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Causing Continued Concern: Forests

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 his study site 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.

Nicolas Brüggemann auf der Untersuchungsfläche in Kleinhau.Prof. Nicolas Brüggemann from the Institute of Bio- and Geosciences (IBG-3).
Copyright: Forschungszentrum Jülich / Sascha Kreklau

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 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.

Robust tree species wanted

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.”

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.

Kleinhau offers the Jülich researchers the opportunity to observe the effects of drought and spruce removal live, so to speak. Since May 2020, together with colleagues from four other Helmholtz Centres, they have been conducting a several-month measurement campaign of the Helmholtz Initiative MOSES. During the campaign, they record soil moisture in the entire area, determine the exchange of CO2 and water vapour between the soil and the atmosphere, and create 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.

Drought slows down regeneration

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. 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.

Alexander Graf beim Messen in Kleinhau.Alexander Graf measuring in Kleinhau.
Copyright: Forschungszentrum Jülich / Sascha Kreklau

No more buffering of heat

Both at Wüstebach and in Kleinhau, researchers have found: if less carbon dioxide is taken up, less water vapour is released. This has advantages and disadvantages. The air above the area cleared of the spruce trees heats up faster: in summer, when it gets hot, there is nothing to buffer the heat. The sun shines unfiltered onto the open spaces, which heat up up to 60 °C. The result: a great deal of warm air rises into the atmosphere, and the heat wave becomes even stronger. On the other hand, without the forest, there is no thirsty consumer that further draws the scarce water away from the soil.

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