And eventually, the rate of decomposition was higher than the rate of formation of new litter. The soil became a source of emissions.
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Comments

Reason 2: At the same time as the decomposition of litter accelerated, litter input into the soil also decreased. The crown of aging trees is smaller in relation to the trunk volume, so less litter is produced now.
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So, aging reduces the sink in two ways: through slower growth of living biomass and through reduced litter production. Therefore, the carbon input to the forest carbon storage is now continuously decreasing as trees are not getting any younger.
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Reason 3: Perhaps the most important reason for the increase in emissions, however, is the acceleration of decomposition activity due to rising temperatures. This is especially the case in the soils of drained peatlands, where there are huge amounts of dead organic matter ready to decompose.
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So, there are three permanent trends behind the increased emissions, the direction of which cannot be changed. Therefore, emissions from forests are likely to continue to increase.
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This is a good practical example of the fact that biological carbon stocks are only temporary, and therefore their inclusion in the national net zero targets cannot be scientifically justified.
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Thanks for this explanation. How do changing rates of annual forest harvest fit into this? Has there also been an uptick in forest harvest (which would lead to more emissions w potentially mixed effects on removals due to regrowth vs old growth rates) in recent years? Or are harvests not increasing?
With sufficient, diverse biological carbon stocks, they support each other, and store carbon longer. If one part of the stocks is removed, eg wetlands, or tree trunks, or leaves and small branches, the system may collapse. A chair with only 3 legs is not stable if it had 4 in the first place..
This seems inevitable from preservation of mass. There must be a limit of tons of plant biomass per square kilometer and there's nowhere else for the carbon to go.