In Central Europe, the weather extremes of the past years, in combination with insect outbreaks, have already reduced tree vitality and productivity. The extreme drought in 2018 severely affected forest stands in Germany causing the death of about twice as many trees in 2019 compared to 2018 and poor vitality of about 80% of all living trees. As extreme climate conditions are expected to increase over the next decades as a consequence of the ongoing climate change, it is of high interest to study which forest management practices could attenuate the adverse effects of heat waves within forest stands.
Capacity and importance of microclimatic regulation
Forest management such as thinning and harvesting intensity largely define structural characteristics of a forest stand. Such characteristics can significantly mediate forests’ capacity to attenuate extreme temperatures, moderate mean temperatures and reduce temperature variability. This is the conclusions of a recent study lead by Jeannette S. Blumröder and Pierre L. Ibisch from the Centre for Econics and Ecosystem Management.
The study, conducted in northern Germany during the two extreme hot and dry summers in 2018 and 2019, compared on-site microclimatic conditions in Scots pine plantations and European beech forests across a gradient of different management intensities. From all the variables, researchers identified the degree of canopy openness and the quantity of logged trees, both directly controlled by forest management, as the main factors for the reduction of maximum temperatures in the forest interior. They found the maximum temperature to be higher in forests with more cut trees and more open canopy.
Further, the forests’ temperature cooling capacity decreased with increasing wood harvest activities, and the maximum temperature was higher in forests with relative low stand volume. Therefore, high stock and dense canopy would provide an insurance against heat and drought events.
Correct forestry key to mitigate the impacts of the climate crisis
In some regions in Germany, pine plantations cover about 70% of the forested areas. Based on the results found, effective forest management aiming continuous forest cover and more complex structures instead of homogenous even-aged monocultures would provide forests a higher capacity for stabilisation of microclimatic conditions in the forest interior. This means therefore that there is an urgent need to develop into more diverse mixed and self-regulating, cooler and resilient forest ecosystems to counteract extreme macroclimatic conditions expected under climate change.
Forests microclimatic stability is not only important to protect woody species, but also other forests organisms sensitive to temperature fluctuations. In addition, forest microclimate regulation provide key ecosystem services such as improved human health, socioeconomic benefits related to timber production, as well as recreation services.
Thus, the authors advocate keeping the canopy as dense as possible. They suggest a cover of at least 80% by maintaining sufficient overgrowth and by supporting deciduous trees that provide effective shade.