The colonisation of Australia transformed much of its landscape into forests with a high density of gum trees, and this led to the country’s cycles of dangerous wildfires, an analysis of 1000 years’ worth of lake sediment reveals.

“The catastrophic fires that we’re dealing with now are the product of colonisation and the product of land-use change, not solely the product of climate change,” says at the University of Melbourne, in Australia.

The combination of increased temperatures and drought with an explosion in the density of eucalypt forests dramatically enhanced the risk of bushfires and drove the 2019-20 “black summer” fires, which burned 24 million hectares, says Fletcher.

He and his colleagues extracted a metre-long core, representing a millennium of sediment deposition, from the bed of Purghagoolah, also known as Lake Elusive, in Croajingolong National Park in south-eastern Australia.

The lake is surrounded by forest and has a catchment area of around 10,000 square kilometres. It is an area where people are largely excluded and nature is unimpeded by active management.

The researchers examined the core at 0.5-centimetre intervals and found that the sediment deposition suddenly changed 200 years ago.

Prior to the arrival of the British, the local Indigenous community, the GunaiKurnai, had managed the landscape, primarily through cultural burning practices, for thousands of years. Pollen falling into the lake was reflective of open woodland vegetation, with a tree cover of around 10 per cent for 800 years, right up until the local colonisation.

But between 1830 and 1860, European settlers massacred and forcibly removed the Indigenous population and cultural burning ceased.

Early settlers to the region reported being able to easily move herds of cattle through the area, which was then much more open woodland.

Shrub species swiftly moved in and, as charcoal evidence demonstrated, bushfires became hotter, which in turn led to thicker regeneration of eucalypt species and tree cover increased to around 40 per cent.

“Critically, at the same time, we could see the first evidence of soil erosion in the entire thousand years,” says Fletcher. “Fire temperature went up, fire amount went up, eucalypt cover went up, and it trapped this system into a hot, catastrophic fire loop.”

Analysis of other sites throughout south-eastern Australia is revealing a similar story, he says, with up to six times as many eucalypts in the modern period as there were in the past.

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When tree density increases, the canopy begins to connect, creating conditions where flames can spread more easily, while shrub growth on the forest floor means fire can “ladder” up to the crown of the forest.

Although climate change will continue to drive wildfires, a return to a regime of widespread cultural burning would help to reduce their intensity and impact, says Fletcher.

He argues that the “cult of the tree” is having devastating consequences in terms of economic loss and national trauma. Also, these much denser forests don’t hold carbon securely because they are so much more prone to igniting.

“It’s a myth that south-east Australian eucalypt forests can store carbon safely for a long period of time managed as they are today,” he says. “It’s pointless actually trying to do so because it’ll go up in smoke – 2019/2020 proved this.”

at the University of Tasmania, Australia, says the study reinforces other research suggesting drastic vegetation changes following the collapse of Indigenous fire management.

“It will energise debates around sustainable fire management, the pivotal role for including Aboriginal fire groups in that journey as well as asking deep questions around the restoration of overgrown, thickened bushland,” says Bowman. “Australians have a lot of research and development to do to get into a safe space with climate change-enhanced wildfires.”

“It’s probably the clearest example yet that pre-invasion landscapes were substantially altered by the removal of Indigenous fire management, and we are now paying the price for it,” says at James Cook University in Townsville, Australia.

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.

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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.