Trees are important carbon stores and CO2 sinks in the climate system. But the German forest is damaged by drought and heat and its buffer function is disappearing. Now a study shows: At least temporarily, the forest floor can compensate for the decline in CO2 uptake by trees – it stores more carbon in dry years than usual. However, this compensating reaction does not last long.
Forests are an important climate buffer and a sink in the earth’s carbon cycle. When trees grow, they absorb carbon dioxide from the air during photosynthesis and convert it into organic carbon compounds. This carbon remains stored in the tissues of the trees and is therefore removed from the atmosphere and the carbon cycle. However, this carbon does not remain in the tree permanently: some of it finds its way into the soil through the littering of leaves and needles, the decomposition of dead wood and dying roots.
A new look at the forest floor as a carbon store
How long the carbon in the soil remains bound in the organic material depends on many factors, including, above all, the climate: the wetter it is, the faster decomposing microorganisms and small animals decompose litter and dead wood. Part of the carbon is released again as CO2, part remains stored in humus. How much this is and how carbon storage in soils changes with climate change has only been partially clarified so far. That’s why researchers led by Nicole Wellbrock from the Thünen Institute for Forest Ecosystems used data from the 2022 Federal Forest Inventory and the current soil condition survey to determine how much carbon is stored in German forests and forest soils.
The team also examined how the drought in 2018 to 2020 affected soil carbon. During this time, large numbers of trees died in German forests. In most cases the areas were cleared, but in others the dead wood remained on the area. In addition, there were needles and leaves that had fallen to the ground as well as the fine roots that had died in the ground. “The international team of experts on greenhouse gas reporting encouraged us to take a closer look at the effects of the damaging events between 2018 and 2020 on soil carbon,” explains Wellbrock.
Forest soil compensates for tree losses – temporarily
The evaluations showed that the forest floors currently store almost as much carbon as all above-ground parts of the forest trees. A total of around 2,200 million tons of carbon are stored in the soil, which corresponds to 197.4 tons of carbon per hectare. There are around 1,184 million tonnes of carbon in the living biomass of trees, or 108 tonnes per hectare. In addition, there are 46.1 million tons of carbon in dead wood and 936 million tons of carbon in litter and mineral soil, as the researchers determined.
However, the data from the dry years showed something surprising: many people died during this time
Trees fell and thus failed as living carbon stores. At the same time, however, the forest soil experienced a high in terms of carbon storage: According to the modeling, the soils stored so much carbon during this time that they compensated for almost all of the carbon losses from the trees in the first years of damage. In principle, the forest floors took over the function of the fallen trees as CO2 sinks, as the team reports. The dry weather and strong sunlight meant more carbon was stored in the soil than in normal years.
However, the new modeling also shows that the soil does not absorb carbon from the dead biomass unabated. The process weakens again after just a few years. At the same time, new forests grow on the damaged areas, which store more carbon in the biomass. It is therefore still unclear how these fluctuations will affect the sink function of forest areas in the long term. “The results are based on model calculations. Only the evaluation of the third BZE will provide reliable data,” emphasizes Wellbrock.
Source: Johann Heinrich von Thünen Institute, Federal Research Institute for Rural Areas, Forests and Fisheries