FW-R-5: Humus reserves in forest soils

The picture shows the soil profile of a forest. The upper horizon is formed by a thick layer of humus.Click to enlarge
High humus reserves in forest soils reduce the vulnerability of forests to drought.
Source: Photograph: © Petra Dühnelt / Thünen-Institut

2019 Monitoring Report on the German Strategy for Adaptation to Climate Change

Table of Contents

 

FW-R-5: 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 1987 and 2008 there were more areas on which humus reserves in mineral soils had increased than areas where they had decreased.

The stacking column represents changes in the stock of organic carbon between the first and second BZE. Three classes were formed: Decrease to low increase (less than or equal to 0.23 tonnes per hectare per year), moderate increase (between more than 0.23 to less than or equal to 0.45 tonnes per hectare per year) and high increase (more than 0.45 tonnes per hectare per year). For these initial evaluations, the category boundaries were set so that each class has a share of about one third.
FW-R-5: Humus reserves in forest soils

The stacking column represents changes in the stock of organic carbon between the first and second BZE. Three classes were formed: Decrease to low increase (less than or equal to 0.23 tonnes per hectare per year), moderate increase (between more than 0.23 to less than or equal to 0.45 tonnes per hectare per year) and high increase (more than 0.45 tonnes per hectare per year). For these initial evaluations, the category boundaries were set so that each class has a share of about one third.

Source: Thünen-Institut für Waldökosysteme (analyses on the basis of the soil inventory (BZE) in forests)
 

Humus – friend in need

Apart from controlling the composition of tree species, forestry professionals have additional ways of furthering the adaptability of forests to changing climatic conditions. Humus conservation therefore 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.

However, the interactions between climate-related changes and the content or reserve of humus go far beyond the scenario outlined above. Apart from its favourable influence on the adaptability of forests, sufficient humus reserves are also worth striving for in view of protection from the impacts of climate change in view of the fact that soil is one of the most important carbon sinks. Any amount of carbon sequestred in the soil is not contributing to climate change as carbon dioxide in the atmosphere. Humus conservation therefore is both an adaptation measure and a method to counteract climate change. Another interaction that enters discussions is the influence of weather patterns on the activity of micro-organisms in the soil, and in that context the formation and decomposition of humus.

In virgin forests, the organic matter which is 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. In commercial forests, the situation is different and this is because the greater part of carbon is removed from the forest by harvesting. The actual extent depends largely on the proportion of trees remaining in a stand after harvesting. The more leaves, needles or branches remain on the felling 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 felling has taken place. As the demand for energy from timber rises, increasing incentives emerge to make use also of inferior material including crown material. In contrast with agriculture, the 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. This is the only way to ensure that sufficient humus is formed.

As part of forest soil condition surveys (BZE) research is carried out nationwide to assess carbon reserves in the upper 30 cm of mineral soil and the humus cover in the forest. Data collected so far permit a comparison of the situation in the period between 1987 and 1993 with the period between 2006 and 2008. Overall there were more areas where the carbon reserves in mineral soil had increased since the first survey; the increase was particularly distinct in northern parts of Germany. In southern Germany there was no indication of major changes. However, it is questionable whether the changes observed can be attributed to the targeted promotion of humus formation because any trend may be masked by forest restructuring measures, changes in the deposition of pollutants as well as by liming procedures.

There were no appreciable changes in carbon reserves found in the humus cover on most sites between the two survey periods. It must be said, however, that it is not possible at the moment to interpret the survey results without a degree of ambiguity. The organic carbon in the humus cover is subject to comparatively rapid formation and decomposition processes thus exposing it to particularly strong external impacts. Furthermore the humus levels fluctuated quite strongly from site to site which makes the interpretation of rates of change difficult. This is why the indicator does not illustrate changes to carbon reserves in the humus cover.

For the purpose of assessing the changes to carbon reserves in the upper 30 cm of mineral soil, the first two surveys differentiated between the class boundaries for a decrease or slight increase; a moderate increase; and a high increase in such a way that the forest area assessed was divided up evenly among these three classes. When the soil condition survey (BZE) is next updated; the documentation will illustrate changes by reflecting shifts in the area proportions allocated to the three classes. 

 

Interfaces

BO-R-1: Humus content of arable land

 

Objectives

Forests are to retain their function as CO2 sinks. (Waldstrategie 2020, p. 11)

Existing carbon reserves should be conserved and augmented and the formation of new carbon reserves should be encouraged. (European Forest Strategy, item 13)

Protecting the ecological efficacy of soils by means of […] conserving organic matter, and implementing site-adapted land use strategies in order to reduce detrimental effects caused by changes in the formation of soil and humus (DAS, ch. 3.2.4)