Dynamics of Picea abies mortality and CO2 and CH4 fluxes from spruce trees decomposition in the southwest of the Valdai Upland, Russia

A mass decline of Picea abies (hereinafter – spruce), often associated with outbreaks of Ips typographus, is one of the main reasons for the reduction of spruce forests. In turn, dry and fallen trees can be both stock and source of greenhouse gases at various stages of decomposition. In our study, using an unmanned aerial vehicle, we evaluated the dynamics of spruce decline in two forest types in the southwest of the Valdai Upland (Central Forest State Nature Reserve, Russia), namely Sphagnum-bilberry forests and nemoral spruce forests. It was found that the rate of decline in Sphagnum-bilberry spruce forest was much higher than in nemoral spruce forest. By the fourth year after a windfall on 0.13 km2, 913 spruce individuals had withered in Sphagnum-bilberry forest and 66 ones in the nemoral spruce forest. Based on direct measurements of greenhouse gas fluxes by chamber method on dead trunks and coarse woody debris, it was found that in relative values the highest amount of CO2 is emitted by coarse woody debris of the decay classes 3–4 (800–1800 mg CO2 × m-2 × h-1). Deadwood and coarse woody debris from the first decay classes are assumed to be a source of CH4 (0.0008–0.0070 mg CO2 × m-2 × h-1), and from classes 3–5 they are a stock (from -0.0070 mg CO2 × m-2 × h-1 to -0.0009 mg CO2 × m-2 × h-1). When converted to the total surface areas of deadwood and coarse woody debris of the study sites, it was found that coarse woody debris of the decay classes 3–5 (2.3–13.6 kg CO2 × h-1) made the highest contribution to the integral CO2 emission, and deadwood (67 mg CH4 × h-1) made the highest contribution to the CH4 emission. Significant differences in greenhouse gas fluxes were found both between deadwood and decay classes of coarse woody debris, and between fluxes from deadwood and coarse woody debris of individual decay classes in various forest types. The results have shown the importance of considering deadwood and all available decay classes of coarse woody debris when estimating greenhouse gas fluxes from dead timber and the contribution of debris to the carbon cycle in forest ecosystems.