The Dying of the Black Forest: Climate Change Impacts on Tree Growth and Survival

A long-term study by forest scientists at the University of Freiburg revealed how climate change affects the growth and survival of trees in the Black Forest, a mountainous region in southwest Germany.

The study, published in the journal Global Change Biology, uses a consistent time series of 68 years (1953 to 2020) to analyze the annual mortality of all trees in an almost 250-thousand-hectare area of the public forests in the Black Forest.

The researchers compared these data with another time series of 140 years (1881-2020) on the climatic water balance, which is the difference between precipitation and potential evapotranspiration.

Climate change reduces climatic water balance and increases tree mortality

(Photo : PATRICK HERTZOG/AFP via Getty Images)

The study found that climate impacts, such as dry, hot summers, reduce the growth and increase the mortality of trees in the Black Forest because they negatively influence the climatic water balance.

The climatic water balance shows a continuous downward trend over the past 140 years, indicating that the region is becoming drier and warmer.

This affects the physiological processes of trees, such as photosynthesis and respiration, and makes them more vulnerable to biotic and abiotic stress factors, including but not limited to insect or fungal infestation, atmospheric deposition, frost, or drought.

The study also revealed that tree mortality peaked in 2019, reaching more than seven times the average mortality rate in the period from 1953 to 2017.

The researchers attributed this to the extremely dry summers in recent years, which increased the susceptibility of trees to bark beetle attacks.

The most affected tree species were spruce, beech, and fir, which are dominant in the Black Forest.

The study estimated that 40% of the sustainable annual growth died off in 2019, compared to a maximum of 12% during the forest dieback at the end of the last century.

Sustainable growth describes the sum of the average annual growth of wood which is used as a basis for calculating the annual allowable sustainable timber harvest.

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Climate change alters the periodicity of cool-humid and warm-dry periods

Another interesting finding of the study was that there was a certain regularity in the occurrence of cool-humid and warm-dry periods that repeated themselves in the past around every 14 years.

However, this periodicity has been disrupted by climate change, as the cool-humid periods are becoming weaker while the warm-dry periods are getting more severe.

This meant that trees have less time to recover from drought stress and are exposed to more frequent and intense heat waves.

The researchers concluded that climate change is a key driver of tree mortality and reduced tree growth in the Black Forest and that this trend is likely to continue in the future.

They suggested that adaptive forest management strategies are needed to cope with these challenges and to maintain the ecological and economic functions of forests.

They also emphasized the importance of long-term monitoring and research to better understand the impacts of climate change on forest ecosystems.

Adaptive forest management strategies for the Black Forest

Adaptive forest management (AFM) is a concept that aims to preserve and develop the functionality of specific forests as a prerequisite for fulfilling the future need for forest ecosystem services under changing environmental and social conditions.

AFM involves a continuous cycle of planning, implementation, monitoring, and evaluation, which allows forest managers to learn from their experiences and make adjustments to management practices as needed6.

Some examples of AFM strategies for enhancing the resilience of forests in the Black Forest are:

•  Increasing species diversity by promoting native broadleaved trees such as oak, maple, ash, or hornbeam, which are more tolerant to drought and pests than conifers.
•  Reducing stand density by thinning or harvesting trees to improve water availability and reduce competition among trees.
•  Implementing silvicultural treatments such as pruning or fertilization to improve tree vigor and resistance to stress factors.
•  Establishing protective zones or buffer strips along streams or wetlands to conserve soil moisture and biodiversity.
•  Applying adaptive harvesting regimes that consider climate variability and risk factors such as fire or windthrow.

These strategies require careful planning and monitoring based on scientific knowledge and local experience.

They also need to balance multiple objectives such as timber production, carbon sequestration, biodiversity conservation, recreation, and protection from natural hazards.

Therefore, AFM requires collaboration among different stakeholders such as forest owners, managers, researchers, policy makers, and civil society.

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