Recent forest cover loss in Germany after the 2018-2020 drought years

Kurzfassung

Vast areas of Central and Northern Europe experienced a pronounced drought in 2018. Germany, among other countries, was heavily affected. In some parts of the country, exceptionally dry conditions continued into spring 2021. The effects of the 2018 drought had a strong impact on Central European forests, particularly in the Czech Republic and Germany. Extensive droughts cause severe stress to trees, which is amplified by the specific situation in Germany, where forests are often located in hilly regions or on poor soils, and many trees are planted at the margins of their climatic niche. Once stressed by drought, trees are generally more susceptible to insect damage. While deciduous trees often have the potential to recover from insect infestations, the situation is different for coniferous trees. The European spruce bark beetle (Ips typographus [L.]) is one of the most damaging pest insects of spruce forests in Europe: successful infestation is typically fatal to trees. During the 2018-2020 drought, bark beetle management in Germany had a strong focus on the prevention of outbreak expansion by massive salvage and sanitation logging in outbreak areas and their surroundings. Actual numbers of the associated forest loss are provided based on statistical sampling and are not spatially explicit. Besides, the temporal development can only be traced at annual intervals. Remote sensing has proven to be valuable in detecting forest changes, particularly stand replacing changes. However, annual change maps typically use annual best-pixel composites or temporal metrics. These can lead to some ambiguity in correctly assigning a change to a particular year, as images taken under optimal conditions are typically weighted more heavily than winter acquisitions. Hence, changes happening late in a year are likely attributed to the next year. Common silvicultural practice in Germany avoids large-scale clear-cuts. This has changed in response to the recent drought. Clear-cuts are common practice to implement salvage logging. To our knowledge there is currently no comprehensive, spatially-explicit assessment of clear-cuts and tree loss in Germany. We demonstrate an efficient method to map clear-cuts in temperate Central European forests with high spatial (10 m) and temporal (monthly) resolution. We present a first spatially-explicit assessment of the tree-loss areas in response to the 2018-2020 drought in Germany. To achieve this goal, we used time series of Sentinel-2 and Landsat 8 data and a spectral index largely insensitive to illumination conditions, the disturbance index (DI, Healey et al., 2005). The dense time series was aggregated to monthly composites, thereby removing outliers. From the monthly time series (January 2018-April 2021), we computed anomalies with respect to a reference period (2017) and applied simple thresholding to separate clear-cuts and dead trees from healthy and stressed forest stands. We identified changes (i.e. tree loss) persisting over the monitoring period, determined tree loss dates at per-pixel scale and aggregated the results to different administrative levels. Our results reveal that about 588,489 ha of forest were lost in Germany between January 2018 and April 2021, corresponding to more than 5 % of the total forest area. This figure contains also dead trees that were not yet logged, but mainly refers to cleared forests. In 2018, the tree loss area was still rather low as it took some time for the trees to die in response to the heavy 2018 drought. Most of the cleared areas of 2018 are likely the result of the removal of windthrown trees in the aftermath of 2017 summer storms (e.g. near Passau in Bavaria, South-East Germany) and 2018 winter storms such as “Friederike” (e.g. in Northern and Eastern Germany). Drought induced mortality in beech and spruce trees started in 2018, and was accelerated by bark beetle infestation in spruce trees, which started in 2018 and continued in several outbreak phases until 2021. Salvage logging as radical management strategy started already in 2018 in some federal states such as Saxony-Anhalt, but accelerated through 2019 and 2020, particularly in Hesse and North Rhine-Westphalia. Consequently, the spatial pattern of tree loss changed from larger areas in Eastern and South-Eastern Germany in 2018 to dominant changes in Central and Western Germany in 2019, 2020 and 2021. Considering all forest types, tree loss was evident throughout Germany, even though Northern and Southern Germany were less affected than Central Germany. Central Western and Eastern Germany were most heavily affected with regard to forest loss in coniferous forests. In a belt ranging from the western to the eastern borders of the country, a large share of the coniferous forests was cleared, in some areas more than three quarters. At district level (Landkreis), the pattern becomes clearer than on federal state level. The district of Soest in North Rhine-Westphalia, for example, lost two thirds of its coniferous forests. While existing annual crown condition assessments are a valuable source to identify general (long-term) forest health developments, spatially-explicit mapping of tree loss is still missing in Germany. We aim to support forest management and scientific understanding with this first assessment of tree loss after the 2018-2020 drought years.