CHAPTER 9

The Growing Dead

Death came to the United States in 1878, in the guise of a cargo of chestnuts from Japan. The shipment included boxes of sweet chestnuts and shoots from beautiful Asian trees. Smaller than its American cousin, the Japanese chestnut tree is cultivated both for its ornamental qualities and for its edible nuts. Samuel B. Parsons Jr, the landscape architect who took delivery of the shipment in Manhattan, began selling saplings to orchards all over the country. Unbeknown to him, he was selling another species together with the trees — the Cryphonectria parasitica fungus.

Japanese chestnuts are resistant to this fungus, which lives under their bark. Since the blight is undetectable from the outside, Parsons’ mistake was easily made — but it was catastrophic for the native American species. In the eastern half of the United States, from Mississippi in the south to Maine in the north, American chestnuts were so abundant that they accounted for a quarter of all deciduous forest trees. There are descriptions of hillsides that appeared to be blanketed in snow when the trees were decked in the white blossom of spring. Chestnuts provided food for everything from squirrels, passenger pigeons, and insects to people. They were considered tastier than the European variety, and were ground into flour for cakes, roasted over open fires, candied, or used in brewing. The timber was used in house building and the bark for tanning leather.

American chestnuts were stately trees that could grow to 30 metres and live for over 100 years, yet they were defenceless against the new blight. After colonising a space between the bark and the wood of the trunk, the parasite exudes an acid that kills the tree’s tissues, allowing the fungus to feed on the dead remains. The dead tissue forms a kind of canker, which spreads over the trunk, blocking the transport of nutrients and water between the roots and the leaves. The effect is the same as killing a tree by ringbarking.

The disease spread like wildfire. Within a mere 50 years, it had killed three billion chestnut trees, almost all the chestnuts in the American forests. A few great, majestic chestnuts remain, but nearly all of them were deliberately planted, and they are a long way from the areas to which American chestnuts were once native. They can be found, for example, in the west-coast states of California and Washington. However, the root systems of some of the affected trees still survive in the great forests of the eastern United States. They are like zombies, constantly reawakening and producing the odd sapling that manages to grow for a few years, until the blight takes hold and kills it again. The parasite can survive in the bark of various other species of trees without harming them, so the eastern forests will never be blight-free.

In practice, this means the American chestnut is an extinct species. There may be survivors, but the forests have lost everything the chestnut trees once provided, from pollen and nectar to insects in spring and the bountiful harvest of sweet chestnuts in autumn. The American landscape changed dramatically when these trees disappeared, and today the forests are full of other species.

Yet there’s reason to hope the American chestnut will make a comeback.

‘In another five years or so, we plan to start planting out blight-resistant chestnuts in the forests again. The only thing left is the legal approval process,’ says William Powell, a scientist at the State University of New York who has spent the last 25 years trying to develop a blight-resistant chestnut tree. Cheerful and enthusiastic, William breaks into laughter from time to time during our conversation. He seems to be genuinely delighted that yet another journalist wants to interview him about his beloved chestnuts.

‘When we started, we thought we’d cover this in five years, and of course it turned into 25. You know — young faculty and over-enthusiasm,’ he jokes. His involvement with chestnut trees began when he was a doctoral student, though it took another few years for this project to get under way. Now he has only ten years to go to his pension. The trees he hopes to plant in the next five years are his life’s work.

For quite some time — since the early 1980s — a project that involves crossing American chestnuts with a blight-resistant Asian species has been in progress. The aim is to produce a hybrid like the American chestnut but resistant to disease. But Asian chestnuts are much smaller than the American variety, so, ever since the first hybrid was created, scientists have been backcrossing various hybrids with American chestnuts, trying to minimise the share of genetic material inherited from Japanese trees, while preserving the resistance trait. This is a laborious task, as many hybrids inherit undesirable genes from the parent trees. Sometimes, for example, crossing produces a small, non-resistant tree — not a big, resistant one.

William’s approach was different. He began by searching for a trait that would enable the chestnut to shield itself against the parasite. When the fungus first takes hold under the bark, it starts producing oxalic acid (the substance that makes wood sorrel and rhubarb taste sour) to kill off the tree’s tissues. William identified a gene in wheat that enables the wheat plant to neutralise acid and thus to resist the fungus that produces it. Wheat has this gene because the plethora of fungal diseases that deploy the acid technique forces many plants to develop defensive mechanisms.

‘This is a very common gene in plants. It’s found in strawberries, bananas, and other things, as well as wheat.’

This is the gene he has spliced into the genome of the chestnut tree, using the same method that scientists apply to create genetically modified crops. Initially, William considered taking genes from the Japanese chestnut, thinking they might be more effective, as the two species are so closely related. However, the Japanese chestnut’s resistance to blight depends on several genes, so it was easier to identify a single gene in a different species that would do the job. In addition to the wheat gene, the team have also spliced in a genetic marker enabling them to check whether the change has really taken effect and had the desired impact. William calls the latest variant of his resistant chestnut ‘Darling 54’.

‘We’ve managed to produce a type of chestnut that’s even more resistant than the Asian variety. People ask me whether I’ve created a new species, but that’s not the way it is. When you cross two different trees and get a hybrid, you’ve got a new species, but this is a much smaller change than you’d get by crossing.’

He shows me a film of an experiment in which three separate groups of plants are infected with fungus at a very early stage in their growth. The leaves of the ordinary American chestnuts roll up, shrivel, turn grey, and fall off. The Asian chestnuts do rather better, though the plants are smaller. Their leaves begin to droop a little and turn pale yellow. Some of the plants look as though they’re dying, while others look almost healthy. In between stands the group marked ‘Darling 54’. These plants have shot up and are covered in luxuriant dark-green foliage, a sign of health that could hardly be clearer. Darling 54 is the best variant so far, and William is clearly proud of the result.

He is just embarking on the legal process that has to be gone through before he and his colleagues can plant out the genetically modified trees in the open. This is the process that GM crops have to undergo in the United States before they can be planted in the fields. It’s a laborious procedure and will take between three and five years, but, once it’s over, the trees can be planted anywhere in the country. Today, the scientists are allowed to grow genetically modified trees for research purposes in a limited number of places. They have to clip the male catkins off these trees or tie bags over them, to stop any pollen escaping into the natural environment.

In the wild, it takes between seven and eight years for the trees to start producing chestnuts, but the team have managed to speed up the process in the laboratory and their plantations. There were not many from this first harvest, and they have all been sent to a laboratory to have their nutrient content analysed.

‘We’re being very, very careful with this — we want to make sure of success,’ William says.

The hope is that the next nuts to be harvested can be planted and produce more seedlings. William is quite sure the team will get the approval they need. There’s nothing to suggest otherwise.

William has tried to examine the role the modified trees would play in a forest. He wants to rule out any unintended consequences. The team has looked into a wide range of issues, such as whether the insects that feed on the flowers are affected in any way, and how the leaves that fall to the ground are broken down into leaf mould.

More than likely, many creatures will benefit from the trees’ flowers and nuts, but William is clear that there will be losers, too. He just doesn’t know which species will be affected. In their experimental plantations, the team have found an unusual beetle that depends on the chestnut tree. Presumably, it will become more common if the trees spread, like other species surviving on the stumps that remain in today’s forests. But more chestnuts will presumably mean fewer oaks, so there’s a risk that oak-dependent species may decline.

Planting out transgenic trees and letting them spread in the open is a sensitive matter. There is strong resistance to genetically modified organisms, and it’s common for activists to sabotage research sites. I ask William whether there has been any criticism of the project from environmentalists or other organisations.

‘Not that much, really. I give a lot of talks each year to explain what we’re doing, and I haven’t come up against that much criticism. The trees we’ve planted on the campus haven’t been damaged either. So I think people can see there are some good uses for genetic engineering.’

Yet it’s hard to say how people will react when William and his fellow scientists start planting out trees in the forests.

The aim is for the transgenic chestnut trees to do precisely what worries so many people about GM crops. The scientists want the new genes to spread to wild trees. William hopes the trees they plant will crossbreed with as many as possible of the specimens that are still just about surviving in the forest. Most of the ‘living dead’ trees in the forest wither away before they can produce nuts or pollen, but now and then it does happen. If they were to crossbreed with the transgenic trees, blight-resistance would spread to their offspring. That would enable the species to recover.

‘There’s still some genetic diversity in the stumps that are left. We want to try and rescue them by getting them to crossbreed with the transgenic trees. Our aim is to make use of the diversity that still exists in the wild, because the transgenic trees are genetically all very similar.’

The gene the scientists have spliced into the chestnut trees is dominant, so a chestnut seedling has only to inherit that gene from its parents for the resultant tree to be blight-resistant.

The blight-resistance William has engineered doesn’t mean the fungus dies. Rather, it can continue to live within the tree, just as with Asian chestnuts. Surely that’s a problem, I say; won’t it just make things worse if the fungus can spread at any time?

‘Not at all,’ William replies. ‘Actually, it means there’s less risk of competition between the fungus and the host trees. If a genetic change in the tree kills off the fungus, there’s huge evolutionary pressure on the fungus to evolve and find a way around the problem. If the fungus is neutralised instead, but it can carry on living, there’s less risk it will find a way to overcome the tree’s resistance.

‘You can add more genes to reduce the risk even further. We’re thinking about a few different options. But in practice, there’s probably very little risk the fungus will become resistant.’

All this means the American chestnut has stolen a march on all other projects to resurrect or recreate extinct species. If the application for a permit is successful, the new chestnuts will be the first revived species to spread again, though it can of course be argued that American chestnuts never really died out completely.

At the moment, William and his fellow scientists are waiting for the regulators to approve the next stage, subject to studies of how the trees affect insects and other organisms in their environment. But his team is already planning to start cultivating the seedlings to be planted out once the permit comes through. The idea is to run the project on a not-for-profit basis. There are no patents on the genetic changes they’ve introduced, and they plan to sell their plants at cost price. Their aim is to have 10,000 plants ready for dispatch as soon as they are given the green light. The idea is that anyone can plant chestnut trees, possibly in their own gardens, though the team really hopes the trees will be planted out in the forests again.

Abandoned open-cast mines are one option. Mining companies have a duty to restore the natural environment when mining is finished. William thinks such sites would be ideal places to plant both chestnuts and other species of trees. There are also plenty of abandoned fields and patches of waste land currently returning to nature that he thinks would be suitable for chestnut trees.

‘People ask me: “So what are you going to do? Are you going to go out and cut down other trees so you can plant chestnuts?” The answer is: absolutely not! But there are plenty of places where forests are making a comeback, and there are always openings in forests where old trees have been brought down by a tornado or a small fire. Those are places where you can start planting chestnuts.’

There will probably need to be a great many chestnuts in the forests before they can begin to spread by natural means; exactly how many is unclear. One difficulty with establishing the new population is that so many forest-dwellers like to feed on chestnuts. If there are only a few trees, nearly all the nuts will be eaten by squirrels and insects, rather than growing into new plants. Once there are enough trees, however, at least a few will manage to reproduce.

‘But by then I’ll be retiring, and I’ll get a couple of acres of land and plant a few chestnut trees on it,’ William laughs.

When will it all be finished? I ask. How long will it be until chestnuts can sustain themselves without human support? How long will it be till there are enough to be felled and used for timber once more?

‘You know, I haven’t actually thought about that, because I’ll probably be dead by then,’ says William, laughing again. ‘I always tell people we’re going to start this restoration project, but it’s a century-long project. It’s going to take a big effort. If we don’t have people to plant these trees out, they’re not going to spread on their own. You might have a few yard trees here and there,’ he continues, rather more seriously.

He still sees the wound in the forests where the chestnuts disappeared a hundred years ago, the species that have struggled to survive. This is an ongoing crisis that he hopes he can use his trees to resolve. His approach offers a way to restore something infinitely precious that we have almost — but not quite — lost. What’s at stake is not just the trees themselves, but all the effects they have on the ecosystem. In this sense, the project resembles Ben’s dream of bringing back the passenger pigeon, though the image of majestic trees covered in white blooms is far less alarming than that of huge flocks of pigeons sweeping across the countryside, devouring vegetation and leaving dung behind.

I believe William will make his dream come true. For one thing, the chestnuts disappeared so recently that there are powerful scientific and biological arguments for restoring them to the forests. It looks as if the trees will do well, and in all probability they will benefit many other species, too. Above all, though, I believe this project will be a success because it is so easy to love trees. There are already committed volunteers who are keen to help plant them out, and, when it comes to goodwill, it’s hard to beat a project to restore land devastated by open-cut mining. This goodwill is strong enough to convince even sceptics of genetic engineering. I admit, I am among those who have fallen in love with the idea of restoring the American chestnut to life.

Listening to William talking about the catastrophic loss of the United States’ chestnut trees, I can’t help thinking about the forests of Sweden. Death has visited Sweden, too — several times, in fact. Dutch elm disease, another fungal infection spread by beetles, has destroyed vast quantities of beautiful trees, in Sweden and the rest of Europe. While a healthy elm lives for four or five hundred years, an elm with this deadly disease can die in a matter of months.

‘Nearly all of Sweden’s elms are infected. But I don’t think elms will disappear altogether, as they often manage to grow into saplings and disperse their seeds before they become infected. So I think the species will survive, but it’ll change; there won’t be any big trees any more,’ says Johanna Witzell, a researcher at the Swedish University of Agricultural Sciences in Lund who specialises in fungi that grow on trees.

The fungus is so widespread that the island of Gotland is now the only part of Sweden where anyone is even trying to combat it. Since 1997, the European Union has supported a project to try to preserve genetic variation in elms and assess whether there are any disease-resistant trees. The participants have collected hundreds of clones of trees from all over Europe in efforts to find a possible way of helping European elms recover.

‘A lot of work has gone into producing disease-resistant hybrids and clones. Many of them are already available commercially. The question is: can these hybrids replace woodland elms, for instance, in terms of their ecological role? That’s something I’m not at all sure about,’ says Johanna. The hybrids are ‘small and square’, she says, more suited to parks and gardens.

Moreover, it looks as if the very fact that they’re resistant to fungal diseases may be problematic. Trees are covered in multiple types of fungi that live within and on them, just as we humans harbour millions of bacteria. These fungi fulfil a function in that they affect various processes within the tree. Johanna’s research has shown that elms that are resistant to Dutch elm disease also host fewer species of fungi in general.

‘We need to ask what will happen to these disease-resistant trees in the natural environment. When they die and it’s time for them to rot, this may involve different processes with fungi and bacteria. It could have a cascade effect on the ecosystem.’

Dutch elm disease is not the only condition to affect trees in Sweden. Ash trees in the south of the country are currently dying of a fungal infection first identified in Poland in 1992. Ash dieback (caused by Hymenoscyphus fraxineus) attacks shoots, making them wither and die. At the moment, there’s no effective cure for the disease, and nothing can be done to protect infected trees. The disease has spread rapidly. In Sweden, it was first discovered on the island of Öland, but ever since 2005 the fungus has been found in all the areas where ashes grow. It looks very much as if it will kill most of Sweden’s ash trees.

However, scientists in Denmark have discovered that a small proportion of ash trees are resistant to the disease. This means there’s some hope that the species will survive and resistant trees can begin to spread, perhaps with human help. Although there are no firm plans so far, some European scientists think the disease-resistant trees that have been identified could be used to develop a new population. Scientists in France and Germany are urging that samples be collected from the remaining ash trees, both those that have survived infection and those that are uninfected.

William thinks another possible approach would be the one he’s taken: identifying genes in other species that could provide protection against both ash dieback and Dutch elm disease.

‘My project is definitely relevant to ash dieback and Dutch elm disease. But the question is: are people prepared to accept genetically engineered trees in Europe? I don’t know if they are; they seem more worried about it than people here in the United States.’

By 2009, a quarter of all ash trees in Sweden were either dead already or seriously affected. And the disease has taken hold even more since then. The damp woodlands where ash trees grow are doubly threatened because virtually all the elms that once grew among them have already succumbed to Dutch elm disease. If the ash disappears, a whole range of other species will disappear along with it.

‘I think genetic engineering is a great way to restore species without making a big change to them. But the caveat there is you probably only want to use it if you think you’re going to lose all your trees. Essentially, you’re starting from scratch, you’re making a resistant tree and you’re restoring, not preventing the disease,’ says William, who doesn’t see genetic manipulation as a panacea.

Johanna doesn’t see it as a credible solution.

‘I think it’s far too slow and too dependent on chance. It’s not as if genes are the solution to everything; there are other factors involved. I don’t think this approach would be effective enough, and it costs too much in relation to the benefits it could bring. That’s my view, and I’ve been working in plant biology since the 1990s, when people were just beginning to talk about the opportunities gene technology offered.’

The main problem, in her view, is that approaches such as gene editing take too long and arrive far too late in the day, once the diseases concerned are well-established.

‘I’d go to the root of the problem. As far as Dutch elm disease is concerned, one of the reasons for the epidemic is the fact that we’ve used genetically identical elm clones everywhere in forestry. That creates ideal conditions for disease. So I’d begin by changing the way we use trees and forests. But then we’d need to scale back our expectations as regards the economic role of forests. I think we need to work towards increasing their underlying genetic diversity.’

But does that mean we should just accept the damage these diseases can cause?

‘Maybe we need to accept the fact that forests are going to look different; there are other invasive diseases that are going to change our woods. We may be on the way to losing our great beeches, by the way, as they’re succumbing to another disease.’

She starts telling me about the fungal disease that has just begun to threaten Sweden’s beech forests, and that really worries me. Having grown up in the northern part of Skåne, Sweden’s southernmost region, I feel as if I’ve spent half my life surrounded by beeches. I love these majestic forests, both in the gold of autumn and in the almost-other-worldly light-green of spring. Johanna and I are in her study at Alnarp, a campus of the University of Agricultural Sciences outside Lund, with a park full of colossal beech trees.

The disease now posing a threat is another fungus, which grows in the soil and is related to the potato blight that led to the Irish famine of the mid-19th century. The fungus spreads through the earth, attacking tree roots and hindering the absorption of water and nutrients. It’s already present in the soil at Alnarp, Johanna tells me, and has started to weaken the trees, depleting their foliage.

‘When we found out that the beeches in Malmö’s Pildammsparken were showing this sort of damage, a lot of worried people rang us. Of course this is bad news. We’ve only looked into it on a limited scale, but we’re finding the fungus everywhere; Söderåsen National Park is full of it, for instance. It’s very hard to say what will happen to the forests with this sort of damage, as it’s a slow process, and the trees may be more resistant than we think now. But if the worst comes to the worst, it could be very serious.

‘But I hope there’s enough genetic variation within our tree populations for there to be some tolerance, so not all the trees will die, as some of the people we talked to feared. I hope we can avoid the worst-case scenario,’ Johanna continues.

Six years have passed since scientists first discovered the disease, but it has almost certainly existed here for longer.

‘We’re always a few steps behind when it comes to forest pathology and damage to woodland. Research doesn’t get under way till the damage is already visible and it’s started to have a big impact. By the time forests are that badly affected, it’s generally too late. It’s terribly difficult to eradicate these diseases; we can’t get at them, and they’re very hard to fight,’ says Johanna.

The only solution is to stop such infections entering the country in the first place.

‘As regards damage to forests and tree-based diseases, the international trade in plants is one of the main sources of new pests which our native species can’t fight off. The trade in plants should just be stopped — there’s simply nothing else to be done if you want to avoid these problems.

‘No one’s prepared to pay for a plant that’s been screened, or for one that’s been grown in a way that ensures it’s free of disease — that pushes the costs up so much. But you really can do something as an individual; you can ask where plants come from, avoid buying from big plant nurseries in Germany or the Netherlands, and try to buy plants grown here in Sweden. That’s a way to curb the spread of disease to some extent.

‘If things carry on as they are, we’ll have a lot more pests and invasive species from other parts of the world. So I hope people can develop a more long-term view and be more prepared to accept lower profits. It’s probably very naive of me to say so, but I’d like to believe it could happen. If we carry on pushing the system to its limits, there’s not much hope.’

I ask her what she thinks Sweden’s forests will look like in the future.

‘I think they’ll be much younger; there are quite a few forces pushing forests in that direction. With diseases and other factors, we just won’t have the same ancient woodland we have now. For instance, we won’t be able to cultivate such old or such extensive beech or oak forests as those we’ve had up to now.’