The Long-Term Effect of Ongoing Spruce Decay on Carbon Exchange in Taiga Forests

The long-term effect of spruce stand decay on the CO2 balance was studied in the taiga of Valdai (Russia, Novgorod oblast). The CO2 emission from soil, the respiration of coarse wood debris, the total respiration, and the net fluxes of CO2 and water vapor were evaluated. In areas of dead spruce forest affected by xylophages and saprotrophs, as well as by windfall, a noticeable decrease in specific gross respiration is absent. This is related to the fact that reduced CO2 respiration of living trees is compensated by emission from coarse wood debris and a sharp (3- to 3.5-fold) and long-term (no less than six years long) rise in soil respiration (hotspots) under dead-standing spruce trees. The contribution to carbon emission from soil hotspots per unit area in dead spruce stands under dry standing trees is greater than the role of decomposition of coarse wood debris (14.4 and 9%, respectively). The observation period is characterized by a significant decrease in the annual net CO2 sink in the area studied: from –300 to –95 g C m–2 year–1, which is followed by reduced evapotranspiration (0.0116 g H2O m–2 s–1 in May–October 2018 as compared to 0.0142 g H2O m–2 s–1 in the same period of 2010). We assume that the observed decrease in carbon uptake is related both to the reduction in the primary spruce productivity and to an increase in gross respiration from larger areas of decomposed spruce trees. The calculations show that if the area of the dead spruce stands is further increased to 27%, the resulting net balance of the territory will switch to a CO2 source for the atmosphere. This should be taken into account for remote forecasting.