1. Introduction

The forest is a large carbon store. 50% of the organic dry mass is pure carbon (C) (Ref. 06). Through tree growth, carbon dioxide CO₂ is removed from the atmosphere and the C is stored in the trunk and all other parts of the tree. The formation of biomass is referred to as a CO₂ sink. When a tree dies, it is decays and the C is released again as CO₂ (CO₂ source). Without CO₂ sinks, the greenhouse case reduction targets of the Paris Agreement cannot be achieved. The biological sequestration of carbon is considered one of the "nature based solutions" (Ref. 50).

On average in temperate zones, the managed forest contains as much carbon in the soil as in the living tree biomass, or even more (Ref. 10, 27, 51, 52). Exceptions are waterlogged soils up to and including moors, as well as organic soils of high mountain ranges and the boreal zones, in which considerably more C is stored in the soil than is present in the tree biomass. In the natural forest of temperate zones, over larger areas, the growth and decay of tree biomass are in balance with a constant average biomass stock of wood (Ref. 14). However, in the soil, even in the natural forest, carbon continues to accumulate (Ref. 28).

If a forest is managed sustainably, the cycles of forest development are significantly shortened compared to natural forests. The forest is deprived of the stock-rich ageing and decay phases. This happens because as the trees age, there is also a loss of timber quality. Thus, a 100-year-old spruce is logged, although it could remain standing for another 100 to 200 years. In a sustainably managed forest, the growth and use of wood are perfectly balanced. The average wood stock, however, is half lower than in the natural forest at equilibrium (Ref. 14).

There is significant silvicultural flexibility for deciding on stock in the managed forest. For ecological reasons, parts of the forest can no longer be managed and are left to the forest's own dynamics as reserves, or old wood islands are preserved, which in both cases leads to higher average stocks (e.g. Ref. 13, 15, 18, 20, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67). For spruce, there have been tendencies for some time to reduce the rotation period to meet the timber industry's demand for thinner wood assortments and to reduce the risk of storm damage (Ref. 41). This leads to generally lower average stocks. In high altitudes or hard-to-access locations, on the other hand, timber use is often not profitable and the use is below growth, resulting in an increase in stocks. In easily accessible locations, stocks are again partially reduced, partly due to the increasing demand for energy wood (e.g. Ref. 18, 30, 31). Calamities such as storms and bark beetle gradations can also lead to stock loss. In small private forests, usage is often below growth because the management itself is not economically significant for the owner. All these partially contradictory developments are reversible and react sensitively to the timber market. When forest wood prices rise, utilisation increases.

If the forest ecosystem is considered a C reservoir, then usage and mortality are C sources, growth causes a C sink effect. Forest owners can manage the biomass reserve of their forest through the intensity of timber utilisation and climate-optimised forest management.

The forest is also counted as a C reservoir in the national climate balances (National Inventory Reporting) according to international agreements (Kyoto, Paris) (e.g. Ref. 40). Forest owners in Europe do not yet participate in the value of this sink functionality, although the ownership of the C reservoir forest is assigned to them (e.g. Ref. 11, 38).

The aim of the method is to make the biological sequestration of carbon (sink capacity) in the forest calculable and verifiable by a mandatory partial renunciation of its use. The forest is also made more resilient to the effects of climate change by promoting suitable tree species, and biodiversity is promoted (impact improved forest management IIFM, conservation and enhancement of forest sinks and reservoirs, resilience improvement, biodiversity enhancement). The starting point and reference scenario are a forest managed according to usual practice.

In a forest climate protection project, a forest owner commits to a higher stock maintenance deviating from usual practice while adhering to the legal and silvicultural rules. Climate protection projects in the forest, following recognised methods, allow the generation of verified CO₂ emission reductions (VER verified emission reductions) through the sink functionality of the forest.