The tall, slender beech trees with their dark green, dense crowns – the very symbol of the temperate forests in Europe – may have disappear from many landscapes by the turn of the next century.
Today, the beech thrives in the temperate deciduous forest zone, which stretches from southern Sweden to central France. But in the future, the beech – like many other tree species – will face increasing exposure to climatic conditions beyond the conditions in the climate zone they occupy today.
This is shown by a new global study, led by Aarhus University and Wageningen University in the Netherlands. For beech, this means that in the future, summers in much of lowland Central Europe will be warmer, drier and reminiscent of the Mediterranean climate – conditions that the beech tree will have difficulty coping with.
This is according to Jens-Christian Svenning, professor of biology and Director of the Danish National Research Foundation's Center for Ecological Dynamics in a Novel Biosphere (ECONOVO) at Aarhus University. He is one of the researchers behind the ECONOVO research study, which has been published in the scientific journal PNAS .
"Denmark – in the northern part of the temperate deciduous forest zone - is in a better position than many other countries, but we need to think carefully when planting trees as part of reforestation and restoration efforts or for forestry. It would be a bad idea to plant too many trees from the cool and humid part of the temperate climate zone – such as Norway spruce and beech – because they may not thrive. Instead, we should focus on a broad mix of native species – e.g., pedunculate oak, European hornbeam, wild cherry, and Scots pine – and species that are currently found south of Denmark, such as sweet chestnut, walnut, Turkish hazel, Turkey oak, and wild pear. Generally speaking, diversity is the safest choice," he says.
The study can contribute useful knowledge to the implementation of the green tripartite agreement. An agreement that, among other things, involves replacing Danish agricultural land with more forest.
"Taking a long-term view is absolutely essential when it comes to planting trees, both as part of forestry operations and from a biodiversity perspective. When planting new trees, we need to factor in our future climate," he says.
He emphasises that it is important to look at what is being done elsewhere in the world.
"In Austria, they are now planting trees such as Turkish hazel – which conventionally is only considered native further south, notably the Balkans – because the resident native trees that are there are suffering drought stress."
Population loss for thousands of species
The research team has analysed the current distribution of more than 32,000 tree species worldwide and assessed how they will much they will be exposed to future climates markedly outside the climates they currently thrive under.
"Under a realistic climate scenario, 69% of species in at least 10% of their current geographic range are predicted to be exposed to climatic conditions that differ significantly from current conditions," says Jens-Christian Svenning.
In these areas, there is a high risk of tree species extinction.
"We're already seeing this in Germany, for example, where Norway spruce is dying as a result of heat and drought stressing the trees. This makes them vulnerable to diseases and attacks by pests," says Jens-Christian Svenning.
Fortunately, according to the researchers, there will still be areas left for each individual tree species which the climate should remain suitable. These areas can be considered as climate refugia and can become crucial for the survival of each individual tree species.
Widespread risk of forest death
But even though a majority of tree species likely will be able to survive in such refugia, from a forest ecosystem perspective the news is less positive.
"The study also points to dramatic consequences for large areas of northern forest (for example, taiga) as well as key tropical forests such as the Amazon. Here, large proportions of tree species in many areas will be exposed to unprecedented high heat with a substantial risk of these forests collapsing. Such collapses will not only affect biodiversity, but also accelerate climate change due to the release of large amounts of stored carbon," says Jens-Christian Svenning.
In the past, biodiversity strategies have focused on expanding the number of protected natural sites. But according to Jens-Christian Svenning, it is clearly not enough if the climate in these areas changes so that the species can no longer survive.
"For forests on the edge of a climate zone, the change can be catastrophic for the majority of the trees, for example if the climate changes from a Mediterranean climate to a desert zone, as is currently happening in parts of southern Europe. Therefore, we must think long-term and in terms of assisted migration of tree species – otherwise we risk major species losses and ecosystem collapses. And at the same time, we must protect the forests that are not exposed to as severe climate pressure."
Unfortunately, we need to act quickly, adds Jens-Christian Svenning.
"In southern Europe, for example, wildfires are becoming increasingly frequent and severe. Such fires can quickly ravage trees in large areas, and recovery may be difficult due to shifts towards more stressful climates."
Coline C. F. Boonman – now a docent at Wageningen University in the Netherlands, who was responsible for the analyses, adds:
"Our research maps 'exposure hotspots' – i.e. areas where the highest proportion of the local tree species will be most exposed to significantly different climatic conditions. At the same time, we identify areas where relatively few tree species will be exposed to novel climatic conditions. These areas can serve as climate refugia for the world's tree species in a rapidly changing world. It's important that we protect these areas from destruction from deforestation and logging, so that they can become true refugia for the endangered tree species," says Coline C.F. Boonman.
