A stork builds a shaggy nest atop an old oak tree on the Knepp Estate in East Sussex. Storks made headlines in 2020 when they hatched their first chicks in the UK after being absent for centuries. They are one of many species, along with bison, beavers and white-tailed eagles, that have been reintroduced to the UK in recent decades as part of efforts to reintroduce animals to ecosystems where they had been extinct. In contrast, the oak tree has been here for 12,000 years.
But ecologist Charlie Gardner worries that one of them may not survive on the land: the oak. By 2050, London's weather could resemble that of Barcelona, with prolonged summer droughts. These ancient trees are not built to thrive in such an environment. “More and more trees will die, and their reproductive success will decline,” Gardner says. Around the world, millions of organisms are migrating, facing unprecedented increases in temperature and habitat loss. The climate crisis is causing a great many species to move north, from algae to butterflies, woodlice to birds. A 2011 study found that species are moving north at an average speed of 17 km per decade. This average equates to 20 cm per hour, two to three times faster than previous estimates.
A butterfly larva feeds on nettles. The species is migrating north as heatwaves and droughts increase. Photo: blickwinkel/Alamy
Some creatures migrate even faster. The Japanese spiny-butterfly, for example, migrates north about seven miles a year. But trees are at the other end of the spectrum. We don't tend to think of trees as migratory because, unlike insects, birds and mammals, they are slow-growing and rooted in the ground. But forests also migrate slowly, over generations and centuries, as seedlings planted in warmer climates thrive and those in harsher conditions die. The problem for forests now is speed. Many trees planted today won't reach maturity for 100 years, but the changes caused by the climate emergency are too fast for trees to adapt. Faced with this problem, Gardner is part of a growing number of ecologists and scientists proposing a radical, and controversial, solution: we help the trees grow.
“If we assume things are going to stay the way they are, they won't,” Gardner says. “The lesson of climate change is that the future is not going to be the same as the past.”
Impact on forest ecosystems
The heat problem for trees is already imminent: the 2022 drought caused Kew to lose more than 400 trees, up from 30 in a normal year. English oak, beech, European birch and holly are particularly vulnerable to rising temperatures and prolonged dry spells, with research showing that more than half of all Kew's tree species are at risk from the climate crisis. As the world warms, young, newly planted trees in the south of England will be the first to be affected, with more frequent and intense summer droughts causing shoot drop and rising mortality.
The head of tree collections at the Royal Botanic Gardens, Kew, London, holds fallen leaves from a plane tree suffering from drought-induced mortality in 2022. Photograph: Toby Melville/Reuters
In the United States, there are clear signs that northern red oaks and balsam firs are migrating northward, with significant implications for forest ecosystems. The Northwest may soon become too hot for sugar maples, whose sap has for centuries been considered a gift from the Creator by the Native American Abenaki people.
If temperatures get too high, trees start to get sick. Signs of stress include crown dieback, reduced growth, leaf discolouration and, in some cases, death. Models show that a worst-case warming of 4°C by 2100 could damage birch, ash, oak and elm in several parts of the UK in the 2080s. There is a high degree of uncertainty in this data; different climate projections show different ways in which trees might respond. “We can't be sure about any particular species,” says Andrew Stringer, head of environment and woodland planning at Forestry England.
Environmentalists are divided
In East Sussex woodlands, the songs of blackbirds and chiffchaffs ring out from the trees, and in future may be joined by the sounds of Mediterranean cicadas and crickets. Gardner is one of several ecologists who believe that English woodlands must be made more resilient to heatwaves and droughts by introducing species from the south.
This method, known as “assisted relocation” or “assisted colonization,” is controversial. Conservationists have long opposed the introduction of invasive species, arguing that it involves interfering with nature and posing the risk of accidentally introducing invasive species that can cause severe damage to native ecosystems.
But some argue that this argument fails to address the speed and scale of changes caused by a warming climate.
Ecologist Charlie Gardner is concerned that these oak trees on the Knepp Estate in East Sussex may not survive in a warming climate. Photo: Louise Jasper
The UK government's environmental watchdog, Natural England, has set up a taskforce to look at the feasibility of assisted relocation. “This is the first time we've been able to talk openly about moving things around,” 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 historical baselines. But as I became more self-reliant in my research I realised that wasn't possible. You can't do that anymore. We're changing the climate so much. We've got to be a bit more inventive.”
You can't eliminate all risk, but there are risks to doing nothing. Sarah Dalrymple
Some scientists advocate moving individual plant species, both domestic and international, hundreds of kilometres north. The Kew Gardens proposes replacing the most climate-sensitive plant species with Iberian alder (native to Portugal and Spain), Montezuma pine (native to Central America) and moose oak (native to Mexico). Southern England might have the sclerophyllous shrubs and small trees that are typical of the dry lands of southern France, which are more resistant to wildfires, and southern European oaks, which are more tolerant of intense heat than native British oaks.
Some have argued that whole ecosystems should be transplanted, with several species moved at once. James Brock of the UK Centre for Ecology and Hydrology says he could create a number of “trial” species of Mediterranean-style woodland and grassland in southern England that could be introduced more widely over the coming decades.
Gardner points out evidence of damage to the trees on the Knepp property. Photo by Louise Jasper
“We might want to try out different ecosystems and see which ones cope well with a changing climate. It's an experiment,” Block says. A big challenge is that we don't know what the future holds: a habitat that was thriving in southern England in 2050 may no longer be suitable by 2070. “Things are going to keep changing,” Block says. “We need to start addressing these issues and thinking about them carefully, rather than just saying, 'That's a bit risky' or 'That's a bit worrying' and brushing them aside.”
Climate change is a major challenge for forestry globally, and “inaction is not a wise option,” researchers said in a Forestry Commission report.
English Forestry's Stringer thinks predictions about widespread tree disease are “pessimistic”, but says increasing species diversity in woodland is a “great idea”.
Commercial foresters are already preparing for the change. At a place called Pleasant Forest in Kent, they are bringing in seeds of small-leaved lime and hornbeam from logs in France, and seeds of Mediterranean alder from Italy. Elsewhere, they hope to get oak seeds from France, which are accustomed to hotter, drier conditions.
According to Stringer, transplanting native species from Europe is relatively safe because these species have evolved together. Without the Channel, many northern European species would have established themselves in the UK naturally. Species coming from further away are more likely to become invasive and cause damage because they have evolved in different ecological niches.
Richmond Park, London, during the August 2022 drought. Long summer droughts could become the norm by 2050. Photo: Malcolm Park/Alamy
Unexpected Results
Many conservationists are concerned about the unintended consequences of assisted relocation and its irreversibility. “Large-scale ecological shifts should be a last resort because they are irreversible,” says Andrew Allen of the Woodland Trust. “Forest wildlife often depends on native trees; for example, more than 300 species are entirely dependent on native oak trees,” he adds. Allen believes the priority should be helping native trees adapt to a changing climate by promoting natural regeneration, expanding and connecting existing forests, and helping landowners improve forest health.
“Climate change will change the composition of some forests over time, but that doesn't mean we can just introduce species from other parts of the world,” he says.
The idea of assisted relocation was first proposed in the 1980s, but it took nearly 20 years for it to be seriously discussed. Due to a lack of research in the field and a long-standing aversion to moving species, the outcomes of assisted relocation are still poorly understood, and there are few reliable case studies, especially for such extreme proposals to completely transform ecosystems.
Fir trees in eastern France suffering from the drought of 2019. Photo: Frédéric Florin/AFP/Getty Images
“Examples of assisted relocation are often when the species we're working with are so close to extinction that they have no other choice,” Dalrymple says. In 2016, captive bog turtles in Western Australia were moved 200 miles south to wetlands they had never lived in before after experts said it was the only way to ensure the species' long-term survival. It is thought to be the first time a vertebrate species has been moved to a new habitat because of the climate crisis.
“There's a lot of inertia in conservation. In a way, we're worthy of the name Conservation Community – Conservative with a little 'c',” Dalrymple says. She would like to see assisted relocations considered earlier, to reach species before they become low and unhealthy, and to study more about what works.
“You can't eliminate all the risks, but there are risks to doing nothing. We have to balance the risks of taking action against the risks of losing these species from the ecosystem,” she said. “The risks of doing nothing are increasing every day.”
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