Interesting facts about the fourth National Forest Inventory

Just as every company regularly checks its inventory, Germany's forests have been systematically checked since 1986. Specially trained inventory teams visit the forests every ten years. The Federal Forest Inventory is a task organized jointly by the federal and state governments in accordance with §41a of the Federal Forest Act. The Thünen Institute of Forest Ecosystems carries out the Federal Forest Inventory on behalf of the Federal Ministry of Food and Agriculture, and in 2022 for the fourth time throughout Germany.

The Scandinavians were the first to introduce national forest inventories based on sampling more than 100 years ago. Today, there are national forest inventories based on sampling in almost all European countries. Each procedure is slightly different because national traditions, objectives of the inventory and local forest conditions are taken into account. However, in order to be able to compare country data internationally, they must be harmonized. This is the task of the European National Forest Inventory Network (ENFIN), which was founded in the 1990s. It brings together the institutions responsible for national forest inventories in each country. Harmonized information on forest areas, wood stocks, biomass and wood growth can now be provided.

A working group at the Thünen Institute of Forest Ecosystems prepares the inventory scientifically, coordinates it and analyzes the data. The permanent random samples are distributed throughout Germany in a basic network of four by four kilometers. The federal states can increase the density of the sampling grid to 2.83 by 2.83 kilometers or even two by two kilometers in order to increase the statistical reliability for their areas. At each grid point in a forest area, the teams record the tree species, diameter and height of selected sample trees - more than 500,000 in total - as well as data on stand structure and deadwood. In total, more than 150 characteristics are recorded. One of the most important parameters is the tree diameter at a height of 1.30 meters. It takes a total of two years to collect all the data.

The field data from the National Forest Inventory is a valuable source of data: scientists can use it to calculate, for example, how much wood stock there is in the forests, how much carbon is stored in them and how these values have changed in recent years. The data series also allow projections to be made on how wood stocks and carbon storage could develop in the coming years under certain predefined assumptions. They form the basis for decisions in politics and business, for example when it comes to the challenges of climate change or sustainable forest management. The data is also an integral part of international reporting obligations such as the Kyoto Protocol and the Framework Convention on Climate Change.

So-called remote sensing data is used for various work processes of the National Forest Inventory. For example, they can be used to provide so-called small area estimates, i.e. evaluations for smaller spatial units such as districts or municipalities.

Aerial photographs and satellite data are also used to check whether the sampling points are actually located in the forest before the inspection. Finally, a forest mask from 2023 is placed over the surveys carried out on site in 2021 and 2022 to check whether the sampling point is still stocked. This procedure in turn forms the basis for modeling forest development and timber volumes.

Data collection, data derivation, extrapolation: it takes time to generate meaningful data sets from the samples. It takes almost two years to record the 80,000 or so random samples and check them for errors and plausibility. It takes another two years to extrapolate the data and create an overall view from the samples.

An example: In order to determine the volume of the sample trees, the diameters of all sample trees are measured at breast height (BHD at 1.3 meters height). The entire height of around a third of the trees is also measured. For all other trees, the height is modeled. Once all breast height data and modeled tree height data are available, the volume of the trees can be modeled. In this way, numerous other parameters must be derived after the field surveys, such as the closeness to nature, stocking types, mixture proportions or volume increases.

No. The National Forest Inventory is a random sample. As such, it is not representative of individual forest areas. However, the individual samples of a region are combined to form so-called estimated values. These then allow statements to be made about forests in a federal state, for example, or about beech forests aged 60 to 80 years. For some federal states, the scientists at the Thünen Institute will also use so-called small area estimators following the standard evaluation in order to provide estimated values for special target values in growth areas or districts.

The National Forest Inventory is a random sample in which measurements on a part of the population are aggregated into estimated values. These estimated values are associated with estimation errors. These so-called sampling errors are a measure of the uncertainty of the estimate. The larger the sample size, the smaller the associated estimation error. Auxiliary information, such as that provided by satellite data or special statistical methods, also reduces the uncertainty of the estimated values.

If the sampling error for a characteristic is more than 20 percent, this estimated value should be handled with care. If the sampling error is greater than 50 percent, the estimated value is too unreliable to make statements on its basis. Economic or political decisions based on estimates with a high sampling error can therefore lead to wrong decisions.

The timber stock in Germany currently amounts to 3.67 billion cubic metres. The annual increase in timber stock is 101 million cubic metres, the decrease is 110 million cubic metres. This means that the amount of wood in the forest should have decreased. In fact, however, it has increased by 242,000 cubic metres. This discrepancy is known as the balance gap.

It is caused by the method of counting the sample trees, which are repeatedly recorded. Trees are counted as disposals if they were measured in the previous inventory and are no longer found in the current inventory. The stock includes all trees that are known from the previous inventory and are still present and all trees that have newly grown into the current sample.

Each sample tree also represents a certain number of trees per hectare due to its diameter. In the case of surviving and established trees, the number of trunks represented refers to the current point in time and therefore to the current population. However, in the case of the outgoing trees, i.e. the utilised or naturally dead trees, it refers to the previous inventory. If one now combines removals and increments to form a balance, this figure inevitably differs from the classic balance because a population is compared at different points in time. The classic balance sheet is the derivation of a change from the difference between the status value at time two minus the status value at time one.

The balance gap could be closed. However, this would require significantly more effort, both in terms of measurements in the forests and in analysing the data. In addition, more data does not automatically mean that growth and removals can be better interpreted. The current estimation method, on the other hand, is easy to interpret and comparable with the classic forestry target values.

No. The National Forest Inventory can answer the question of how many trees have died due to natural mortality since the last inventory and how many have died due to calamities such as pest infestation.

The question of forest health is answered by the forest condition survey, which is carried out annually in the federal states. For this purpose, crown thinning is recorded on a selection of individual trees. The data is also collated and analysed at the Thünen Institute of Forest Ecosystems.

The National Forest Inventory assesses the tree species composition of old and young stands at each sampling point. The naturalness of the stand (tree population of an area) is derived from this. The closeness to nature is based on the natural forest community that would be found at this location without anthropogenic intervention.

The closeness to nature of old-growth stands changes only slowly across Germany as a whole. The process of change is fuelled, for example, by regular logging or calamities such as storms. No timber has been felled on 44 per cent of forest areas in the past ten years. Young stands are now much more natural than old stands. On the one hand, this is due to the successful active forest reorganisation towards more climate-resilient forests, but on the other hand it is also related to the calamities of the past five years, such as bark beetle infestation.

Forests that are damaged do not simply disappear. Of the 500,000 hectares of forest that have died in recent years as a result of drought and pest infestation, only 100,000 hectares are so-called bare areas, i.e. forest areas that are temporarily without trees. On the remaining areas, a new generation of trees is already present, which previously stood under the canopy of the old, dead trees. These areas are therefore not unstocked and therefore not open spaces.

Forest land in Germany is only lost if the forest is converted to another form of use, for example for infrastructure projects, following prior official authorisation. However, replacement measures are prescribed for this. These can also be the renaturalisation of moors.