Science

Forests: tree mortality increases in Europe due to climate change

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Deforestation due to logging is already a major problem in itself, and when natural phenomena (influenced or not by humans) are added to it, it can be greatly accelerated. According to a new study covering 3 million observations, global warming (mainly due to human activities) is said to be dangerously drying the soil of the European continent, causing the death of more and more trees.

The study of tree death is a very difficult task when it must be done on a large scale. Therefore, many research teams use satellite surveys or only focus on small areas. But with satellite analysis, it is difficult to know if a tree died naturally or by human hand … It is mainly for this reason that a large field study was necessary to take stock of the situation. Situation, and this is where Jan- Peter George and his colleagues from the Tartu Observatory in Estonia intervened.

The results of their study, which are now available on the bioRxiv pre-launch server, come as more and more evidence from researchers around the world shows that increasingly severe droughts due to climate change are killing more trees in Africa and Europe. In 2018 alone, for example, 110,000 hectares of forest area were damaged by drought in Germany, a particularly affected country.

A growing tree mortality rate since 2012

For this new large-scale study, the researchers analyzed about 3 million field observations made as part of the ICP-Forests initiative, established in the 1980s. With this data set, the team was able to rule out deforestation. . The analysis also excluded ash trees, which are disappearing under the combined effect of a fungal disease and an invasive beetle.

Researchers have found that annual mortality rates are increasing for all tree species. The common fir is the most affected, with mortality rates 60% higher on average between 2010 and 2020 compared to the period 1995-2009. In the case of Scots pine, rates have increased by 40%, European beech by 36% and oak by 3.5%.

What’s more, for all species and all regions, annual mortality rates have been positive since 2012. A positive annual mortality rate means that more trees are dying than usual, compared to the long-term average. This could mean that European forests reached a critical point in 2012.

The researchers also looked at soil moisture, estimated by a model powered by rainfall and runoff data. They found that abnormally low soil moisture was the main factor in tree mortality over the following year.

Soil moisture anomaly models for 2003, 2018 and 2019. The year 2002 is given as a reference. The average SMA (soil moisture anomalies) is shown from April to August for each year. Red indicates drier conditions and green indicates wetter conditions. White areas have been excluded due to unrealistic high values. © Jan-Peter George et al.

In northern regions like Scandinavia, many people still believe that local forests will not be affected by droughts due to warming, but all forests in Europe are already affected, including northern boreal forests, the researchers say.

Scientists say they still don’t know exactly why trees often die one or two years after drought phases, but there are several hypotheses: the main one is that “severe water stress” deprives trees of resources and damages them. . makes them vulnerable to other types of stress such as pests and diseases … For example, drought-stricken conifers produce much less resin than normal, but the resin is what protects them against bark beetles, the beetles that They are found under the bark of trees, the larvae feeding on the sap, which can cause their death.

Forests increasingly vulnerable to fires

Increased tree mortality will have many consequences. First, forests will absorb less carbon as old trees are replaced by young trees. Furthermore, Europe is already experiencing increasingly frequent and severe forest fires, and the increasing number of dying trees will only increase this danger. As dry and dead matter accumulates, forests will be much more vulnerable to wildfires.

The timber industry could also be affected. One of the reasons why the common fir is the most affected species is that it has been planted in areas outside its natural range for wood production, generally in monoculture plantations. According to the researchers, forest owners should start planting a mix of species to make plantations more resilient.

There could also be financial consequences. In fact, wood prices in Europe are already high due to demand from the United States and China. Increased tree losses could drive prices up even further and impact the construction industry and beyond.

Lack of water due to global warming is far from the only threat to trees in Europe. Exotic pests and diseases have decimated elms and now ash trees, and threaten other trees, especially oaks. Wood extraction has also increased by 50% since 2016.

This study shows once again that to protect our forests from climate change, to make them more resilient, we must diversify them. For some experts, this study is a “wake-up call”, but it could mean that tomorrow’s forests will indeed be more biodiverse, if we follow the advice of scientists.

bioRxiv

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