As 90mph winds whip across the UK and Europe this week, new research shows forests containing diverse tree species are more likely to stay standing than monocultures when hit by extreme weather.
Forests with two or three tree species are on average 35% more resilient to storms than forests with only one species, simulations created by researchers found. The type of trees also matters – forestry plantations are typically made up of fast-growing tall trees such as conifers, but they are more vulnerable to high winds than slower-growing hardwood species such as oak.
Extreme weather events such as Storm Isha – which caused deaths, power cuts and flooding across the UK – are likely to hit more frequently as the climate crisis goes on, because warm air holds more moisture, which in turn fuels storms. The destruction of trees in storms is becoming an increasing problem for foresters – in 2021, winter storms destroyed about 12,000 hectares (30,000 acres) of forest in Britain.
“Monocultures of fast-growing species such as pine, although valuable from an economic point of view, are more susceptible to storm damage,” said lead researcher Dr Julien Barrere from the French National Research Institute for Agriculture, Food and Environment (Inrae). “In a context of increasing storm losses across the continent, our study therefore argues for forest management practices that promote diversity and slow-growing tree species,” he said.
The benefits of more tree diversity were even more pronounced under extreme conditions, such as in hot and dry Mediterranean regions and the cold areas of northern Scandinavia, according to the paper, published on Thursday in the journal, Functional Ecology.
Researchers created simulations based on data from more than 90,000 real forest plots in Europe to look at how they resist and recover from extreme weather events. They say fieldwork is still needed to back up the findings. Barrere said: “These are scientific conclusions, not practical recommendations.”
However, their simulations support observations from conservationists in November 2021 when millions of trees were brought down by Storm Arwen as 100mph winds flattened conifer plantations. Experts at the time said many trees that were knocked down were single-species, single-age conifer plantations, which are more vulnerable to storms than native, mixed woodland.
“The findings are exactly what one would expect,” said Prof Martin Lukac from the University of Reading, who was not involved in the research. “Trees growing in a mixture not only produce more biomass, but the forests they create are better able to ride out the impact of adverse factors, including wind storms.
“We need to plant trees in mixtures – same as the old saying about not putting all eggs into the same basket. Most natural forests grow as mixtures, clearly able to tolerate disturbance better than monocultures.”
Conservationists and foresters are working out how they can reduce the damage when the next storm hits. The National Trust is looking towards more natural tree management. Ancient oaks, for example, lower their branches as they age to increase their stability, and conservationists hope to leave these limbs where possible, rather than cutting them back. Storms can also increase forest diversity by increasing the amount of wildlife-rich dead wood and creating more variation within plantations.
“The study seems very timely considering the recent weather in the UK,” said Prof John MacKay from the University of Oxford, who did not take part in the research. “It is important to keep in mind that diversifying the makeup of forests can be acted on now but will take many years and even decades to produce benefits given the time it takes for trees to establish and develop,” he said. “It appears crucial to act in the short term for their future.”
Previous research has shown that species diversity has other benefits including more resistance to increased pests and diseases, as well as drought. At the Royal Botanic Gardens in Kew in London, the head of tree collections, Kevin Martin, says he wants to plant species that can cope with conditions projected for the coming decades and beyond.
He said Kew’s research had demonstrated the “importance of genetic and species diversity of treescapes for urban forests, not only for building climate resilience but also to ensure pest and disease resilience – another growing threat”.
“It is therefore vital that we utilise findings like these … and plant the right tree in the right place,” Martin said. “Without this, we risk creating landscapes that will fail in a short space of time.”
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