At the top of an ancient oak at Knepp estate in West Sussex, a white stork has made a scruffy nest. The birds made headlines in 2020 when, after an absence of centuries in the UK, the first chick hatched. Alongside bison, beavers and white-tailed eagles, the storks are one of many species reintroduced to Britain in recent decades in an effort to restore animals to ecosystems where they had been wiped out. The oak tree, by contrast, has been here continuously for 12,000 years.
But ecologist Charlie Gardner is worried one of them may not have a future here – and it’s the oak. By 2050, London’s weather could resemble that of Barcelona, with long stretches of summer drought. These ancient trees were not designed to thrive in such conditions. “More and more individual trees will die and reproductive success will fall,” says Gardner. Around the world, millions of creatures facing unprecedented temperatures and habitat loss are on the move. The climate crisis is causing a vast array of species – from algae to butterflies, woodlice to birds – to shift northwards. Species are travelling north at a median rate of 17km a decade, according to 2011 research. That average equates to 20cm an hour – two to three times faster than previous estimates.
Some creatures move faster – the comma butterfly, for example, has been travelling north by about 11km a year. But trees are at the other end of the scale. We do not tend to think of them as migratory: unlike insects, birds and mammals, they are slow-growing and rooted to the ground. But forests, too, make slow moves over the course of generations and centuries, as saplings seeded in temperate climates succeed and those in harsher conditions fail. Their problem now is a matter of speed: many trees planted today will not reach maturity for 100 years and the changes caused by the climate emergency are too quick for trees to adjust. Faced with this problem, Gardner is one of a growing number of ecologists and scientists proposing a radical, controversial solution: we help the trees on their march.
“If the assumption is that things will stay the way they are – they won’t,” says Gardner. “The lesson from climate change is that the future will not be like the past.”
Impacts on forest ecology
The trees’ heat problem is already pressing. In the drought of 2022, the Royal Botanic Gardens at Kew lost more than 400 trees. In a normal year it loses 30. English oak, common 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 heats, young, newly established trees in southern England will be among the first affected, with new growth falling and mortality increasing during more frequent and intense summer droughts.
In the US, northern red oaks and balsam fir are showing clear signs of moving northwards, with significant impacts on forest ecology. The north-east of the country could soon be too hot for sugar maple trees, where for centuries the Indigenous Abenaki people considered their sap a gift from their creator.
When it gets too hot, trees start to sicken: signs of stress include canopy dieback, reduced growth, leaf discoloration, and in some cases, death. Models show under a worst-case trajectory of about 4C of heating by 2100, downy birch, ash, oak, and elm and could be vulnerable across multiple regions of Britain in the 2080s. This data comes with high levels of uncertainty – different climate projections tell us different things about how trees will respond. “We can’t be certain about any one species,” says Andrew Stringer, head of environment and forestry planning at Forestry England.
Conservationists divided
In the west Sussex woodland, songs of blackbirds and chiffchaffs can be heard from the trees. In the future, they could be joined by the chants of Mediterranean cicadas and crickets. Gardner is among the ecologists who believe we must make English woodlands more robust to heatwaves and drought by introducing southern species.
This practice is called “assisted migration” or assisted colonisation – and it is divisive. Conservationists have long argued against introducing non-native species. To do so is to meddle with nature, they say – and risk accidentally bringing in invasive species which can do vast damage to native environments.
But others say those arguments fail to deal with speed or size of change caused by the heating climate.
The UK government’s environment watchdog, Natural England, has created a taskforce to look at how assisted migration could work. “It’s the first time we’ve been able to talk openly about moving things,” says Sarah Dalrymple, a conservation ecologist from Liverpool John Moores University, who is part of the taskforce. “Earlier in my career the narrative was all about restoring past baselines, and then, as I got more independent in my research, I realised that’s impossible – we can’t do that any more. We are changing the climate so much, we have to be a little more inventive.”
Some scientists advocate moving individual species hundreds of kilometres north, within the country and from abroad. Kew has suggested its most climate-vulnerable species could be replaced by Iberian alder (from Portugal and Spain), Montezuma pine (from Central America) and spoon oak (from Mexico). Southern England could be home to hard-leaved shrubs and small trees typical of dry landscapes in southern France, which cope better with wildfires, as well as southern European oaks which might deal better with extreme heat than those native to the UK.
Others say entire ecosystems should be translocated, moving multiple species at once. James Bullock from the UK Centre for Ecology and Hydrology says a variety of “pilot” Mediterranean-style woodlands or grasslands could be created in southern Britain, then be implemented more widely in the coming decades.
“We may want to try a variety of ecosystems and see which ones are responding well as the climate changes. It’s an experiment,” he says. The big challenge is that we don’t know what the future holds. A habitat that thrives in southern Britain in 2050, by 2070 could no longer be appropriate. “The conditions will carry on changing,” says Bullock. “We’ve got to start addressing these issues and think about them carefully, rather than just saying ‘it’s a bit dangerous’, ‘it’s a bit worrying’, and pushing it to one side.”
According to a report by the Forestry Commission, the changing climate is a significant issue for forestry globally: “To do nothing,” say the researchers, “is not a sensible option.”
Stringer, of Forestry England, believes predictions of widespread tree sickness are “pessimistic” – but says it is “an excellent idea” to increase the diversity of species within a forest.
Commercial foresters are already preparing for change. At a site in Kent called Pleasant Forest, they are bringing in small-leaved lime and hornbeam seeds from parent trees from France, and a Mediterranean species of alder from Italy. At other sites they are hoping to get sessile oak seeds from France, because they will be used to hotter, drier conditions.
Native species translocations from Europe are relatively safe, says Stringer, because these species co-evolved together. Many northern European species would have naturally colonised the UK if not for the Channel. Species from further away are more likely to become invasive and cause damage because they have evolved in a different ecological niche.
Unintended consequences
Many conservationists have concerns about unintended consequences of assisted migration – and its irreversibility. “Wholesale moving of ecosystems can’t be undone, so should be very much a last resort,” says Andrew Allen from the Woodland Trust. “Our woodland wildlife often depends on native trees: for example well over 300 species are completely dependent on our native oak trees,” he adds. Allen believes we should prioritise helping native trees adapt to changes in the climate by encouraging natural regeneration, expanding and connecting existing woodlands, and supporting landowners to improve the health of their woodlands.
“Climate change will see the makeup of some woods change over time. That doesn’t mean we should just sub in species from elsewhere in the world,” he says.
The idea of assisted migration was first floated in the 1980s but wasn’t properly debated until two decades later. Due to a lack of research in this area and a longstanding aversion to moving species around, outcomes of assisted migration are still largely unknown. There are few reliable case studies, especially for the more extreme proposals of shifting ecosystems wholesale.
“When we have got examples of assisted migration, it’s often with species we’re working with which are absolutely on the brink of extinction and have run out of other options,” Darymple says. In 2016, a captive-bred swamp tortoise from Western Australia was moved 200 miles south to wetlands where they had never lived 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 is quite a bit of inertia in conservation. In some ways we’re well named – we’re conservative as a community with a little ‘c’,” Darymple says. She wants to see assisted migration explored earlier, working with species before their populations become so small they become unhealthy, and putting more research into what may work.
“You can never get rid of all the risks, but there is also risk in inaction. We have to balance the risk of doing things against losing these species from the ecosystem,” she says. “The risk of inaction is increasing every day.”
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• This article was amended on 19 August 2024. An earlier version located the Knepp estate in East Sussex, whereas it is in West Sussex and a reference to the north-west of the US and the Abenaki people, should have been to the north-east region.
