CH₄), the net primary productivity (NPP) and annual net carbon sequestration of vegetation (VNCS) of four different harvest treatments (control-D, low intensity selective cutting 15%-Q_z, high intensity selective cutting 45%-Z_z and clear cutting-J) of Alnus sibirica var. hirsuta swamp were measured synchronously using the static chamber gas chromatography method and the relative growth equation, as well as related environmental factors (soil temperature, water level, thawing depth, soil carbon and nitrogen content, etc.) at Nanwenghe in Daxing’anling discontinuous permafrost region of northeastern China, in order to reveal the influence of harvesting disturbance on the carbon source/sink of the cold temperate forested wetlands according to the balance of net carbon budget of the ecosystem.

Result(1) Z_z and J made the mean annual fluxes of CH₄ (0.008−0.019 mg/(m²·h)) significantly lower than D by 52.6%−57.9% (P < 0.05), Q_z is close to the D (− 10.5%, P > 0.05), and their seasonal dynamic trends took on two types (D, Q_z bimodal-low emission; Z_z and J bimodal-low absorption); (2) Q_z, Z_z and J all made the mean annual fluxes of CO₂ (103.69−133.65 mg/(m²·h)) significantly decreased by 14.4%−22.4% (P < 0.05) compared with control, their seasonal dynamic trends also existed two types (D, Q_z unimodal type-peak at late summer; Z_z and J unimodal type-peak advance to midsummer); (3) The soil CH₄ flux was controlled by soil temperature, water level and snow thickness; soil CO₂ flux was controlled by soil temperature, soil organic carbon content and freezing depth synthetically; (4) Both NPP (5.07−8.83 t/(ha·year)) and VNCS (2.10−3.83 t/(ha·year)) showed a declining trend with increasing harvesting intensity, among them, NPP and VNCS of Q_z were similar to those of D, Z_z and J, made NPP and VNCS significantly decreased by13.7%−36.9% and 14.2%−43.5% compared with control (P < 0.05), respectively, and those of J were significantly lower than that of Z_z by 26.9% and 34.2% (P < 0.05); (5) Net ecosystem carbon budget (− 0.42−1.30 t/(ha·year)) of four treatments existed significant differences. D, Q_z and Z_z were all carbon sinks, and the sink strength of Q_z was significantly higher than that of D and Z_z by 1.6 and 1.2 times (P < 0.05), but J had been converted into carbon source (− 0.42 t/(ha·year), P < 0.05).

ConclusionTherefore, after eight years of harvesting, Alnus sibirica var. hirsuta swamp had restored the carbon sink function at selective cutting site, while it still maintained the carbon source at clear cutting site in the cold temperate discontinuous permafrost region, so selective cutting should be adopted in the management of wetland carbon sinks and to avoid clear cutting.