Healthy trees have a dense crown. However, a walk through the forest reveals that the green canopy is generally quite sparse. In recent years more and more deforested areas have become evident. Forests are our green lungs, create habitats for a diverse world of flora and fauna, provide timber, and protect against floods and slope collapse. How large is the loss caused by forest die-off and tree removal? Researchers at the German Aerospace Center (DLR) are addressing this question with the help of satellite earth observation data. For the first time, they have made visible how much of the forest inventory has been lost throughout Germany. The results are alarming: from January 2018 to and including April 2021, some 501,000 hectares of tree loss have been noted in Germany. This is an amount almost equivalent to five percent of the entire forested area, and thus considerably more than previously assumed. The triggers are primarily considered to be unusually intensive periods of heat and drought in these years, which in turn favoured infestation by harmful insects.
Support from space
Access to comprehensive forest information is important for forest management staff in order to map tree species, differentiate the causes of damage, and prevent negative developments by early detection. Earth observation satellites can supply the requisite spatial and temporal resolution. The potential inherent in satellite images is however not yet being fully utilized by public authorities. The DLR research team at the Earth Observation Center (EOC) has contributed its expertise. In order to precisely estimate tree loss, it used as data sources the Copernicus Sentinel-2 satellite of the European earth observation programme and the US Landsat-8 satellite.
The satellite data presently available can be analysed to precisely determine large-scale loss in the forest canopy. The images also make drastic damage readily visible, such as the complete die-off of groves. Another advantage is the frequency of updating. Satellites can provide data of high temporal density.
Monoculture, weather extremes and insects
The view from space shows that it is mainly central Germany with its conifer forests that is affected, from the Eifel and Harz mountain ranges to the Thuringian Forest and the heights of Saxon Switzerland. In three years, the state of North Rhine-Westphalia alone lost more than a quarter of its pine forests and, in some counties, over two-thirds. Trees died or were sacrificed in large-scale distress felling. Deforestation is often the last measure in cases of massive pest infestation. It is undertaken in the case of pine trees in order to remove the bark beetle food supply, thereby preventing their further spread.
While deciduous trees like oak can recover from insect infestation, this is frequently not the case for conifers. After the second world war, Germany was reforested with pine trees as a top priority, sometimes in unsuitable locations, because they were the most important lumber source. These forests thus have similar age and growth structures and as monocultures are less resilient. Between 2018 and 2020 all of central Europe was ravaged by several unusually long periods of drought and heat waves. This weakened the green giants—the deficits in ground moisture are still measurable today. At the same time the dry heat provided ideal conditions for bark beetles, so their populations increased explosively.
Not only pine forests are affected by the consequences of drought. "Our analyses show that oak, beech and spruce—in addition to pine the most common tree species in Germany, show considerable damage. The same is true for less common species like sycamore and larch," explains Dr. Frank Thonfeld of the DLR Earth Observation Center (EOC). "The annual forest survey reports drawn up by public authorities now make it evident that the condition of German forests has continuously deteriorated already for a long time. But the damage caused in the last few years is unprecedented."
In addition to pest infestation, German forests also suffered blowdown loss. The DLR research team identified these wind-damaged areas with the help of highly precise satellite images derived from Sentinel-2 and Landsat-8 data. The assessments revealed, for example, the extent of storm damage in eastern Bavaria, Saxony-Anhalt and Saxony. The present Germany-wide storm situation will probably again require the removal of wind-damaged wood in many locations.
Big data on forest development
The remote sensing experts in Oberpfaffenhofen evaluated over 20,000 datasets. In this way they were able to identify forest die-off and newly felled areas on a monthly basis. The result is a differentiated forest survey for all of Germany at a resolution of ten meters. The processing of the Sentinel-2 and Landsat-8 data archives was completely automated. This highly complex procedure was developed at EOC and is being optimized for other applications.
The methodology for interpreting the forest inventory can also be applied in other countries and regions. Large-scale forest damage is not only a German, but also an European concern. Neighbouring countries like the Czech Republic and Austria are facing similar challenges. In the mid-term it can also be expected that more forest stands will be lost. It will take decades before the economic repercussions are surmounted, and it can take even longer for the forest ecosystem to recover. It is therefore urgent that Germany and Europe promptly initiate efficient measures to protect forests. Satellite-supported earth observation can provide researchers and decision makers with the needed data basis.
Just as the new methodology can be customized for particular users, the new maps can be also be brought up to date at any time. In the future, the DLR research team could provide satellite data on forested areas to forest authorities on a monthly basis and thus provide long-term support of ongoing forest management in all regions.