If caterpillars have munched through a lot of an oak tree’s leaves one year, then, the following spring, the tree’s buds open three days later. This delay means the caterpillars don’t have food available when they hatch, and so many die, halving how many leaves get eaten.

In spring, longer, warmer days drive trees to start growing again, opening buds and unfurling young leaves. Many species time their life cycle to match this, so some caterpillars, for example, hatch when the leaves are new and soft, so they can start eating immediately.

Now, at the University of Würzburg in Germany and his colleagues have discovered that oak trees have a way to fight back. They analysed the condition of tree canopies in images from radar satellites for a 2400-square-kilometre area in the northern Bavaria region of Germany between 2017 and 2021.

The forests there are dominated by two species of oak: the pedunculate or English oak (Quercus robur) and the sessile oak (Quercus petraea). Each pixel in the satellite images showed an area of 10 by 10 metres – about the size of the crown of one tree – and the team looked at 27,500 pixels in total.

In 2019, there was a massive outbreak of gypsy moths (Lymantria dispar), the hairy caterpillars of which feed on tree leaves, causing extensive damage when they are plentiful.

The satellite data showed which trees were stripped of leaves and how they responded. If an oak tree was heavily infested by caterpillars, the following spring, its leaves would emerge three days later than those of trees that hadn’t been badly eaten.

Free newsletter

Sign up to The Earth Edition

Sign up to newsletter

This delay slashed the damage caused by feeding on the tree by 55 per cent compared with the year before. This is because the caterpillars still hatch at the same time, but they emerge to a bare cupboard rather than a feast of young leaves, leading many of them to die, says Mallick.

Oak trees also have other defences, including making leaves tougher to chew or that may attract other organisms to prey on the caterpillars. “The delay in bud opening seems to be more efficient than all these other defence mechanisms,” says Mallick, who thinks other deciduous plants may do it, too.

“It’s very plausible,” says at the University of Alberta in Canada, but he says the delay in bud emergence after the caterpillar outbreak is a correlation, and evidence of causality isn’t yet there. The delay could be caused by decreased plant vigour as a result of the leaf loss, he says, but having data from more than one outbreak would help work out what’s going on. “It certainly deserves more research.”

Mallick says the delay could be explained by physiological constraints such as resource depletion, but because it was seen across dozens of tree populations and was strongest in forests where a delay most effectively reduced herbivory, he thinks it isn’t just a physiological response by individual trees, but an adaptation.

“The mechanisms are intriguing and are a key aspect requiring further research,” says at the University of Eastern Finland.

Forests sometimes turn green later in spring than computer models predict they will based on temperatures, , and this study explains why, says Mallick.

“This point that plants respond to much more than climate change is very important,” says Cahill.

Ash trees in the UK are rapidly evolving resistance in response to ash dieback disease, DNA sequencing of hundreds of trees has shown.

The finding is good news, says at the Royal Botanic Gardens, Kew, in the UK, but it is unlikely that ash trees will become completely resistant in the near future. “We probably need a breeding programme so that we can help nature along and finish the job,” he says.

Ash dieback is caused by a fungus (Hymenoscyphus fraxineus) native to Asia that slowly destroys trees’ ability to transport water. It began spreading in Europe in the 1990s and reached the UK in 2012.

The death of ash trees leads to the release of carbon dioxide and affects hundreds of species that rely on these trees for their habitat. Falling trees are also a threat to people and property. “There’s a lot of ash close to footpaths and roads that is now quite dangerous,” says Buggs.

Because the fungus takes much longer to kill large trees than young ones, Buggs’s team was able to compare the genomes of 128 adult European ash trees (Fraxinus excelsior) and 458 saplings at a site called Marden Park in Surrey. This revealed that thousands of variants his team had previously shown to be linked to resistance were more common in the young trees – probably because those that lacked them had died off.

Free newsletter

Sign up to The Earth Edition

Sign up to newsletter

It is the most detailed genetic picture of evolution in action ever obtained in the wild. “What’s original about this study is we’ve been able to characterise the genetic basis and then demonstrate a shift happening in a single generation,” says Buggs.

However, each of the gene variants has only a tiny effect, rather than conferring complete resistance. The rate of evolutionary change will also slow in the future as large ash trees die off and fewer fungal spores are produced, meaning young ash trees will have a better chance of surviving, says Buggs.

“It’s a massive problem, but they’re not going to disappear,” he says. “I think our results encourage us that some of these young ash trees will hopefully make it through to adulthood, and hopefully have another generation of natural selection.”

Ash dieback hasn’t yet spread to North America, but an introduced insect pest, the emerald ash borer (Agrilus planipennis), is spreading and killing ash trees there. It isn’t clear what will happen if ash dieback and the emerald ash borer both arrive in the same region, but it could make the situation much worse.

“Globalisation is mixing up the world’s insects and microbes, and so we are increasingly seeing these new tree epidemics, and it is very hard for the trees to keep up with it,” says Buggs. “Trees are facing threats that they’ve never faced before, coming at them at speeds that they never have before.”

He thinks we need to step in to help trees survive the onslaught, for instance by crossing native trees with exotic species to create resistant hybrids.

“One of the answers is to be moving the genetic diversity of trees around the world as well, to keep up with all of the pests and pathogens that we’re moving around,” he says.

It would be nearly impossible to plant enough trees to compensate for the climate impact of burning through the world’s fossil fuel reserves. Offsetting the estimated 182 billion tonnes of carbon held in the reserves of the world’s largest fossil fuel companies would require covering more land with trees than the entirety of North and Central America.

The analysis, by at ESSEC Business School in France and his colleagues, assessed the cost and viability of offsetting the carbon emissions from burning the oil, gas and coal reserves held by the largest 200 fossil fuel firms around the world.

The researchers found that an area greater than 24.75 million square kilometres would have to be planted with new trees to offset the impact of burning these reserves, swallowing the equivalent of the entire landmass of North America, Central America and parts of South America combined.

This would be impractical to achieve in reality, requiring the displacement of settlements, farmland and other existing natural habitats.

“There simply isn’t enough land available for the level of afforestation that would be needed to offset fossil fuel-related emissions,” says at UK energy analysts Carbon Tracker. “Pursuing anywhere near that level of afforestation risks increasing food prices – if farmland is diverted to afforestation – or indirectly causing deforestation elsewhere to meet global food demand.”

Meanwhile, the financial cost of such a mass-scale afforestation scheme would be ruinous. The cost of tree planting is around $16 per tonne of CO₂ equivalent that is offset. At this price, using trees to offset emissions from fossil fuel reserves would eliminate the entire market value of 64 per cent of the largest fossil fuel companies, the team found. This doesn’t include the cost of acquiring the land.

Free newsletter

Sign up to The Earth Edition

Sign up to newsletter

If a higher carbon price is used, in order to account for the negative social and economic consequences of fossil fuel combustion, the findings suggest all of the companies would essentially become bankrupt.

Naef and his colleagues acknowledge there is little chance of fossil fuel companies voluntarily offsetting the emissions of their reserves. Instead, they say the work is a thought experiment to underscore that offsetting cannot be used to allow polluting industries to continue business-as-usual operations. “Our key message from this paper is that this oil and gas should remain in the ground,” Naef told a press briefing on 18 June.

at Trillion Trees, a UK-based conservation group, agrees. “Planting trees is not a substitute for rapid phase-out of fossil fuels and must not replace action to decarbonise our economies,” he says.

An average mature oak tree grows hundreds of thousands of leaves each year. When those leaves fall, their nutrients return to the soil to nourish the tree that grew them, as well as other living things in the forest. “They essentially eat themselves every year,” says artist Clare Hewitt. “There are these little symbiotic relationships happening between all of the forest.”

Hewitt produced these oak leaf prints over five years of regular visits to a group of trees tucked within the Birmingham Institute of Forest Research, UK. In 2019, she started spending time there after reading about an epidemic of loneliness in rural areas, suspecting the trees may hold lessons about connection and sharing resources. After enough time spent with them, “you come to know them as you would know a friend,” says Hewitt.

She wasn’t allowed to remove anything from the ecosystem, so the forest became her studio: each fallen leaf was placed on expired photo paper, then exposed to sunlight, before being returned to the oak grove. “A lot of the photographic process is a scientific process,” says Hewitt. “It’s really about time and light.”

The prints are part of a larger exhibition of Hewett’s tree art titled , at the Impressions Gallery, Bradford, UK, until 23 August, where they appear alongside other work from the grove, including a hand-crafted, biodegradable book made with mushroom-based paper and plant-based inks.