1. Introduction

The forest is a significant carbon store. 50% of the organic dry mass is pure carbon (C) (Ref. 06). Through tree growth, carbon dioxide CO₂ is withdrawn from the atmosphere and carbon is stored in the trunk and all other parts of the tree. The formation of biomass is called a CO₂ sink. When a tree dies, it decomposes, and the carbon is released back as CO₂ (CO₂ source). Without CO₂ sinks, the climate gas reduction targets of the Paris Agreement cannot be achieved. Biological carbon sequestration is considered one of the "nature-based solutions" (Ref. 50).

In temperate zones, on average, the same amount of carbon is found in the soil as in the living tree biomass, or even more (Ref. 10, 27, 51, 52). Exceptions are waterlogged soils, such as bogs, and organic soils of high mountains and boreal zones, where significantly more carbon is stored in the soil than in tree biomass. In the natural forests of temperate zones, growth and decay of tree biomass balance each other over larger areas, maintaining a constant average biomass stock of wood (Ref. 14). However, carbon continues to accumulate in the soil even in natural forests (Ref. 28).

If a forest is managed sustainably, the cycles of forest development are significantly shortened compared to natural forests. The forest loses the stock-rich phases of old age and decay. This happens because as trees age, there is also a loss of wood quality. Thus, a 100-year-old spruce is used, even though it could remain standing for another 100 to 200 years. In a sustainably managed forest, the growth of wood and its use are ideally in balance. The average wood stock is, however, about half as low as in natural forests in balance (Ref. 14).

There is a considerable silvicultural potential for stockpiling in managed forests. 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, both leading to higher average stocks (e.g. Ref. 13, 15, 18, 20, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67). In the case of spruce, there has been a trend for some time to reduce the rotation period to meet the wood industry's demands for weaker timber grades and to reduce the risk of storm damage (Ref. 41). This leads to generally lower average stocks. In high or hard-to-access altitudes, wood use is often not profitable, and use occurs below growth, resulting in increasing stocks. In easily accessible locations, stocks are sometimes reduced due partly to the growing demand for energy wood (e.g. Ref. 18, 30, 31). Calamities such as storms and bark beetle infestations can also lead to a loss of stock. In small private forests, usage often occurs below growth because management is not economically significant for the owner. All these sometimes contrasting developments are reversible and respond sensitively to the wood market. When wood prices rise, usage intensifies.

When the forest ecosystem is viewed as a carbon store, uses and mortality are carbon sources, whereas growth has a carbon sink effect. Forest owners can regulate the biomass reserves of their forests through the intensity of wood use and climate-optimised forest management.

The forest is also accounted as a carbon store in national climate balances (National Inventory Reporting) according to international agreements (Kyoto, Paris) (e.g. Ref. 40). So far in Europe, forest owners do not participate in the value of this sink capacity, although the ownership of the carbon store forest is awarded 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 demonstrable through a partial or complete renunciation of use. The initial situation and reference scenario is a forest managed according to usual practice.

In a forest climate protection project, a forest owner commits to a higher stockpiling than usual practice, while adhering to legal and silvicultural rules. Climate protection projects in forests, observing recognised methods, allow the generation of verified CO₂ emission reductions (verified emission reductions - VER) from the sink capacity of the forest.