Emerging from the Forest Crisis: Cooling Green Oases in a Warming Landscape

The German forest landscape is facing critical challenges due to recent climate changes. Years of drought, specifically from 2003, 2015, and 2018 to 2021, have left significant marks on the ecosystem. The combination of below-average precipitation and increasing frequency and intensity of heatwaves poses a threat to remaining ancient beech forests.

Researchers from the University of Eberswalde highlight the pressing need for a future-oriented approach to forest management, emphasizing that the survival of forests cannot be separated from the surrounding landscape. Larger, contiguous forest areas stand a better chance of thriving compared to isolated patches surrounded by agricultural land.

Forests serve as natural coolers, often maintaining temperatures up to ten degrees lower than adjacent open areas on hot days. This cooling effect significantly benefits agriculture, particularly during heatwaves. However, with ongoing global warming, the protective capabilities of forests may diminish.

Utilizing satellite data gathered from five ancient forested areas between 2017 and 2023, scientists assessed the surface temperatures above the tree canopies and the coloration of leaves to gauge the vitality of these ecosystems. Higher levels of chlorophyll in leaves indicate healthier trees.

In situ measurements were taken every ten minutes to monitor humidity and temperature near the ground. This data allowed for a detailed analysis of the microclimate across different zones within beech forests. The focus was primarily on how these forests respond to extreme heat days--characterized by temperatures exceeding 30 degrees Celsius, which are particularly detrimental to trees.

The findings, published in the journal Nature Scientific Reports, revealed varying temperature responses among the five forest sites. For instance, the Grumsin forest in Brandenburg recorded an average canopy temperature of 43.2 degrees Celsius on 237 hot days, the highest among the studied forests. Other areas, such as the Kellerwald in Hesse and the Hainich in Thuringia, also experienced elevated temperatures, while the Jasmund forest on Rügen maintained relatively cooler conditions at 36.1 degrees Celsius.

The Grumsin forest is situated in a largely uniform agricultural landscape, which heats up significantly during hot spells, creating harsh conditions that adversely affect nearby forests. In contrast, the Jasmund National Park benefits from the cooling influence of the Baltic Sea, leading to healthier forest ecosystems.

Historically, Europe was predominantly covered by beech forests, but today's remaining patches are classified as endangered habitats. The UNESCO World Heritage designation for beech forests, initiated in 2007 in the Carpathians, was expanded to include German forests in 2011, now encompassing 93 forest sites across 18 countries.

Research indicates that dense, mature forests with well-defined edges are generally cooler and healthier. Core areas of these forests exhibit greater resilience to heat stress compared to their outer zones, allowing trees to recover more effectively.

Forest ecosystems create their own internal climates, which are vital for survival. The size of the forest and the length of its perimeters are critical factors. For example, the Kellerwald has fragmented edges and lacks a substantial core area, making it more vulnerable to climate change. Smaller forests like Grumsin, surrounded by agricultural land, face heightened risks from climate impacts.

To mitigate warming effects in both forests and agricultural landscapes, researchers recommend converting portions of farmland into green spaces and planting shrubs. Every small addition of greenery contributes to the cooling effect. To preserve ancient forests, the team advocates for the cessation of all forms of resource extraction.

Furthermore, the establishment of new forests is deemed essential. For instance, connecting the Grumsin forest with the Angermünder Stadtwald could enhance the survival chances of both forested areas, acting as a form of ecological insurance.