Study in northern Germany: Forestry contributed to the warming of forests during extreme summers
In a study published with colleagues from Leuphana University in Lüneburg and Freie Universität Berlin in the journal Ecological Solutions and Evidence of the British Ecological Society, scientists from Eberswalde University for Sustainable Development (HNEE) show that forest utilisation has a significant influence on the cooling capacity of forests and thus also their sensitivity to climate change.
In extremely hot and dry years, it is not only the weather that influences forest vitality. Forest utilisation can significantly reduce the cooling capacity of the forests and thus increase the sensitivity of the forest to climate change. In the FFH area ‘Forests near Feldberg with Breitem Luzin and Dolgener See’ in the neighbourhood of the Heiligen Hallen, Mecklenburg-Western Pomerania, old exposed beech trees in particular have been dying off after forestry interventions for several years, June 2020.
Light pine plantation in Brandenburg.
In a heavily thinned beech forest interspersed with logging roads and a disintegrated canopy, the microclimatic regulation typical of forests is lost. This results in severe heat and drought damage and the death of old exposed trees in particular. FFH area ‘Forests near Feldberg with Breitem Luzin and Dolgener See’ in the neighbourhood of the Heiligen Hallen, Mecklenburg-Western Pomerania, June 2021.
Forest ecosystems influence the climate on a global and local level. An important characteristic of forests is the regulation of the ecosystem microclimate. The shading provided by the trees, the evaporation of water, the storage of heat in the biomass and the conversion of energy through photosynthesis mean that forests cool themselves and their surroundings when it is hot. This means that harmful maximum temperatures can be avoided, especially during long-lasting heatwaves.
Jeanette Blumröder, the HNEE scientist responsible for the study, states: "Increased logging and a correspondingly larger opening of the canopy drive up the maximum temperatures in the forest. This also increases the vulnerability of forests to climate change. The extensive series of measurements in beech forests and pine forests in northern Germany from the hot summers of 2018 and 2019 very specifically confirm the fears that emerged from previous studies. If the canopy is opened by 10 %, the average maximum temperatures rise by around half a degree Celsius. Pine forests show a below-average cooling capacity as soon as the canopy is less than 82% closed."
In biomass-poor pine forests (177 m3 per hectare), the average maximum temperature was 9°C higher than in relatively wood-rich beech forests (> 565 m3 per hectare). If only pine plantations are considered, there is also a significant influence of utilisation intensity: during the hottest day in 2019, the difference in temperature peaks between those with a relatively dense canopy (72 %) and those with a particularly open one (46 %) was more than 13°C. Project leader Prof. Dr Pierre Ibisch summarises: "The conclusion is that forest managers have some control over how much the forests entrusted to them heat up and are potentially damaged by climate change. Higher biomass stocks and a closed canopy are an insurance policy against extreme weather conditions".
The published study also critically discusses and casts doubt on previously frequently made silvicultural recommendations for increased thinning of forests. Water losses and the risk of heat damage increase as a result of increased thinning. The authors recommend keeping the canopy as closed as possible (at least 80 %) and utilising the forests with appropriate caution. They also confirm the well-known demand to develop the simply structured conifer monocultures into structurally rich mixed deciduous forests as quickly as possible. Forest managers should fulfil their outstanding responsibility for landscape temperature management in the face of climate change.
The study was largely funded by the project "Ecological and economic evaluation of integrated nature conservation measures in forest management to secure ecosystem services and forest ecosystem functions (Gläserner Forst) - Subproject 3: Ecological evaluation and ecosystem services" of the Federal Ministry of Education and Research (Bundesministeriums für Bildung und Forschung, BMBF). The project partners are the Brandenburg Ministry of Agriculture, Environment and Climate Protection (Ministerium für Landwirtschaft, Umwelt und Klimaschutz, MLUK), the University of Göttingen and the German Nature and Biodiversity Conservation Union (Naturschutzbund Deutschland e.V., NABU).
The forests analysed are located in the Schorfheide-Chorin biosphere reserve and on areas belonging to the NABU National Natural Heritage Foundation in Brandenburg, as well as in the Heiligen Hallen conservation area in Mecklenburg-Western Pomerania.
Original study: Blumroeder, Jeanette S., Felix May, Werner Härdtle and Pierre L. Ibisch (2021) Forestry contributed to warming of forest ecosystems in northern Germany during the extreme summers of 2018 and 2019. Ecological Solutions and Evidence. DOI 10.1002/2688-8319.12087 .
Authors of the study
Jeanette Silvin Blumröder and Prof. Dr Pierre L. Ibisch
Centre for Econics and Ecosystem Management at Eberswalde University for Sustainable Development
Dr Felix May
AG Theoretical Biology, Institute of Biology, Free University of Berlin
Prof Dr Werner Härdtle
Institute of Ecology, Leuphana University of Lüneburg
Note on image material
The photos available here may be used for reporting purposes. Please note the copyright notices © © HNEE / Jeanette Blumröder or © HNEE / Pierre Ibisch