Aerial view of the spruce forest near Kleinhau in the Eifel – the trees, already severely damaged by drought and the bark beetle, stand out clearly.
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Research
Concern for the forest
Research
Concern for the forest
Looking out at the tree death with concern: Nicolas Brüggemann
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It has been too dry and warm in Germany for the third year in a row. Many trees are suffering, especially spruce. One of the consequences: they are more susceptible to pests. Jülich researchers are observing the development in the Eifel and measuring the effects, for example on the atmosphere’s CO2 content.
When Nicolas Brüggemann looks out over the northern Eifel from the forest in Kleinhau, he is worried: “In the past, we couldn’t see as far as 100 metres because of all the trees, but now, when the view is clear, I can see the Siebengebirge – and that’s 60 kilometres as the crow flies.” For the head of the Jülich research unit, Plant-Soil-Atmosphere Exchange Processes, and Professor of Terrestrial Biogeochemistry at the University of Bonn, it is beyond all question: the forest in the northern Eifel is thinning. “This is primarily due to combined damaging factors as have never before been seen: they range from drought and heat, forest fires and storms to pest infestation.” The result: trees die, many have to be cut down.
Image above: Looking out at the tree death with concern: Nicolas Brüggemann
Spruce trees are the worst affected. Drought and the bark beetle cause this flat-rooted plant great trouble, particularly as the bark beetle, due to the milder winters, gradually reaches the higher elevations in the central uplands. In Kleinhau, which is situated at an altitude of about 400 metres – the Helmholtz initiative MOSES studied the effects of the drought on the forest and soil there from May to October 2020 – the forest owner had almost the entire stock of spruce felled; the bark beetle had infested these conifers too much. On the other hand, at Wüstebach in the Eifel National Park, at an altitude of around 600 metres and where Jülich researchers operate a permanent measuring station, the damage caused by the beetle is still manageable.
However, not only spruce trees have considerable problems. “In August 2020, in terms of colour, the beech trees in the national park already looked as they would usually look in autumn. The oaks are suffering as well – either from insects or fungi,” says Brüggemann. He nevertheless finds it misleading to speak of a forest dieback: “It is not the forest as an ecosystem that is dying, but certain tree species will disappear.”
Drone flight over the cleared spruce forest in Kleinhau Affected areas are to be reforested, however, in order to preserve their function for the climate and environment. But many foresters are unsure which native tree species they should continue to plant. Foreign tree species are an alternative. Burkhard Priese, forest manager of the Kleinhau forest, is now relying primarily on the indigenous sweet chestnut and North American red oak, as these are less susceptible to pests, heat and drought. “Forests such as the one in Kleinhau, which are used economically, must be reforested immediately. We cannot wait and see if and what might grow back naturally,” he explains.
Alexander Graf measured how the CO2 exchange between the soil and the atmosphere developed after the spruce had been removed in Kleinhau. Kleinhau offered the Jülich researchers the opportunity to observe the effects of drought and spruce removal live, so to speak. During the MOSES measurement campaign, together with colleagues from four other Helmholtz Centres, they recorded, for example, soil moisture in the entire area, determined the exchange of CO2 and water vapour between the soil and the atmosphere, and created thermal images from the air. The data provide a decent addition to the extensive measurements at Wüstebach, where Jülich researchers have been collecting data since 2009. The site is part of the Germany-wide observation network TERENO, which is another Helmholtz initiative for studying the earth and the environment.
At Wüstebach, the national park administration had already cleared an area of around eleven hectares of spruce trees by autumn 2013. The idea was to accelerate the transformation of the spruce monoculture into a near-natural deciduous forest. “The cutting down has had a massive impact on soil, water quality, exchange processes and the composition of flora and fauna. This gives us an insight into the regeneration of the existing forest,” explains Dr. Alexander Graf from the Institute of Bio- and Geosciences (IBG-3). For the carbon dioxide balance of the area, for example, removing the spruce trees was a bitter blow at first.
Drought inhibits regeneration
Normally, forests take up CO2 from the atmosphere. Without the forest, however, the former carbon sink area has become a carbon source, because the soil continues to process humus from dead animals and plant parts, thereby releasing CO2. It will be several years before the area becomes a carbon sink again. Even so, the Jülich experts were amazed at how quickly the area was recovering in the first few years – until the drought periods hit. “At present, the plants are stagnating in growth and are absorbing less CO2 than in previous years,” says Alexander Graf.
1per cent
is how much the CO2 removal was decreased by terrestrial ecosystems in 2018.
Even so, compared to other ecosystems, the forest was still the best at resisting the drought: on average, CO2 take-up by terrestrial ecosystems decreased by 18 per cent in 2018. In the case of arable land, grasslands and moors, the decline was much greater, at up to 40 per cent, whereas in the case of forests it was only 8 per cent. Graf sums up: “For various reasons, this should not be overestimated, but having a lot of forest is good when it comes to taking up a lot of CO2.”
Katja Lüers/Christian Hohlfeld
By August 2020, about 90 per cent of the spruce trees in the approximately 328-hectare private forest had been cut down.
In Kleinhau, researchers from four Helmholtz institutes – among them Jülich expert Nicolas Brüggemann – have investigated how the drought has affected the forest and soil. In order to measure the exchange of energy and trace gases between the land surface and the atmosphere, for example, a so-called eddy covariance tower was built.
With the help of a drone, the researchers took thermal images and multispectral photographs from the air.
Jülich researcher Heye Bogena explains how a stationary cosmic-ray sensor works. The device measures neutrons near the ground. In simple terms: the more neutrons measured, the drier an area of soil is.
With the mobile Cosmic-Ray Rover, Heye Bogena and his Jülich colleague Jannis Jakobi (left) can determine soil moisture practically as they drive by.
A new forest is soon to stand on the cleared area near Kleinhau.
Burkhard Priese, forest manager of the forest in Kleinhau, relies primarily on the native sweet chestnut and the North American red oak for reforestation.
The Eifel National Park had already cleared the Wüstebach forest of spruce in 2013. A mixed forest has meanwhile been planted there. Due to the drought years, however, the growth of the new trees has been stagnating.
Photos: Forschungszentrum Jülich/Sascha Kreklau, Forschungszentrum Jülich/Montzka, Forschungszentrum Jülich/Schmidt Video: Forschungszentrum Jülich