The carbon cycle describes the constant exchange of carbon between different carbon pools, such as the atmosphere, the soil and the plants.

In forests, carbon is primarily absorbed through photosynthesis by trees and ground vegetation and is stored in wood, leaves, and roots. After plants die, carbon is released or transformed through decomposition processes. A portion can be stored in the soil for long periods. Forest ecosystems thus provide a climate protection function.

Various surveys within the intensive forest monitoring programme provide information on parts of the carbon cycle.

In forest ecosystems, the largest carbon pool is in the soil. On the Level II sites, soil samples are taken and analyzed every ten years. From this data, carbon storage in the soil can be calculated and its dynamics can be assessed.

Litterfall represents an important carbon pathway in forest ecosystems. On the Level II sites, litterfall is continuously sampled to determine how many leaves, needles, branches, fruits, and other organic materials fall to the ground.

The growth rate of forest trees is not only an important parameter to describe the vitality of trees, but also provides information on how much carbon is stored in the biomass. On the Level II sites, the growth in thickness of a selection of individual trees is measured continuously; diameters of all trees on the sites are measured at least every five years.