Battenberg cake is one of my favourite cakes. But when I tell people that one of the things I like most about this cake is its E-number, they tend to look at me strangely.

It contains the raising agent E541, acidic sodium aluminium phosphate, which is one of the most restricted food additives in the UK. This is due to health concerns about human exposure to aluminium.

According to the UK government regulations, E541 is permitted in almost nothing except “sponge cakes produced from contrasting coloured segments held together by jam or spreading jelly and encased by a flavoured sugar paste”.

The reason for this curious exception is that Battenberg manufacturers managed to persuade the authorities that the additive was essential for achieving the cake’s chequered pattern, and that aluminium exposure from such a niche product wasn’t a health risk.

This level of specificity in the regulations surprises most people, who assume that additives are loosely regulated, casually thrown into food by manufacturers cutting corners. It’s part of a massive counter-movement against additives, driven by fears over ultra-processed foods and a desire for “clean” labels. Though these concerns are understandable, my view of the scientific evidence is that the reality is a little different.

Additives, of course, are compounds used to enhance foods in different ways, but mainly they are about control. When you cook in your home kitchen, the results can be very variable – which is often part of the fun. No-one expects two homemade cakes to look or taste the same, while breads out of your kitchen oven can be very different if you make them from different batches of flour.

For commercial foods, it’s a different story: industrial production relies on predictability, while consumers expect consistency. Few of us would be happy if supermarket breads, biscuits or burgers varied from batch to batch. So commercial foods are usually standardised — and additives help to achieve that.

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Welcome to our series exploring the cutting edge of food science. From the latest advances in meat alternatives to weird and wonderful new additives, science is transforming what we eat like never before. This series will bring you up to speed on all the latest and give you plenty of, er, food for thought.

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Their number one role is preservation: extending shelf life, making it possible to produce foods further away, and often making them more affordable in the process.

Propionic acid (E280) stops bread from going stale and mouldy. Antioxidants like vitamin C (E300, aka ascorbic acid) and vitamin E (E306/tocopherol) prevent fats from oxidising and going rancid. Emulsifiers stabilise products such as mayonnaise which would otherwise quickly separate.

Creating a specific texture and flavour is also very important. Thickeners improve the creaminess of yoghurts and some cheeses. Emulsifiers also help to make ice-cream smooth and chocolate soft. Citric acid (E330) can help zing up a salad dressing, while also dialling down the earthiness in some plant-based milks.

Another use of additives is to overcome the problem of raw products of different quality. Wheat from a wet, cold year can behave very differently from the same variety in a dry, hot year. This can mean differences in protein content and gluten formation during baking, which can affect how much breads rise, for instance.

Ascorbic acid and synthetic fats (mono and diglycerides of fatty acids (E471) can compensate for this and ensure the quality of bread is consistent. Likewise, adding E500 (bicarbonate of soda) and E336 (cream of tartar) – in other words, baking powder – ensures cakes and muffins are light and fluffy.

In the same way, jam depends on the pectin content of fruits, which is affected by factors that can’t be controlled – extra sunlight during growing reduces it, for instance. So manufacturers add pectin (E440) during jam-making to ensure it gels properly.

Or take meat, whose water retention can be improved by adding phosphates like E451. This ensures that natural fluids don’t leak out in the supermarket chiller section (phosphates are also more dubiously used by some meat producers to enable them to add water to cheaper cuts so that they look more attractive to consumers). As for milk products, you can avoid the risk of separation by adding carrageenan (E407), a seaweed gum, which strengthens their protein network.

Indeed, whole product categories would barely exist without certain additives. Many low-fat spreads rely on gelling agents to mimic the texture of butter. Without sweeteners, there would be a much smaller selection of low calorie and low sugar foods.

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Read more: Sweeteners and the quest for the perfect alternative to sugar

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Very little of this is about deceiving consumers — and it should not be. The need to ensure the consistent quality of produce using additives is probably much greater than most people realise.

The remarkably long history of additives

While we’ve only referred to them as additives for about a century, these compounds have been around a great deal longer. Findings from the Qesem Cave in Israel suggest ash was being used to preserve foods as much as 200,000 years ago.

In times when the food supply has been unreliable, meaning most of our history, preserving food was essential to human survival. Entire empires have been built on the trade of another vital preservative, salt, to be used either as brine for pickling or as crystals/powder for drying.

Nitrates (E251) and nitrites (E250) have also been used as additives for many hundreds of years. In Roman times they were used as a constituent of rock salt, both for preserving meat and to keep it nice and pink. With a little more chemical know-how, these compounds are still used today.

Another preservative in use since Roman times is sulphur dioxide (E220), both for wine and foods such as fruits. The Romans obtained it by mining brimstone and heating it into a liquid that could be made into sulphur candles. These could then be burned to produce sulphur dioxide gas in the presence of whatever was being preserved.

The Romans also used additives to make dull, bland food more palatable. Garum — the ubiquitous Roman fish sauce – provided salty and umami flavours by fermenting fish guts. Lead acetates, obtained from boiling grape juice in leaden pots and kettles, were used as a non-calorific sweetener – in wine, for example. It wasn’t until many centuries later that it was proven that this additive was poisonous.

Towards the end of the 18th century, we developed a better understanding of how additives work thanks to advances in food chemistry. It also became possible to extract these compounds in a laboratory, such as lecithin from egg yolk to be used as an emulsifier, pectin from apples, and agar, another gelling agent, from seaweed.

A few decades later, chemists started synthesising additives, which often proved cheaper and easier than obtaining them naturally. Salt of hartshorn – also known as Baker’s ammonia (E503) – could be made by heating ammonium sulphate obtained from soot in combination with limestone (calcium carbonate).

Previously extracted from deer antlers, this was used as a raising agent and preservative for things like digestive biscuits and lebkuchen, a German gingerbread. Much like making aspirin in a laboratory instead of extracting it from willow bark, or digoxin instead of foxglove tea (for heart problems), synthetic and purified additives allowed much better control over their use.

Scientists’ better understanding of chemical and physical mechanisms in the 19th century also allowed new additives to be developed to suit the food industry and consumer demand. These included dyes like aniline purple and martius yellow, both obtained from coal tar, and new flavours like amyl acetate, a byproduct from fermenting alcohol, which gives a sense of pear or banana. These were unveiled at the Great Exhibition in London in 1851 for the first time.

Other additives allowed the use of cheaper ingredients or extended shelf lives. Borax, also known as sodium borate, was harvested from lake beds starting in the late 19th century and added to sub-standard milk and butter to mask sour tastes and kill bacteria.

Additives also began to allow the development of new products that could be used as cheap alternatives, such as margarine. Though originally made from beef fat and skimmed milk in the 1870s, it became vegetarian around the turn of the century after chemists worked out how to make food oils solidify by forcing hydrogen into them at high temperatures (known as hydrogenation).

Margarine solved one problem – the affordability of butter – but food scientists realised a few years later that it created another: it lacks the vitamins naturally found in dairy. So from the 1920s onwards, manufacturers also started fortifying it with vitamins, as well as making it more attractive by incorporating emulsifiers, flavours and colours.

Elsewhere, the use of additives has put consumers at risk, often due to unscrupulous characters. They made bread whiter with chalk, for instance, at a time when many people were already suffering from malnutrition. They sweetened wine with lead salt long after it was known to be toxic. There have even been cases in more recent times of wine being sweetened with anti-freeze.

One other notorious case was the Bradford sweets poisoning of 1858. At the time, sugar was very expensive due to high taxes, so confectionery manufacturers would mix it with plaster of Paris to keep costs down. In the Bradford case, an apprentice muddled up plaster of Paris and arsenic, which led to around 20 people being killed from eating toxic peppermint lozenges.

The modern era

The 20th century brought massive changes to the food system: a rapidly growing population needed to be fed, scientific advances provided a better understanding of food processing, and the rise of synthetic chemistry made hundreds of new compounds available. The upheaval of two world wars accelerated these processes as new products such as canned meats, sliced cheeses and even packets of crisps were all made possible by additives.

The same goes for British sliced bread. The Chorleywood process, named after the village near London where it was invented in 1961, revolutionised production of the daily loaf. By adding lard, ascorbic acid and extra yeast to dough, then mixing it at high speeds, there was no more need to wait hours for the mixture to rise.

This allowed the mass production of bread, which was extremely helpful in meeting ever growing demand. It also allowed the use of British-grown flour, which often has a lower protein content, meaning its dough traditionally didn’t rise as well. In subsequent years, other additives have also become common ingredients in this mass-produced bread, including E471 (mono- and diglycerides of fatty acids) as emulsifiers to improve dough handling and E282 (calcium propionate) as a preservative to enhance shelf life.

The post-war decades saw dramatic societal changes: rising incomes, longer working hours, and more women entering the workforce. These all created demand for foods that saved time and preparation. Ready meals, packet soups, instant desserts and salad dressings all reached supermarket shelves for the first time. This wouldn’t have been possible without emulsifiers, stabilisers, gelling agents and preservatives – even if such details were quietly hidden on labels that often sought to make these products look as traditional as possible.

At the same time, synthetic coal-tar dyes made sweets vivid and soft drinks luminous. Artificial flavours like strawberry, banana, pineapple, sometimes not tasting remotely natural, could be added to almost anything at negligible cost. And artificial sweeteners made it possible to indulge without sin … or at least calories.

For several decades between the 1950s and 1970s, all these new products were seen as unambiguous progress. At a time when science was enabling everything from tower blocks to rocket ships to the pill, it made perfect sense that it was doing the same for our eating habits.

The backlash

The enthusiasm for this modern food was short lived. In 1973, Californian allergist Benjamin Feingold suggested that additives – in particular artificial dyes and flavourings – were responsible for hyperactivity in children. His book, Why Your Child is Hyperactive, became a bestseller. The so-called Feingold diet became popular among parents, eliminating artificial colours, flavours and other additives.

In Britain, Maurice Hanssen’s E for Additives (1984) was another major blow to this industry when it became a bestseller in the 1980s. It detailed the purpose and health risks of all the E-numbers, which were devised as a European mark of safety, so that only additives considered safe were allocated one. Now they became culturally inverted into a warning sign.

The case against additives wasn’t quite what many began to believe, however. There was little proof that Feingold’s diet could reduce hyperactivity, even after extensive research. This may have been because of the strong genetic component in the condition.

In 2007 the UK’s Food Standards Agency commissioned a research study to investigate the effect of some artificial food colourings on hyperactivity. Children in the study showed some increase in hyperactivity from consuming the compounds in question – tartrazine (E102), sunset yellow (E110), quinoline yellow (E104), carmoisine (E122), ponceau 4R (E124) and allura red (E129).

Yet the European Food Safety Authority decided there was sufficient uncertainty that there was no need to change the permitted use of these additives. Instead European lawmakers mandated that foods containing them carry the warning “may have an adverse effect on activity and attention in children”, which applies today in the UK and across the continent.

In parallel, the additives industry has tried to adapt to consumers’ concerns about the things that go into foods. For the past several decades, we have seen a plethora of products which claim to be free from “artificial colours and flavours” – and sometimes even “free from chemicals” (although that is impossible).

Much of this “clean-label” movement has been about replacing E-numbers with ingredients that sound more natural, though in many cases the manufacturers just replace additives with plant extracts that contain the same compounds.

Acerola extract contains vitamin C, for instance, but sounds better than E300. Seaweed extract sounds much more natural than E407 (carrageenan) and turmeric extract better than E100 (curcumin). Similarly, vegetable salts or celery extract in processed meat replace nitrite (E250). The nitrite in these compounds will still act — turning the meat more pink and killing bacteria – but the consumer is not always aware that this additive is even present.

In the EU and UK, whether something counts as an additive legally depends on what it does, not where it comes from. However, this often depends on interpretation. Acerola cherry extract, for example, is almost always added for its vitamin C content, which acts as an antioxidant. However, it usually appears on the label as a fruit ingredient rather than as ascorbic acid (E300). Clean labelling probably employs as many lawyers as food scientists.

One example of how this plays out is Naked Bacon, made by Northern Irish company Finnebrogue. The goal was to remove nitrite from bacon to make it healthier – not just create a nicer label. As an alternative to nitrite and nitrate, they used a proprietary fruit and spice extract called Natpre made by Spanish manufacturer Prosur. This was initially marketed as free from E-numbers. The Food Standards Agency, however, took the view that the ascorbic acid (vitamin C) in the mix was an additive that had to be declared. After a lengthy dispute the company agreed in 2018 to list “antioxidant: ascorbic acid” on the label. The remainder of the extract is still declared as “natural flavouring”.

As well as these kinds of uncertainties, there’s also a potential problem with plant extracts as alternatives to existing additives: a rigorously tested compound is being replaced by something less understood, and still potentially harmful, in the name of a cleaner label. They can contain natural toxins, for instance, while the levels of the additive can vary by batch.

The case for additives

Another part of the stigma against additives is that they enable industrially produced food, which has come to be seen by many as inherently unhealthy. This was abetted by the likes of US journalist Michael Pollan’s bestseller, In Defense of Food (2008), the documentary Food Inc (2008) and TV chefs like Jamie Oliver.

This mindset probably partly derives from all those unsavoury practices I mentioned in the early food industry, where chemicals were used to disguise spoilage, extend products or improve appearance.

Wrapped up in this is a romanticism of natural food, viewing chemical intervention with suspicion. Many people still see natural as better and healthier, though there is actually little evidence for this claim.

Whether industrially produced food is unhealthy depends very much on what is in the food. Foods high in fat, salt and sugar increase the risk of chronic disease, whether it is a home-baked cake or an industrially manufactured cookie. Sure enough, a recent study found no difference in health outcomes between processed and non-processed foods that meet current dietary guidelines.

With today’s focus on ultra-processed foods, it is true that certain industrial additives like emulsifiers and artificial colours and flavours are sometimes used as evidence that a product falls into this category. But the health impact of these foods is more tied to issues like high calorie-density and low fibre content than the toxicity of individual ingredients, so the additives are rarely the primary culprits.

It’s also worth emphasising that without additives, natural food can spoil much more quickly. Almost 20% of foods are lost to spoilage in retail, and more than 30 million years of healthy lives are lost every year to food-borne diseases. Without additives, this situation would be much worse. And at a time when the cost of living is such a burden for so many people, it’s hard to deny that additives make food much more affordable.

Regulatory issues

Unlike with pharmaceuticals, there is no formal system for checking whether additives are causing harm after they start being used in food manufacturing. Regulators rely on the scientific literature to keep them up to date on any risks, as well as commissioning research studies if there are reasons for concern.

The situation isn’t helped by the different classification frameworks that exist. A recent example is aspartame. In 2023 the International Agency for Research on Cancer (IARC) classified this sweetener as possibly carcinogenic to humans, causing a huge amount of concern.

Yet at the same time Jecfa, the Joint FAO/WHO Expert Committee on Food Additives, whose pronouncements influence food regulation around the world, reaffirmed aspartame’s accepted daily intake of 40mg per kg of body weight. It concluded that there was no convincing evidence of adverse effects at current consumption levels.

This sounds contradictory, but actually the IARC and Jecfa ask very different questions. The IARC asks about hazard: could this substance possibly cause cancer? Jecfa asks about risk: could this substance possibly cause cancer at the amount people eat? Both can be right. A substance can probably cause cancer, only not at the amounts people generally consume.

The IARC, for instance, classes the Sun as a group 1 carcinogen, meaning it’s known to cause cancer in humans. Yet we still recommend sun exposure so that our bodies produce vitamin D. We manage the risk by reducing exposure — either by looking for shade or using sunscreen. As for aspartame, it’s worth noting that the IARC classification put it in the same category as aloe vera and pickled vegetables.

Emulsifiers are another additive currently receiving a lot of attention. Some studies have shown that they can alter the human gut microbiome and disrupt the intestinal barrier. Whether this causes harm is obviously an important question, but there’s much that isn’t yet clear: we don’t know if these effects are harmful, or whether they’re transient or lasting.

The trouble is that changes in the gut microbiome are difficult to interpret. It can change in response to all sorts of alterations to someone’s diet, and while there is some good data on links between the microbiome and health, we do not yet have a clear framework for interpreting what specific changes mean.

To be clear, change in itself isn’t a negative: fermented foods and prebiotics are recommended especially for their ability to change the gut microbiome. Regulators in the UK and elsewhere are actively investigating how to incorporate microbiome data into safety assessments, but the science is not yet at a stage where firm conclusions can be drawn.

It might still seem appealing to ban emulsifiers in the meantime as a precaution, but that might have consequences that eclipse the intended benefits. For example they could be replaced by natural lecithins from sources like egg yolks and sunflower seeds, but these are much more expensive. The consumer would bear the brunt – and there is no evidence that these alternatives are better.

There is also a question of consumer choice. Provided a food is safe to consume within established guidelines, there is a strong argument that individuals should be free to make their own choices, rather than having them imposed on the basis of uncertain evidence.

Alcohol, for instance, is another product categorised by the IARC as a group 1 carcinogen, and there is overwhelming evidence that drinking it can harm your health in numerous other ways. Yet while there are regular calls for tighter regulation of additives with a much better risk profile, calls for a ban on champagne or claret are much rarer.

This isn’t to say that the need for proportionality should always justify inaction. Regulatory agencies should and do act to protect consumers when the evidence warrants it. In 2007 data emerged that red 2G (E128), used as a colouring in sausages and burgers, was potentially carcinogenic and toxic to people’s genes. This was rightly considered serious enough that it was swiftly withdrawn in Europe and elsewhere.

The future of additives

One current area of rapid development has been plant-based foods, where achieving the texture, appearance and mouthfeel of meat and dairy requires a specific set of additives. For example, to produce plant-free dairy products that melt and taste like the real thing, food scientists add synthetic wheys and caseins made using a technique called precision fermentation. This involves inserting synthetic copies of dairy DNA into yeasts or fungi.

The first large-scale application of precision fermentation was actually in medicine in the early 1980s, producing insulin using genetically modified E. coli. The food industry got onboard in the early 1990s, with chymosin, an enzyme essential for cheesemaking to separate the curds and whey. It became the first major precision-fermentation product, and is now used in many milk-based cheeses instead of traditional rennet from calves’ stomachs, producing more consistent results and also vegetarian products into the bargain.

In the last couple of years, companies are now offering the entire suite of dairy proteins including whey, casein and lactoglobulins made in a fermentor as opposed to a cow. This is enabling products that are bio-identical to dairy rather than just plant-based alternatives, such as Bored Cow milk in the US. The line between additives and food itself is also increasingly blurring as different companies and research groups compete not just to produce individual proteins but entire muscle tissue in fermentation tanks.

Meanwhile, AI is somewhat inevitably being leveraged to help drive additives forward. One promising line of enquiry is discovering compounds that can fool human receptors into thinking they’re consuming salt or sugar that isn’t really there, or mask other tastes such as bitterness.

Also promising is a technique called nano-encapsulation, which raises the prospect of wrapping additives in microscopic biopolymers or bubbles of lipid (fat) to prevent them from spoiling. This could prevent colours from fading on supermarket shelves, or vitamins from being damaged by heat, for instance.

Whatever you think of these developments, food additives will probably be around for as long as humans need to consume food that is not freshly prepared. They are neither inherently good nor inherently bad, just tools that can be used to make food safer, more affordable and longer lasting.

Next time you enjoy a slice of Battenberg cake, indulge me by sparing a thought for E541 — and for the system that decided exactly where it could be eaten.

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For you: more from our Insights series:

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• Kyrgyzstan’s Dordoi Bazaar: a world made from shipping containers and a city within a city

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• Free will: does refusing to believe in it make you a ‘bad’ person? Research is challenging this old idea

To hear about new Insights articles, join the hundreds of thousands of people who value The Conversation’s evidence-based news. Subscribe to our newsletter.

Gunter Kuhnle has received research funding from Mars, Inc., a company engaged in flavanol research and flavanol-related commercial activities. As a previous member of the EFSA ANS panel and the Committee on Toxicity (FSA), he was involved in the evaluation of food additives. He is currently a member of ACNFP. Gunter also had discussions with Finnebrogue ahead of the release of their Naked Bacon product. He has no financial interest in either Finnebrogue or Mars. The views expressed are his own.

High above the rainforest floor, tiny ponds form in the leaves of plants perched on tree branches. Frogs breed in these ponds, alongside insects, microbes and even tiny crustaceans, creating miniature ecosystems suspended high in the canopy. These are the bromeliads – the remarkable plant family that also gave us air plants, the towering 12-metre “Queen of the Andes”, and the pineapple on your terrible pizza.

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Many people think of plants as nice-looking greens. Essential for clean air, yes, but simple organisms. A step change in research is shaking up the way scientists think about plants: they are far more complex and more like us than you might imagine. This blossoming field of science is too delightful to do it justice in one or two stories. This article is part of a series, Plant Curious, exploring scientific studies that challenge the way you view plantlife.

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Bromeliads are one of the great evolutionary success stories of the neotropics, the tropical ecosystems of the Americas. The ancestor of all living bromeliads existed just 20 million years ago, and since then, they have branched into about 3,800 species. To put this into perspective, hydrozoans – a lineage of predatory jellyfish-like animals – took more than 500 million years to evolve about 3,800 species.

Explaining how the remarkable plant biodiversity of the neotropics evolved remains one of the greatest challenges in evolutionary biology. Bromeliads, which are almost entirely unique to this region, provide one of the best natural experiments for investigation. My colleague Elizabeth Forward and I have published a new study that confirms hummingbirds played a crucial role in this plant family’s evolution.

Over the past 20 million years, bromeliads repeatedly evolved new adaptations to the point that some of them are hard to recognise as close cousins.

Many abandoned the forest floor to grow on tree branches as epiphytes (meaning they use their host as a perch and don’t harm it). Others evolved leaf formations that trap rainwater. Some evolved Cam photosynthesis, the same water-saving strategy used by cacti living in the harshest deserts.

Plants that use Cam photosynthesis only absorb carbon dioxide at night, when it is cooler. These adaptations allowed bromeliads to maximise new ecological opportunities at high speed.

However, one idea that dates back to Charles Darwin is perhaps the strongest, and that is the influence of different pollinators.

The hummingbird hypothesis

Most flowering plants, including the earliest bromeliad ancestors, are pollinated by insects such as bees. However, at least half of all bromeliad species today are pollinated by hummingbirds, and some others rely on bats, butterflies or moths.

Scientists have long suspected that the repeated evolutionary shifts between pollinators helped accelerate the formation of new bromeliad species. But exactly when these shifts occurred, and what effect they had on diversity, has remained uncertain.

My colleague and I tested this idea. We compiled pollinator records for more than 400 bromeliad species, and reconstructed the history of how bromeliads switched between pollinators through 20 million years of evolution.

Our analyses support the idea that the earliest bromeliads were pollinated by bees. Following this, there were repeated shifts, primarily to hummingbirds, but also bats and butterflies. Bromeliads experimented with different pollinators as they spread into new habitats across the neotropics. Our results reveal a more dynamic evolutionary history than scientists previously suspected.

One pattern stood out. The bromeliads pollinated by hummingbirds split into new species at almost double the normal rate.

New species usually arise when populations become separated and gradually stop exchanging genetic material. Given enough time and isolation, enough genetic differences accumulate that they become unable to reproduce with each other. Pollinators can influence this process because they help shape which populations of plants are genetically connected and can reproduce together.

One possibility is that hummingbirds move pollen around differently to insects as many species move larger distances, which changes how genetic material flows across landscapes. Different pollinators also favour different flower colours, shapes and flowering times. Hummingbirds often prefer long, bright red and tubular flowers with copious nectar rewards, which match their long, slender beaks and hovering feeding behaviour. Over thousands of generations, these small differences build up. In mountainous regions, where many hummingbirds live, and in which valleys and peaks naturally separate populations, these effects may become even stronger.

Like most evolutionary stories, there is unlikely to be a single explanation that explains everything. Other innovations, such as epiphytism, also accelerate diversification rates. However, our study suggests that hummingbirds were of particular importance for bromeliads, helping to accelerate one of the fastest known plant radiations.

An ongoing evolutionary story

This high-speed evolutionary story of bromeliads has not finished. Our reconstruction shows that many shifts between pollinators occurred surprisingly recently. For example, the hummingbird-pollinated Hechtia iltisii, which looks a bit like a cross between a spider plant and an aloe vera. Its closest cousins are bee-pollinated.

In fact, about a quarter of bromeliad species changed their pollinator from that of their most recent ancestor in the last few million years, suggesting that bromeliads are still experimenting today. But they, and hummingbirds, face unprecedented pressures from habitat loss and climate change, threatening not only individual species but the evolutionary interactions that generate biodiversity across the neotropics.

Up to 81% of bromeliads are predicted to be possibly threatened with extinction. And dozens of hummingbird species are considered endangered.

Every pineapple in the supermarket, every air plant on a bathroom windowsill, and every pond-forming bromeliad clinging to a rainforest tree is the result of 20 million years of high-speed evolutionary experimentation. Tiny hummingbirds in particular have helped transform a single ancestral plant population into one of the richest bouquets of neotropical plant biodiversity.

Jamie Thompson receives funding from The Leverhulme Trust.

If you visit the Royal Botanic Gardens at Kew, on the edge of London, you will see a brightly painted skeleton of a dead oak tree. The tree, known as the climate changed oak, succumbed in the heatwave of 2022. Instead of removing it, Kew left it in place as a reminder that climate change is already taking its toll on Britain’s trees.

The 2022 heatwave killed 400 of Kew’s trees and predictions show that up to 50% of Kew’s existing 11,000 trees could be vulnerable to climate change by 2090.

A similar story is playing out across the UK. Mature trees shedding limbs and branches during intense periods of hot weather, their canopies thinning, and some dying back altogether. Although these changes can appear sudden, they are often the final visible signs of a much longer process of decline that has been unfolding inside the tree.

The problem is not simply these high temperatures, but that the combination of both extreme heat and drought, increasingly referred to as a “hot drought”, pushes trees beyond the limits of what they can cope with.

Trees struggle to keep cool

To understand why heatwaves are so damaging to trees, one needs to appreciate how trees keep themselves cool. When water is absorbed by the roots, this travels through the xylem (the network of microscopic tubes that carries water from the roots to the leaves), where it evaporates through stomata (tiny pores on the leaf surface). This cools the leaves in much the same way that sweating cools humans.

This cooling system works well when water is freely available. A clear example can be seen in Fremont cottonwood (Populus fremontii), which is often found close to rivers. This tree used this process to keep its leaves between 2°C and 5°C cooler than the surrounding air during Arizona’s record-breaking heatwave in 2023, even when air temperatures exceeded 48°C.

As air temperatures rise, the atmosphere draws water from leaves more rapidly, in a process plant scientists call the vapour pressure deficit. Concurrently, dry soil makes it harder for roots to replace those losses, placing trees under increasing water stress and limiting their ability to maintain normal physiological functions.

By closing their stomata, trees conserve water. But this in turn restricts carbon dioxide uptake, slowing photosynthesis and reducing sugar production required for growth, repair and for defence against pests and diseases. When stomata are open, this allows for continued cooling, but comes at the cost of unsustainable water loss.

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Read more: Heatwaves: expert tips on redesigning UK homes to cope with hotter temperatures

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Every leaf has a thermal limit beyond which its ability to photosynthetise suffers irreversible damage. Surviving a heatwave therefore depends on maintaining enough evaporative cooling to stay below that threshold, although this safety margin is known to disappear surprisingly quickly. A recent study on poplar trees found that only 72 hours of reduced soil moisture and access to water was enough to disrupt leaf cooling. The leaves that had previously remained cooler than the surrounding air temperature became hotter than the air itself, reaching temperatures damaging to photosynthetic tissue.

Why branches fall

As heatwaves persist, problems can often spread beyond just the leaves. With xylem water under constant tension, when water supplies become limited, tiny air bubbles known as embolisms begin to form within the xylem in a process called cavitation. This results in a blockage impeding the transport of water. Ultimately, if enough xylem vessels become blocked in this manner, the tree experiences hydraulic failure, with water no longer able to reach the canopy tree effectively. This causes branches or entire sections of the crown to die back.

Some trees effectively sacrifice parts of their own canopy, reducing the need for water to keep the main stem alive. This strategy is known as hydraulic segmentation and helps explain why seemingly healthy trees may lose substantial numbers of limbs during prolonged hot weather.

Trees can continue to pay a price long after a heatwave is over, as this extreme weather depletes overall carbohydrate reserves and reduce growth potential. These effects can persist for years, leaving trees less resilient when the next drought arrives. A tree may therefore survive one exceptionally hot summer, only to succumb after another as the stresses accumulate over time. Water-stressed trees also become more vulnerable to insect pests and fungal and bacterial diseases because they have fewer resources available to defend themselves from attack.

Britain’s trees evolved under relatively cool, wet summers, and temperatures above 40°C were unheard of until recently. Many are now being exposed to climatic conditions well outside those under which they evolved. Research following the 2003 European heatwave suggests that vulnerability depends not only on how hot it becomes, but also on how well species can adapt to local conditions. As heatwaves become more frequent, many traditional species may increasingly find themselves outside their climatic comfort zone.

What can help?

Fortunately, there are several practical steps that can help, particularly for younger and more susceptible trees. Occasional deep watering is much more effective than frequent light watering because it encourages roots to grow deeper into the soil, where moisture remains available for longer.

Mulching, creating a thick layer of hardwood chip or compost, reduces soil surface evaporation rates, moderates soil temperatures and, as it breaks down, improves soil structure and water holding capacity. Soils with high concentrations of organic matter can absorb water/rain more effectively, which supports more resilient root networks.

Managing mature woodlands to increase their resilience to future heatwaves will require new strategies. Increasing woodland species diversity, reducing the concentration of trees in drought-prone areas and planting species better suited to future climates should be considered. A greater diversity of species helps resilience to hot weather.

Many of Britain’s mature trees have stood for centuries – these may be the most effective ways of protecting them for the future.

Glyn Barrett does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.

The story of Stonehenge starts around 5,000 years ago. But its famous old stones, some of which were transported all the way from south-west Wales to Wiltshire, are just part of the story.

Close to the beginning of its use, the site was a cemetery, with dozens of people laid to rest. That so many changes have been made to the site since – almost all involving huge commitments of human labour and creativity – is testament to Stonehenge’s significance to countless generations of people.

Its continuing fascination was demonstrated by a 2022 British Museum exhibition that attracted 190,000 visitors to its collection of prehistoric objects from 36 European institutions. Now, that physical exhibition has been brought to virtual life in a new collaboration between researchers at the University of Reading, including myself, and the museum.

Users of The Virtual World of Stonehenge – released to coincide with the summer solstice – can go deep inside Stonehenge and watch it change through time.

They can closely examine one of the bluestones that was very likely brought to Stonehenge from Wales, or an ancient pounder tool used to shape the sarsen stones that were excavated much closer to the site.

With the help of English Heritage, which manages Stonehenge, we have also reconstructed how this megalithic structure might have looked through different phases of its existence.

Users can also travel further afield to view the evolving landscape of ritual monuments around England – for example, to Neolithic flint mines at Grimes Graves in Norfolk. Here are some more of the ancient places and artefacts you can discover today without leaving your armchair.

The Coneybury pit

The pottery, animal bones and flint tools in this display were all excavated from a pit close to Stonehenge – but they pre-dated the stones by almost a thousand years. It was among the less spectacular assemblages in the 2022 exhibition, yet had a fascinating story to tell.

The pit held the remains of a feast enjoyed by hunter-gatherers and the first farmer communities some 5,900 years ago. Those gathered ate farmed beef and hunted venison.

Chemical analysis of the remains shows the cattle and deer consumed came from different places – and that the meat was prepared in different ways. As a coming together of worldviews, languages, customs and traditions, the remains of this shared meal marked the end of thousands of years of a hunter-gatherer way of life.

The virtual experience enables users to watch an animated video telling the story of this meeting, and explains how archaeologists pieced together its story.

Seahenge

In 1998, a well-preserved timber circle emerged from the shores of Holme-next-the-Sea, on the coast of Norfolk. It was soon dubbed Seahenge owing to its similarity to timber circles that had once stood in the Stonehenge landscape.

The Holme circle was originally built on a saltmarsh, between land and sea, and consisted of 55 large oak posts. A narrow entranceway was positioned exactly to align on the Sun’s path, so that the midsummer rising Sun illuminated the interior of the monument.

From a study of the tree rings, it is known that Seahenge was built in the spring or summer of 2049 BC, at a time when stone tools and weapons were rapidly being replaced by metal as the material of choice for social and economic life – and also for offerings to supernatural forces.

During that period, circles of wood and stone were in decline. Seahenge was constructed near the end of a religious tradition that had lasted for almost a millennium.

In the virtual experience, you can listen to the “Seahenge soundscape” created by the artists Rose Ferraby and Rob St John. Their work added a spine-tingling dimension to the British Museum’s display of these ancient timbers.

A 6,000-year-old leaf

A humble object that captured many people’s imagination during the British Museum exhibition was a single elm leaf. This delicate specimen fell around 6,000 years ago near an early farming settlement on the coast of Lancashire.

Deforestation has played a key role in European history ever since then. Hard-won clearances could be filled by trees in a human lifespan, while woodland continued to provide vital resources to support the farming way of life.

Near this ancient leaf, we displayed a wall holding 89 stone axes – each representing a generation of human lives during the primary period covered by the exhibition (circa 4000-1500 BC). Many would have been used to chop down trees thousands of years ago – clearing land for crops and monuments, as well as providing wood for building.

Folkton chalk ‘drums’

These three carefully-carved chalk treasures accompanied the body of a small child buried 5,000 years ago in Folkton, North Yorkshire. The eyes peering out from above abstract motifs on the largest and smallest of the sculptures might have been created with the fate and protection of a loved and vulnerable child in mind.

This enigmatic group of objects was excavated in the 1860s. The drums, placed carefully along the line of the child’s back, work well as a set: their size is incremental and all three have similar decorative schemes.

The patterns share the same repertoire of concentric circles, lozenges, zigzags, chevrons and parallel lines found elsewhere across Britain and Ireland. But intriguingly, two of the drums have a motif of eyes with eyebrows insinuated in the design – hinting at a daring, possibly dangerous, reference to a person or spirit embodied within.

In The Virtual World of Stonehenge, you can explore these fascinating and intricately carved objects in 3D, along with a similar drum found much more recently at Burton Agnes. This one was placed with three children of different ages buried close together, the two youngest poignantly holding hands.

Duncan Garrow receives funding from UKRI (AHRC).