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A Europe-wide study has found that the drying out of soils across the continent as the planet heats up is leading to the deaths of more and more trees.

鈥淓uropean forests are suffering,鈥� says at Tartu Observatory in Estonia. 鈥淚t needs to be made clear to everyone in Europe, regardless of whether you are in the north or the south, that this will become a huge problem.鈥�

There is already growing evidence that more severe droughts due to climate change are killing off more trees in Europe. But most previous studies have focused on particular areas or been based on satellite surveys, where it is hard to know whether a tree has died or been felled.

George and his colleagues have instead analysed about 3 million on-the-ground observations made as part of an initiative called , which was set up in the 1980s.

Using this data set meant the team could exclude tree deaths due to felling. The analysis also excluded ash trees, which are being wiped out by the double whammy of a fungal disease and an invasive beetle.

The researchers found that annual mortality rates are rising for all tree species. Norway spruce has been hardest hit, with mortality rates 60 per cent higher on average between 2010 and 2020 than between 1995 and 2009. For Scots pine, rates are up 40 per cent, European beech 36 per cent and oak 3.5 per cent.

What鈥檚 more, for all species and regions, annual mortality rates have been positive since 2012. A positive annual mortality rate means that more trees are dying than usual, compared with the long-term average. 鈥淭his could mean that European forests reached a critical point in 2012,鈥� says George.

The researchers also looked at soil moisture, as estimated by a model fed with rainfall and run-off data. They found abnormally low soil moisture was the main driver of tree deaths over the following year.

In northern regions such as Scandinavia, many people still think local forests won鈥檛 be affected by warming-driven droughts, says George, but all forests across Europe are already being affected, including northern boreal forests.

鈥淚t is a very unsettling study, but also not surprising,鈥� says Hannah Mowat at Fern, an organisation in Brussels, Belgium, that campaigns to protect forests. 鈥淚t鈥檚 not just the Amazon forest that is drying up 鈥� our own forests are parched, and it is causing them to die back too.鈥�

The researchers cannot be sure why trees often die a year or so after droughts but there are many possible reasons, says George. Severe water stress starves trees of resources and can also damage the water-conducting tissues, making them vulnerable to other stresses such as pests and diseases.

For instance, drought-hit conifers produce much less resin than normal, and resin is what protects them from bark beetles. 鈥淏ark beetles love to attack drought-stressed trees,鈥� says George.

Rising tree deaths will have many consequences. For starters, forests will soak up less carbon as older trees are replaced by younger ones.

鈥淭here is no question that old-growth forests are able to sequester more carbon than young forests,鈥� George says. However, he thinks it is unlikely that Europe鈥檚 forests will become a carbon source in the long term.

Europe is also already suffering from more frequent and severe wildfires, and more dying trees will increase the danger even further. 鈥淎s more and more dry and dead material accumulates, forests will be much more vulnerable to wildfires,鈥� says George. 鈥淭his is the dynamics we usually know from the US and Australia.鈥�

The timber industry could also be affected. One of the reasons why Norway spruce is the species hardest hit is that it has been planted in areas outside its natural range for timber production, typically in monoculture plantations. Forest owners need to start planting a mix of species to make plantations more resilient, says George.

There could be economic consequences, too. Timber prices in Europe are already high because of demand from the US and China, says George. Rising tree losses could push prices even higher and have knock-on effects on the building industry and beyond.

鈥淭his is also yet again evidence that to climate-proof our forests, to make them more resilient, we need to make them more diverse,鈥� says Mowat. 鈥淭his study is a rude awakening, but it could mean that the forests of tomorrow are actually more biodiverse, if we heed the scientists making this wake-up call.鈥�

A lack of water due to global warming is far from the only threat to Europe鈥檚 trees. Exotic pests and diseases have decimated elms and now ashes, and threaten other trees including oaks.

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