FW-R-3: Humus reserves in forest soils

High contents of organic carbon in forest soils further the water storage capacity and improve the supply of nutrients. They also improve the trees’ chances of surviving hot periods with poor precipitation levels. Between 1992 and 2021 the humus reserves in Germany’s forest soils increased. However, this does not exclude the possibility of regional humus depletion.

Humus – essential for storing water and nutrients

Apart from controlling the composition of tree species (cf. Indicator FW-R-1) and the selection of appropriate provenance (cf. Indicator FW-R-2), forestry professionals have additional ways of furthering the adaptability of forests to changing climatic conditions. Hence, humus conservation is an important key to the creation of favourable growing conditions and greater stability. This is because humus or rather organic carbon – its vital component – creates a favourable soil structure thus making it hugely important for the supply of nutrients and water to forest trees. Especially on nutrient-poor and rather dry sites, the conditions for forest trees can be enhanced by ample supply of humus. Nevertheless, weather patterns also influence the activity of micro-organisms in the soil. If temperatures rise at the same time as the water supply is plentiful, the process of humus decomposition is apt to accelerate.

Apart from their favourable influence on the adaptability of forests, sufficient humus reserves are also worth striving for as a means of climate protection. This is due to the fact that soil is the most important terrestrial organic carbon sink, at the same time as being one of the most important natural sources of CO2 in the atmosphere. Carbon sequestered in the soil is the only form of carbon not contributing to climate change by way of CO2 in the atmosphere. Humus conservation therefore is both an adaptation measure and a method of protecting the climate (cf. Indicator BO-R-1).

In virgin forests, the organic matter – available for the formation of humus – is particularly plentiful, as all trees are decomposed when they die, thus ultimately adding to the formation of humus reserves. The situation is different in commercial forests, because harvesting the timber means that there is carbon loss involved in the forest. On one hand, the forest floor is disturbed by such interventions thus potentially leading to increased decomposition of humus. On the other hand, considerable amounts of carbon are removed from the forest by way of timber, the extent of which depends – to a considerable extent – on the amount of debris remaining in the forest after harvesting. The more leaves, needles or branches remain on the logging site in the forest, the more organic matter is available for replenishing the humus reserves. In that context, it also matters how well the harvesting remnants are distributed in the areas where logging has taken place. As the demand for energy from timber rises, increasing incentives have emerged for making use also of inferior material including crown material. This development is driven even more strongly owing to the circumstances arising from the war in the Ukraine and the resulting energy crisis which, as of 2022, caused firewood prices to rise even more. In contrast with agriculture, the technical opportunities of compensation for the removal of carbon and nutrients from the forest are quite limited. It is therefore ultimately crucial to achieve the best possible adaptation of use intensity to site requirements. That is the only way to ensure that sufficient humus is formed.

As part of forest soil condition surveys (BZE-Wald), research has been carried out nationwide to assess carbon reserves in forest soils. However, BZE-Wald surveys are conducted at major intervals only. The first BZE-Wald survey was carried out in the period of 1987 to 1993, the second between 2006 and 2008. The third BZE-Wald survey began in 2022. Process-based modelling – developed on the basis of BZE-Wald data for greenhouse gas reporting at Federal level – facilitates the derivation of annual mean values for organic carbon reserves in Germany’s forest soils.

The time series shows a significantly rising trend prevailing over the past 40 years. However, it is questionable whether the changes observed can be attributed to the targeted promotion of humus management because the trend is liable to be masked by forest transformation measures or liming activities. Furthermore, air pollutant inputs as well as unremittingly high nitrogen deposits are apt to affect the development in respect of reserves of soil carbon in the forest.

The organic carbon contained in the humus cover is subject to comparatively rapid formation and decomposition processes thus exposing it greatly to external impacts. Furthermore the humus levels have been fluctuating quite strongly from site to site which makes the interpretation of rates of change difficult. The indicator illustrated in this report shows changes in organic carbon reserves in the humus layer and in the upper 30 cm of mineral soil.

Humus reserves – regarding their deviation from the nationwide mean – can develop quite differently from region to region. In the Northeast-German lowlands, soil carbon reserves increased comparatively strongly in stands of pine trees. Those areas were reforested massively from the middle of the past century onwards, thus enabling them to build up both biomass and humus reserves. Research projects undertaken in the region of the Bavarian Alps have indicated, however, that distinct humus losses occurred there, most strongly in soil on limestone or dolomite rock and most distinctly in areas particularly affected by warming. The losses arisen might be attributable to increased biological activity which would presumably lead to accelerated decomposition of humus. The management of montane forests has not been adapted to changing site conditions over the past 50 years. It is conceivable that, in this region too, forest management will have to include the promotion of humus generation in order to curb its depletion.¹¹⁹

^{119 - Prietzel J., Zimmermann L., Schubert A., Christophel D. 2016: Organic matter losses in German Alps forest soils since the 1970s most likely caused by warming. Nature Geoscience Vol. 9: 543–548. doi: 10.1038/ngeo2732}

FW-R-3: Humus reserves in forest soils | Umweltbundesamt