The paper provides a brief analysis of well-known works containing evidence of carbon accumulation in old-growth forests. The analysis of the current state of the problem allows us to conclude that old-growth forests continue to accumulate carbon. A map of old-growth forests in Russia, identified on the basis of tree age higher than 200 years, using remote sensing data, is presented, and estimates of carbon pools in these forests are discussed. According to the estimates obtained, the area of old-growth forests in Russia was 163 mln ha as of 2021, and carbon stocks in phytomass reached 7.33 bln t, with the contribution of larch forests and larch woodlands of 86%. It is shown that the most important cause of uncertainties in the estimates of carbon cycles in old-growth forests is the uncertainty of the concept of “old-growth forests.” The mosaic structure of forests, that is, the high horizontal structural diversity, contributes to the accumulation of nitrogen and carbon in soils due to the creation of functioning conditions for various plant species, including light-loving ones, and, accordingly, due to the presence of litter of different quality, which is important for soil biota. Old-growth mosaic forests in Moskvoretsko-Oka Plain accumulated more nitrogen and carbon in soils than forests at an earlier stage of succession with a low mosaicity (in average 80 t/ha versus 60 t/ha in the 30-cm layer). The old-growth fir-beech dead-cover forests of the Northwestern Caucasus, whose tree stand is characterized by the highest productivity in Russia and Europe and high carbon reserves in the tree stand, are characterized by low carbon stock in soils compared to forests at an earlier stage of development (in average 58 t/ha versus 99 t/ha in 30-cm layer). This is due to the low quality of beech and fir litter and the absence of a pronounced window mosaic, which prevents the colonization of light-loving plant species, including with a high quality of litter. It is shown that, along with microorganisms, it is necessary to take into account such agents of decomposition, mineralization and humification as earthworms, which play a key role in carbon cycles. Carbon stock in the litter of northern taiga spruce forests is an order of magnitude higher than in coniferous-broad-leaved forests; in the litter and in the mineral layer of 0–30 cm, the carbon reserves under the crowns of spruce trees for about 200 years turned out to be significantly higher than in the spaces between the crowns, exceeding 80 t/ha.
