Are we seeing a climate-related shift in European forests mortality?

The annual General Assembly of the European Geosciences Union (EGU) was held in Vienna on May 23-27, 2022 with a focus on climate change. The assembly was special for the Tartu Observatory as remote sensing researcher Jan Peter George’s study on forest mortality in Europe was selected for the program from among 12,000 candidates. It was also one of the few presentations selected to be presented at the press conference on the far-reaching consequences of climate change.

European forests are an important source for timber production, human welfare, income, protection and biodiversity. Jan Peter’s retrospective analysis of the last 25 years asks: are European forests currently experiencing a shift in climate-related mortality? During the last two decades, Europe has experienced a number of droughts which were exceptional within the last 500 years both in terms of duration and intensity and these droughts seem to have left remarkable imprints in the mortality dynamics of European forests. However, systematic observations on tree decline with emphasis on single species together with high-resolution drought data has been scarce so far so that deeper insights into mortality dynamics and drought occurrence are still limited on a continental scale.

Jan Peter George and his co-authors made use of the ICP Forest crown defoliation dataset, permitting them to retrospectively monitor tree mortality for four major conifers, two major broadleaves as well as a pooled dataset of nearly all minor tree species in Europe. In total, the researchers analysed more than 3 million observations gathered during the last 25 years and employed a high-resolution drought index which is able to assess soil moisture anomaly based on a hydrological water-balance and runoff model every ten days across the continent.

“We found significant overall and species-specific increasing trends in mortality rates accompanied by decreasing soil moisture,” the scientists write in their abstract. “In particular, previous-year soil moisture anomaly had a stronger influence on mortality rates than current-year soil moisture, suggesting that legacy effects play a key role in actual forest decline. Thus, past environmental conditions can directly affect the structure of the plant as well as the functioning of its cells. The effect can also occur indirectly through other organisms, such as attack by bark beetles.

Remarkable peaks in mortality occurred simultaneously in Norway spruce and Scots pine (2004, 2018, 2019), but were largely asynchronous in broadleaves. Mortality rates in Norway spruce and Scots pine have increased by 60% and 40%, respectively (period 2010-2020 compared to 1995-2009). Oak (Quercus robur and petraea) as well as Silver-fir (Abies alba) showed much lower mortality and only a weak upward trend. The scientists conclude that mortality patterns in European forests are currently reaching a concerning upward trend which could be further accelerated by future global change-type droughts.

The EGU promotes the sustainable future of humanity and the planet. The first EGU General Assembly took place on April 30, 2004 in Nice, France.

Jan Peter’s presentation can be watched here (starting from 14:20): https://youtu.be/XCptvBGJYVA?t=860