The droughts of the past few years in various European regions have ensured that the most important tree species of the continent die above average and the forest ecosystems come under pressure. This is the conclusion of a study by Jan-Peter George from University Tartu in Estonia and his team, which was published in advance on the biorxiv. This is particularly affected by the "bread tree" of forestry, the spruce: their annual mortality rate was 60 percent higher between 2010 and 2020 than in the comparison period from 1995 to 2009.
Pines with 40 percent and beech with 36 percent more dead trees also suffered from the drought, while oak mortality increased by only 3.6 percent. The working group did not rely solely on satellite images for its analysis, as it is not always possible to conclude with certainty from these data whether a tree was felled or died. Instead, they supplemented their dataset with three million on-site observation points collected as part of the long-term International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests (ICP Forests) project.
In all the tree species examined, mortality had been above the long -term cut of the comparison period since 2012, so there were more trees regularly than before. This was mainly caused by soil dryness, as the comparison with moisture data showed: The trees were always thoroughly died in a drought year the following year. The extraordinary drought in Germany is a good example for this: According to estimates by the German Ministry of Agriculture, the drought in 2018 itself destroyed several million trees that would cover an area of around 2450 square kilometers. However, dying continued in 2019, which was also too dry, which further added to the weakened trees. Since almost all of the regions of Europe have been affected by drought in recent years, tree death is a continent -wide phenomenon.
However, the trees are not only stunning because of the direct consequences of the drought: it also makes them more susceptible to diseases and pests. Stressed spruces, for example, are more easily affected by bark beetles because they cannot form enough resin to ward off them. Mass reproductions of the insects then reinforce tree death. Fire can break out and spread out more easily in dry forests.
Some of the problems are homemade, especially in the case of spruce: In many places, it is planted in monocultures atypical of its location and is very susceptible to drought. Their death therefore means heavy losses for the timber industry. George and Co strongly advise the conversion of forests to make them more resilient. However, clear-cutting is delicate, as the open areas can heat up more, which in turn puts newly planted trees under heat and drought stress. In addition, tree death reduces the role of European forests as carbon sinks, which increases climate change – which in turn makes droughts more likely.
