AA white stork has built an uncared-for nest in the top of an ancient oak tree on the Knepp estate in East Sussex. The birds made headlines in 2020 when the first chick hatched after a centuries-long absence from the UK. Along with bison, beavers and sea eagles, the storks are one of many species that have been reintroduced to the UK in recent decades to help reintroduce animals into ecosystems where they had been wiped out. The oak, on the other hand, has stood here continuously for 12,000 years.

But ecologist Charlie Gardner fears one of those species may have no future here – the oak. By 2050, London’s weather could resemble Barcelona’s, with long periods of drought in the summer. These ancient trees are not designed to thrive in such conditions. “More and more individual trees will die and reproductive success will decline,” says Gardner. Millions of creatures around the world are fleeing due to unprecedented temperatures and habitat loss. The climate crisis is causing a wide range of species – from algae to butterflies and woodlice to birds – to move north. According to a 2011 study, species are migrating north at an average speed of 17 kilometers per decade. That’s the equivalent of 20 centimeters per hour – two to three times faster than previous estimates.

Some creatures move faster—the C-wing butterfly, for example, has traveled about 7 miles north each year in recent years. Trees, on the other hand, are at the other end of the scale. We tend not to think of them as migratory: unlike insects, birds and mammals, they grow slowly and are rooted in the ground. But forests also move slowly over generations and centuries, with seedlings planted in temperate climates thriving but dying in harsher conditions. Their problem now is one of speed: many trees planted today will not be mature for 100 years, and the changes caused by climate change are happening too fast for trees to adapt. Faced with this problem, Gardner is one of a growing number of ecologists and scientists proposing a radical but controversial solution: help trees advance.

“If you assume that everything will stay the same, it will not stay that way,” says Gardner. “The lesson of climate change is that the future will not be like the past.”

Impacts on forest ecology

The heat problem for trees is already acute. In the 2022 drought, the Royal Botanic Garden at Kew lost more than 400 trees. In a normal year, it loses 30. Pedunculate oak, European beech, silver birch and holly are particularly vulnerable to warmer temperatures and longer dry spells, and more than half of all tree species at Kew are at risk from the climate crisis, research shows. As the world warms, young, newly established trees in southern England will be the first to be affected, as new growth declines and mortality increases during more frequent and intense summer droughts.

In the United States, there are clear signs that red oak and balsam fir are spreading northward, with significant impacts on forest ecology. In the northwest of the country, it may soon be too hot for sugar maple trees, as the indigenous Abenaki people have considered the tree’s sap a gift from its Creator for centuries.

When it gets too hot, trees start to get sick: signs of stress include crown dieback, reduced growth, leaf discolouration and, in some cases, death. Models show that if the worst warming is around 4°C by 2100, downy birch, ash, oak and elm could be at risk in several regions of the UK by the 2080s. There is a high degree of uncertainty in these dates – different climate forecasts tell us different things about how trees will respond. “We can’t be sure about any species,” says Andrew Stringer, head of environmental and forest planning at Forestry England.

Environmentalists disagree

In the woods of East Sussex, the song of blackbirds and chiffchaffs can be heard from the trees. In the future, they may be joined by the songs of Mediterranean cicadas and crickets. Gardner is among the ecologists who believe that we need to make English forests more resilient to heatwaves and drought by introducing southern species.

This practice is called “assisted migration” or “assisted colonization” – and it is controversial. Environmentalists have long opposed the introduction of non-native species. This is an intervention in nature, they say – and there is a risk of inadvertently introducing invasive species that can cause enormous damage to the native environment.

Others argue that these arguments do not take into account the speed or magnitude of the changes caused by global warming.

The UK’s environmental agency Natural England has set up a taskforce to look into how assisted migration might work. “For the first time, we could talk openly about shifting things,” says Sarah Dalrymple, a conservation ecologist at Liverpool John Moores University who is part of the taskforce. “Earlier in my career, it was all about restoring previous baselines, but as I became more independent in my research, I realised that’s impossible – we can’t do that anymore. We’re changing the climate so much that we need to be a bit more inventive.”

Some scientists argue for moving individual species hundreds of kilometres north, both within the country and from abroad. Kew has proposed replacing the most climate-vulnerable species with Iberian alders (from Portugal and Spain), Montezuma pines (from Central America) and spoon oaks (from Mexico). Southern England could be home to hard-leaved shrubs and small trees typical of dry landscapes in southern France that cope better with wildfires, as well as southern European oaks that could cope better with extreme heat than those native to Britain.

Others say that whole ecosystems need to be relocated, and multiple species need to be moved at the same time. James Bullock of the UK Centre for Ecology and Hydrology says that a series of “pilot” Mediterranean-style forests or grasslands could be created in the south of Britain and then rolled out on a larger scale in the coming decades.

“We should try different ecosystems and see which ones respond well to climate change. It’s an experiment,” he says. The big challenge is that we don’t know what the future holds. A habitat that’s thriving in southern Britain in 2050 might not be appropriate in 2070. “Conditions are going to keep changing,” says Bullock. “We need to start looking at these issues and thinking carefully about them, rather than just saying ‘it’s a bit dangerous’, ‘it’s a bit worrying’ and brushing it aside.”

According to a report by the Forestry Commission, climate change poses a significant problem for forestry worldwide: “Doing nothing,” say the researchers, “is not a sensible option.”

Stringer, of Forestry England, believes predictions of widespread tree diseases are “pessimistic” but says it is “an excellent idea” to increase biodiversity in a forest.

Commercial foresters are already preparing for the change. At a site in Kent called Pleasant Forest, they are bringing in seeds of small-leaved lime and hornbeam from parent trees from France and a Mediterranean alder species from Italy. At other sites, they hope to get seeds from sessile oaks from France, as these are used to hotter, drier conditions.

Translocating native species from Europe is relatively safe, says Stringer, because these species evolved together. Many northern European species would have colonised Britain naturally if the English Channel had not existed. Species from further afield are more likely to become invasive and cause damage because they evolved in a different ecological niche.

Unintended consequences

Many environmentalists are concerned about the unintended consequences of assisted migration – and its irreversibility. “The wholesale shifting of ecosystems cannot be reversed and should therefore only be a very last resort,” says Andrew Allen of the Woodland Trust. “Our woodland animals are often dependent on native trees: for example, well over 300 species are entirely dependent on our native oaks,” he adds. Allen believes we should prioritise helping native trees adapt to climate change by encouraging natural regeneration, extending and connecting existing forests and supporting landowners to improve the health of their forests.

“Climate change will cause the composition of some forests to change over time. But that doesn’t mean we should simply introduce species from other parts of the world,” he says.

The idea of ​​assisted migration was first raised in the 1980s, but was not properly discussed until two decades later. Due to a lack of research in the field and a long-standing aversion to relocating species, the outcomes of assisted migration are still largely unknown. There are few reliable case studies, especially for the more extreme proposals to relocate ecosystems entirely.

“When we have examples of assisted migration, it’s often with species we work with that are absolutely at risk of extinction and have no other options left,” says Darymple. In 2016, a captive-bred pond turtle from Western Australia was moved 200 miles south to wetlands it had never lived in before after experts said it was the only way to ensure the species’ long-term survival. It was believed to be the first time a vertebrate species had been moved to a new habitat due to the climate crisis.

“There’s a certain inertia in conservation. In some ways, our name is justified – we’re a conservative community with a small ‘c,'” Darymple says. She wants to see assisted migration explored earlier, work done with species before their populations get so small they become unhealthy, and more research done to figure out what might work.

“You can never eliminate all risks, but there are risks in doing nothing. We have to weigh the risk of taking action against the loss of these species from the ecosystem,” she says. “The risk of doing nothing is increasing every day.”

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