ARCHIV - 25.09.2019, Brandenburg, Sieversdorf: Abgestorbene Fichten, die die Trockenheit der vergangenen zwei Jahre nicht überstanden haben, stehen zwischen noch gesunden Nadel- und Laubbäumen in einem Waldgebiet im Landkreis Märkisch-Oderland. (Luftaufnahme mit einer Drohne). (zu «Brandenburgs Umweltminister Vogel stellt den Waldzustandsbericht 2019 vor») Foto: Patrick Pleul/dpa-Zentralbild/ZB - Honorarfrei nur für Bezieher des Dienstes ZB-Funkregio Ost +++ ZB-FUNKREGIO OST +++

Arid tree skeletons in green forests drastically show walkers the consequences of climate change in Thuringia, Brandenburg and many other regions of the world. Is it a question of the fate of individual trees or are the forests’ resistance to rapid changes in precipitation and temperatures already overwhelmed in large areas?

Giovanni Forzieri from the Joint Research Center of the European Commission in Ispra, Italy, and his team have now confirmed this fear with a complex evaluation of satellite data in the journal “Nature”: With two exceptions in northern regions, the adaptability of forests to the consequences of climate change seems to be increasing worldwide dwindle

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“An important and disturbing piece of work that unfortunately fits into a number of other studies on the subject,” comments nature conservation professor and forest expert Pierre Ibisch from the Eberswalde University for Sustainable Development (HNEE), who was not involved in the study.

Forests cover around 41 million square kilometers and thus around 30 percent of the earth’s land area. Tree leaves and needles absorb large amounts of the greenhouse gas carbon dioxide from the air and store the carbon it contains in their wood for decades and sometimes even centuries. Forests currently swallow about a third of the carbon dioxide produced when oil, coal and natural gas are burned. However, if their resilience to change decreases, less wood often grows there, the carbon sink becomes smaller and this accelerates climate change.

However, Forzieri and his team also analyzed the adaptability to rapid environmental changes, since forests are central to the water balance and the prevention of soil erosion and are also recreational areas for people. To do this, they used satellite data on red and near-infrared radiation, which is particularly strongly reflected by vegetation. Using this data from 2000 to 2020, the EU group used computer algorithms and machine learning to study how well forests have adapted to changes in their environment and the increasing frequency of extreme weather events associated with climate change.

In the beginning, especially in the cold and temperate latitudes of Europe, Asia and North America, the vegetation even benefited from the increased carbon dioxide concentration in the atmosphere, which has a fertilizing effect on the photosynthesis of the plants. At the same time, the rising temperatures shorten the cold periods in winter and thus lengthen the growing season. The forests in these regions turned green.

However, climate change with more frequent and severe weather extremes such as longer periods of drought is increasingly causing problems for the forests. According to the study, these effects overwhelm the resilience and adaptability of the forests, especially in the tropics, in the relatively dry regions and in the temperate latitudes, and outweigh the fertilization effect. “It shows once again that one should not lightly infer the future from a longer-lasting trend in the past,” explains HNEE researcher Pierre Ibisch.

So far, forests in eastern Canada and in the European part of Russia have benefited from the increasing carbon dioxide content in the air. However, the EU team fears that the negative consequences of climate change could also slow down greening there.

“Forests that have benefited from man-made carbon dioxide fertilization for some time and were quite productive suddenly reach a critical limit and show a reduced ability to recover,” summarizes Ibisch another result of the EU study. Accordingly, 23 percent of the world’s forests are already close to such a tipping point, most of them growing in the tropics. Once this critical point has been exceeded, the affected forests could become increasingly vulnerable, the EU researchers believe.

As frightening as these prospects are, they could still give us a false sense of security: “Unfortunately, there are signs that many trends in climate change and the corresponding reactions of ecosystems could become significantly worse,” says Ibisch. “In Germany and other regions with several years of extreme weather, we are currently learning how, for example, drought and heat waves can reduce the ability of forests to recover within a very short time,” explains the HNEE forest expert.

Entire stands of certain tree species can then collapse. In addition, use can also weaken the forest. Ibisch calls for more restraint: “We shouldn’t keep crushing and demanding the forests while at the same time the effects of the climate crisis are rolling over them more and more violently.”