The German forest is in bad shape. A new EOC method now shows for the first time in high spatial and temporal resolution how its horizontal and vertical structure has changed in recent years.
Dryness and heat waves have greatly afflicted German forests since 2018. Pests like bark beetles and storms aggravated the problem. As a result, foresters had to prematurely cut down numerous diseased and damaged trees to prevent pest propagation. Because of this loss of trees, the forest canopy has become thinner and lower. Forests become more susceptible to threats and lose their above-ground biomass. Primarily coniferous forests of spruce and pine, which comprise about half of Germany’s forests, are affected. Especially these forests have a simple, one-layer canopy structure, which means a more uniform tree height. More diverse forests with two or more canopy layers have several distinct heights, and with this more complex canopy structure can react more robustly to disruptions like storms or climate change.
Now EOC researchers have for the first time used machine learning methodologies to map forest structures and how they have changed in all of Germany’s wooded areas between 2017 and 2022. With ten-meter resolution sensor data, recent problems caused by dryness can be detected and those areas identified that should be primarily preserved. Having time series of consistent data products available, changes in the height and density of the canopy and the amount of above-ground biomass can be quantified.
Several years of monitoring show that since 2018 there have been considerable losses in all derived forest structure characteristics for much of Germany’s forested area. In 2017 forest landscapes in the Harz, Siegen-Wittgenstein and the southern Thuringian Forest still showed high canopy levels (> 20 m), a dense canopy cover (> 80 %) and a high level of above-ground biomass (> 250 mg/ha). These values drastically changed in the following years, especially in 2020. The regions named showed not only a massive decline in canopy heights (< 10 m), but also in their density (< 20 %) and in above-ground biomass (< 100 mg/ha). These quantitative findings are consistent with earlier EOC mapping of the areal loss of German forests in the same time period.
This awareness of forest structure became possible by combining data from several sensors. For example, NASA`s GEDI mission (Global Ecosystem Dynamics Investigation) with three lasers installed on the International Space Station ISS provided a detailed, three-dimensional (3D) map of tree canopy heights and the distribution of branches and leaves. However, only for specific locations. By contrast, Europe’s Copernicus satellites Sentinel-1 and Sentinel-2 do not yield a high-resolution view of vertical forest structure, but their optics and radar sensor can instead provide an inclusive, all-encompassing view of forests. The new EOC methodology uses this full coverage data to model the highly precise laser measurements also for the areas not mapped in the GEDI mission.
The current structure of forests is determined by historic forest cultivation. In most cases rapidly growing tree species were planted in monocultures, and since the middle of the 20th century these were primarily extensive spruce and pine plantations of the same age. Under natural conditions German forests were in the past dominated by deciduous copper beech trees. According to the annual forest survey report of the Federal Ministry of Food and Agriculture the percentage of copper beech trees is now only 16 percent, while coniferous trees make up over half of German forest inventory, mainly pine (23 %) and spruce (25 %) trees. Spruce trees are quite resilient to frigid temperatures, but they are very sensitive to drought. Their natural tree stands are restricted to cold, high altitudes, so during the dry periods since 2018 exceptionally many spruce trees cultivated at unsuitable low elevations have died. But not only coniferous trees are affected. Since 1984 the modestly increasing canopy transparency of all major tree species indicates their poorer health. Also bark beetle infestation has been favored in many places by current climate conditions and led to the extensive die off of the spruce inventory. The consequence has been preventative tree felling to keep such forests healthy, which involves the large-scale removal of the felled trees and deadwood. The resulting cleared areas are today characteristic of the forests in the Harz, Siegen-Wittgenstein and southern Thuringia.
The new data products provide important indicators for evaluating forested areas. For example, the height of the canopy gives an impression of the vertical structure of forests. Intact forests can be distinguished from disturbed ones. The vertical structure is also an indicator of a forest’s age and adaptability to disruption. The density of the canopy, that is, the horizontal forest structure, is also relevant for forest management. It gives insights into how much sunlight reaches the ground and the accompanying dryness. The third modeled attribute, above-ground biomass, combines the information about the vertical and horizontal characteristics of a forest and makes it possible to estimate how much carbon is stored in the forest.
With these EOC data products Germany now has continuous information about losses experienced by the canopy as well as about the overall forest structure. This makes it possible for the first time to study with a high degree of detail (10 m) structural changes like the standing deadwood and cleared areas that result after storms or pest infestation. In the future, it will be possible to reliably identify disturbed structures, for example, also places with standing deadwood. For such purposes, the requisite time series are to be operationally extended also in the future.
With consistent and timely provision of satellite-derived data products on the condition of forests via the EOC Geoservice, government ministries and forestry departments have access to a valuable data basis for developing new forest management strategies. The goals are to avoid deforested areas, to preserve complex forest structures, and to make forests more resilient through more variety in deciduous tree species.
