Our woodlands are under attack from new pests and humansby Clive Anderson / July 17, 2014 / Leave a comment
Woodman, spare that tree!/ Touch not a single bough!/ In youth it sheltered me,/ And I’ll protect it now.” If the 19th-century American poet George Pope Morris were writing today, he would have to address his plea not just to the axe-wielding woodman, but to the many other creatures great and small—human, animal, bacterial and fungal—which seem to be waging war on our trees, woods and woodlands.
I should declare an interest here. For the last 10 years I have been President of the Woodland Trust. My role is not as grand as the title suggests—showbiz patron or celebrity tree-hugger might cover it. When describing the role of the trust, I like to use the phrase “trees, ancient and modern” to emphasise our twin-track approach: protecting ancient woodlands and planting new trees for the present and for the future.
Today, there is a third aspect of tree conservation, which cuts across the other two and is becoming increasingly important. That is coping with the array of new pathogens and diseases infecting trees both ancient and modern and which threaten to destroy many of the country’s trees, or in some cases wipe out entire species.
That may sound melodramatic, but it has effectively already happened to the English elm. For hundreds of years, elm trees were a familiar feature of the British countryside: elegant, tall and attractive to naturalists, painters and furniture-makers. But in the 1960s and 70s most of the 30m elms in this country were brought down by Dutch elm disease, a devastating fungal condition spread by the elm bark beetle.
The English elm is not particularly English, and the disease isn’t in any way Dutch—it probably originated in Asia and is thought to have been introduced to Europe from America in a consignment of logs. But with the exception of a few survivors still growing around Brighton and Edinburgh, there are now no English elms to be seen anywhere in Britain.
The roots of the elm live on, of course, and they do produce new shoots capable of becoming full-grown trees. But once they reach a height of 15 feet or so they are on the flight path of the elm bark beetle which again infects them with the killer fungus, choking off their growth. This is a special case, as the English elm reproduces exclusively vegetatively. Elm trees grow as suckers off each other, so they are genetically identical. As a result whole areas of elm are susceptible to the same pathogen.
A threat to ash trees, potentially every bit as serious Dutch elm disease, emerged a couple of years ago. “Ash dieback” is caused by another fungus, Chalara fraxinea, which causes leaf loss and crown dieback in affected trees. Sooner or later this is likely to be fatal. Young saplings are knocked out immediately. Mature trees, though they cope better at first, are overwhelmed after a year or two. Only a few thousand trees on 655 different sites have been infected by this fungus in Britain so far, but it has been spreading steadily across continental Europe since its effects were first noticed in Poland in 1992. In Denmark, for example, losses are now estimated at between 60 and 90 per cent.
Another fungus, Phytophthora ramorum, identified in the United States as the cause of the ominously named “sudden oak death,” is killing a different species in the UK, the larch. It has caused huge damage to forests in Wales, Scotland and, most recently, the Lake District. There is no cure. Vast areas of diseased trees are being felled in an attempt to contain the problem, but there is no guarantee of ultimate success.
The larch is a commercially grown crop. But there are also diseases afflicting the trees in our towns and suburbs. Horse chestnut trees have been planted in decorative avenues and in other prime spots in leafy areas since the tree was introduced to Britain a few centuries ago. It is not native to Britain, but has been with us long enough for the game of conkers to have been taken up by generations of British school children. But for how much longer? As more or less every school child knows, in recent years the leaves of the chestnut tree no longer wait for autumn to turn brown. They wither and die in the summer months. This unsightly development is caused by the burrowing habits of the larvae of the horse chestnut leaf-miner, a moth recently arrived from Eastern Europe.
And if the leaf miners don’t get them, bleeding chestnut canker almost certainly will. Conker canker is caused by a type of bacteria, Pseudomonas syringae. Possibly lethal in the long run, it has already infected perhaps half of all of our horse chestnuts.
Add in a fungus or two and the horse chestnut may be reaching the finishing post. Steve Woodward, a leading expert on tree diseases at Aberdeen University, recently expressed his opinion at the Cheltenham Science Festival that our horse chestnuts are such a mess that “the best thing to do with them would be to cut them all down now. They are being hit by pests and diseases which are now so thoroughly established that we cannot get rid of them.” And cut down they will be if there is a perceived risk to human life and limb from falling branches. Nor will they be planted as long as they are known to lose their leaves and their looks.
Even the London plane, that robust hybrid between an oriental plane and the American sycamore, which has proved adept at coping with the stresses of life on the streets of polluted inner cities, is suffering from a new disease, massaria, which weakens its branches. The news from France about planes is even worse: Ceratocystis platani—“plane wilt”— is killing the avenues planted along French roadsides and the shade trees of the Canal du Midi. Sooner or later it is likely to make its way here. If it does so, as a report prepared for the City of London Corporation last year spells out, it is likely to lead to the loss of the London plane. The Department for Environment, Food and Rural Affairs has introduced controls on the importing and movement of plane trees, which we can only hope will save them.
Plane, horse chestnut and larch are not native to Britain, but indigenous species are not immune to new infections either—even our beloved oaks. There are two species of oak native to Britain: Quercus robur, the English or pedunculate oak, and Quercus petraea, the sessile or durmast oak. Both are currently threatened by, among other complaints, “acute oak decline” and the oak processionary moth.
Acute oak decline has been observed in British oaks for more than 20 years now. It’s not a pretty sight—symptoms include black weeping patches on tree stems. It appears to be caused by two new types of bacteria, possibly spread by the oak jewel beetle. Some, though fortunately not all, affected trees, die within four or five years.
The processionary moth entered the country on a batch of imported ornamental trees a few years ago. They have been found living in several London boroughs and parts of Berkshire. The moth gets its name from the head-to-tail conga lines formed by their caterpillars. The caterpillars’ food of choice is oak leaves, and a long line of them can strip a whole oak tree of its foliage in short order. This might not kill the tree straight away, but it certainly weakens it. And apart from the damage the caterpillars do to trees, the tiny hairs covering their bodies are harmful to humans, so it is not just tree huggers who want rid of them. Efforts to eradicate the moths have included sucking them off the trees with hoovers and spraying them from the air from helicopters. Mothballs don’t work.
Other trees face different threats: red band needle blight and the pine tree lappet moth are damaging Scots pines, and spruce and juniper bushes face similar problems. Junipers are in a particularly bad way at the moment. Already we have to import most of the berries that flavour our gin and tonics.
Another serious pest, the Asian longhorn beetle, was discovered in Kent two years ago. It was imported inadvertently in wooden packing crates containing patio stone from China. This outbreak was contained, but the beetle would be a danger to a whole range of broadleaf trees if it were to make its way to our shores again.
It is not obvious why so many new diseases should be manifesting themselves at such a rate, but the expansion in air travel and the globalisation of trade and industry must have something to do with it. No man is an island, and nowadays no island is either. Insects, bacteria and fungi hitch a ride from country to country and from continent to continent with human travellers and cargoes. We now import lots of fresh produce from around the globe. It is moved further and faster often in sealed containers. Foodstuffs arrive fresh. As do the pests.
There is also a growing obsession with gardening. A few decades ago garden centres were relatively few and far between and they were largely supplied by local nurseries. We now have massive garden centres and landscape businesses selling the latest plants and varieties, usually shipped in from overseas.
A parasite which causes relatively little harm to its host species in one part of the world can wreak havoc in another part where it finds a new host that has less resistance.
It is possible, up to a point, to control the importing of plants and trees in order to minimise the risk that they will bring disease in with them. But that requires everyone involved to be alive to the danger. The Woodland Trust itself was shocked to discover in August last year that it had planted ash trees infected with the Chalara fungus. The plants were British but had been raised in a Dutch nursery, where they had become infected with the fungus.
Controlling the importing of trees and plants is one thing, but pest species can arrive in boxes of fruit, wood shavings and other packing, wooden products and in hundreds of other ways that it would be almost impossible to legislate against. They can arrive by train, boat, plane, container ship and ferry or on the shoes of travellers or their car tyres, any time of day or night.
Climate change is likely to be playing its part in all this, too. Even slightly warmer weather alters the balance in the natural world. Some species will be able to react quicker than others to seasonal change coming earlier in the year, a process that has already been observed. An insect that has a week or two without predators can rapidly become a problem to the plant on which it feeds. Temperatures failing to drop below freezing will allow some organisms to survive the winter weather that used to kill them off. And once insects or microbes can “overwinter” they can settle in for the long haul.
Does it really matter if trees are suffering from all these diseases? Quite apart from the look of the place, trees are vital to the overall condition of the country and indeed the planet. They turn carbon dioxide into oxygen storing carbon in the process. They provide a habitat for wildlife, give us shade in summer and help cope with floods in winter. They retain riverbanks in the country and help deal with pollution in the city.
Global opinion is understandably exercised by the destruction of the rainforest in Brazil, Indonesia and other regions in the tropics, but there has also been deforestation closer to home. We like to think of our country as a green and pleasant land, but Britain is not rich in trees. Worldwide there are perhaps as many as 100,000 species of tree, but only around 30 are native to Britain.
Britain’s tree population has started to grow in recent decades and currently stands at about 12 per cent of the total land area. Even if the Woodland Trust were to achieve its goal of doubling that figure, it would still leave us well below the European average. The aim of increasing tree cover is obviously not going to be helped by the growth in tree diseases. So what should be done to keep them under control?
I have mentioned trying to prevent the importing of pathogens and have discussed some of the measures that have been taken to contain outbreaks once they occur. These are often pretty desperate measures and you could argue that the best thing to do would be to let nature and evolution take their course.
Oak trees, for example, are much admired by ecologists for the biodiversity they support. There will be something like 280 species of insect and as many as 300 types of lichen living and growing in the vicinity of a mature oak tree, a figure likely to surprise those of us who had no idea there were 300 types of lichen. Given that, couldn’t an oak tree adapt to one more brand of fungus or a particularly hungry caterpillar?
Perhaps, but evolution proceeds slowly and trees built to live for hundreds of years might take millennia to accommodate a new parasite. In the meantime our woods and forests may lose something of their charm and many of their trees.
We need to create woods that are as robust as possible and can cope with the challenges that will inevitably come along. Anybody planting new woodland or even an avenue of trees would be well advised to have a good mixture of species so that a disease attacking one type of tree need not spell the end of the whole thing. Having trees of different ages alongside each also makes for more resilience.
This is one of many arguments in favour of preserving ancient woodland, which naturally has a range of trees of different ages. Ancient woodland is somewhat arbitrarily defined as woodland in which trees have been growing for more than 400 years. Some woods may have been wooded for several centuries before that, but the point is that it takes hundreds of years for the ecology of woodland to fully develop. There is a vanishingly small amount of ancient woodland left in Britain, and more and more of it is disappearing because it is not properly protected by national planning policy.
There are approximately 400 ancient woods currently under threat from development across the UK, including 83 on the proposed route of HS2. As things stand, an ancient wood does not get the same protection from planning law as an ancient monument. Smithy Wood near Sheffield is a good (or bad) example. It is recognised locally as important for nature conservation, but it was cut into four by the construction of the M1 in the 1960s and is now in the proposed path of HS2. In addition, a planning application was put forward in April for a motorway service station to be built in the wood. If this goes ahead, against local opposition, up to eight hectares (20 acres) of the wood would be lost.
Given their vulnerability to human wants and needs, trees need new diseases like they need a hole drilled in them by an oak pinhole borer.