When we talk of species extinction we tend to think of this as something that happens a long way away from us, maybe in another continent, in the jungles of the Amazon or Asia. We also tend to think of species extinction as being the result of human encroachment – the combination of felling, burning, agriculture and urbanisation. While this picture is largely correct as far as it goes in many parts of the world, something just as frightening is happening much closer to home.
Our trees are dying. Not a few of them, somewhere else, but many of them, here, throughout our country. Many of our tree species are facing a wave of diseases that will have almost unimaginable consequences for our countryside. The list of tree diseases in Britain is now very, very long. The government website Gov.UK lists some two dozen, all of them serious (1). The most frightening thing of all is that many of these diseases are new – new to Europe not just to the UK.
The sad loss of the majestic English Elm
Some of us are old enough to remember the second wave of Dutch Elm disease. The first hit during the 1920s and burnt itself out. The second started in the 1970s and by the early 1980s, Elm trees had almost totally disappeared from the landscape. Some twenty million elms had been killed during a single decade. Now they are but a memory with only a few isolated examples remaining.
Anyone younger than forty is probably too young to remember how elms had once been a prominent feature of our landscape, their stately forms punctuated the hedgerows and fields, the galleons of the green. Appearing taller than other English deciduous trees, mainly due to the fact that they put their energy into vertical growth and spread their branches less widely than trees such as the oak or the ash, they reached a height of 30 metres.
The English elm often stood solitary or in small groves and was once the home of choice to rooks (2). Rookeries were found wherever elms stood in a cluster, as they once did on the edge of almost every English village. Then they were as English as cricket on the green. No more. The elms have gone and so too, to a large extent, have the rooks. While they have found other trees in which to build their nests, the colossal rookeries of yesteryear are very much a thing of the past. Elms survive today largely as a hedgerow shrub.
When I was young, a fairly large rookery was housed in a small cluster of elms that stood within earshot of my bedroom. Every morning I was greeted with the raucous squabbling of these delightfully cantankerous corvids. As sunset approached, this performance was repeated, slowly subsiding into a round of gentle cawing as they settled down for the night. Despite their quarrelsome manner, I always found there to be something cheerful and amusing in how the rooks conducted themselves. And while the elms are sadly no more, rooks have remained one of my favourite bird species. The sight of these large bare-billed black birds feeding in a fallow field during winter seems to me to be a scene that could stretch back into eternity.
The slow death of the Common Ash
Many of us have heard of Ash dieback caused by the fungus Hymenoscyphus fraxineus, more properly known as Chalara ash dieback. The disease attacks one of our most common and loved native trees, Fraxinus excelsior, a tree which can otherwise reach 35 metres in height and 400 years in age. There are thought to be some 90-150 million mature ash trees in Britain, and as many as 2.2 billion trees in total, when saplings are included (3).
The disease is of Asian origin and was first reported in Europe in Poland in 1992, where it quickly had devastating consequences. The first symptoms are the weakening, drooping and discolouration of leaves and shoots, followed by premature leaf fall. Stricken trees subsequently develop black lesions in their trunks and bark. Once present, Chalara ash dieback stunts growth and weakens the heart of the tree, making it vulnerable to attack by other tree diseases. This almost invariably results in death, sooner or later (4).
Unfortunately, the disease spreads easily, the spores of the fungus travelling on the soles of shoes or car tyres. The spores can be carried as much as 10 miles or more on the wind. As a result of its ease of transmission, Chalara ash dieback has spread rapidly throughout Europe. It was first identified in England in 2012. However, scientists now believe it had reached the UK as early as 2006, but had gone unreported.
It is expected that the disease will kill somewhere between 95% and 99% of all UK ash trees (5). There is no cure. Because the tree is so widespread, from southern England all the way up to Scotland, this disease will change the appearance of the British landscape even more than the loss of the elm. While few trees will survive in the long term, some have proved resistant to the fungus. This has given rise to some hope and the formation of the Living Ash Project (6)(7).
Why we have a tsunami of tree diseases
Ash dieback is just one unexpected and unfortunate outcome of the absence of proper plant quarantine facilities in the UK. As we have seen with human disease, there is little or no attempt to enforce controls at our borders. This is even more the case when it comes to controlling plant imports. Trade is all in our globalised world.
As a result, Chalara ash dieback is one of more than half a dozen new deadly tree diseases, all of which have arrived on these shores in recent years. I list the main notifiable diseases by the date when they were first identified in Britain:
- 1993 Phytophthora of alder: Lethal to the alder tree; it came here from Germany and France; 20% of all trees are already infected.
- 2000 Horse chestnut bleeding: This is a type of canker caused by two different pathogens – one from the US, the other from the Himalayas. Lethal.
- 2002 Ramorum disease: Arrived from Asia and affects 150 species. Larch are particularly susceptible, resulting in browning of the needles and needle drop, followed by sudden larch death. Stands of larch across Britain and Europe are now permanently brown all year around. They are being felled and have little or no value as wood.
- 2007 Phytophthora of juniper: Arrived from Argentina. Causes dieback and eventual tree death.
- 2009 Massaria disease: Arrived from Europe. Affects the plane tree, including the London plane, leading to branch loss, weakening them and making them unsafe. Councils are forced to resort to felling to protect the public.
- 2010 Phytophthora lateralis: From North America. Kills the evergreen Lawson cypress.
- 2011 Neonectria canker of Abies: From Europe. This disease causes crown dieback and weakens the wood. Affects 13 species of fir as well, it appears, as some spruce and the western hemlock.
- 2011 Sweet chestnut blight: Say goodbye to the sweet chestnut tree. This arrived from the US via Europe. It is estimated that it has already killed 3.5 billion trees in the US during the twentieth century.
By now, if you’re like me, you are no doubt feeling distressed that we could ever have permitted this slow moving disaster to gradually overwhelm our woodlands. Sadly, there might be worse to come.
The big new killers on the block
Containing plant diseases should be a whole lot easier than containing human ones. Plants don’t run, nor do they walk (8). If we wish to, we can prevent things going from bad to worse by adopting proper plant quarantine procedures as the Australians have done for decades (9). Our trees needn’t die in such vast numbers (though, sadly, it might already be too late to save the ash).
There is urgent need to act. The tree diseases on our doorstep are the most frightening yet. And probably the most serious of all is that of Xyella fastidiosa. The photograph above is of an Italian olive grove devastated by this disease.
According to Forest Research, “Xylella is a plant disease which can affect several species of broadleaf trees widely grown in the UK, such as oak, elm and plane, as well as a wide range of other commercially grown plants. Among them are grape, citrus, coffee, olive, almond and blueberry species, and many herbaceous plant species.”(10) I am not looking forward to doing without coffee, wine, or citrus fruit; and this list is just the tip of the iceberg. While it includes some of Xyella’s commercially more valuable victims, it misses off most of the garden plants that will disappear if it arrives on these shores. Currently, some 560 plant species are known to be vulnerable to Xyella!
The disease reached Europe from America, being first detected in Italy in 2013. It has since made its way slowly northwards, spreading to Spain and France which are struggling to contain it. Germany, on the other hand, was successful in eliminating an outbreak in 2018.
The bacterial disease blocks the movement of water and nutrients (through the xylem) in the host plant, thereby causing severe damage and frequently killing the plant. It affects different species with varying degrees of lethality. The disease has already led to the destruction of tens of thousands of olive trees. I find this particularly sad. Olive trees are known to live for well over 2,000 years and are some of the oldest living organisms on the planet (11).
However scary a prospect for our gardens and countryside, Xyella is not the end of the story. Further horrors that have yet to reach our shores await our trees. Some are already knocking at our door:
- Conifer root rot: This has already destroyed 25% of Norway spruce trees throughout Scandinavia.
- Pitch canker of pine: It arrived in Europe from North America. The disease affects spruce trees as well as pine. It reduces timber growth and causes widespread tree death. It is present in Portugal and Spain and is considered eradicated from France and Italy.
- Plane tree wilt: This is widespread in Europe. It kills slowly, taking up to 7 years to finish off its host.
- Oak tree wilt: The name is misleading. The fungal disease can kill oak trees in just a few years. If it ever came here, it could do immense damage to our two native species of oaks, which are already are already under severe stress from threats such as acute oak decline, chronic oak dieback, and oak processionally moth.
This post focuses on the new tree diseases brought to Britain and Europe from other countries. Take pitch canker of pine. This disease is prevalent in southeastern USA, as well as coastal Monterey and California. It is thought that it might have originated in Mexico, however, and is also found in Chile and Haiti. From the Americas, as well as Europe, it has also spread to South Africa and several countries in Asia, including Japan, Korea and possibly Iraq. In addition to the felling of the great jungles of South America and Asia, the unrestricted trade of recent decades has had some unexpected and terrible consequences for the world’s trees.
Is our number one carbon sink sunk?
People everywhere are calling for the planting of millions (or trillions) of more trees to act as carbon sinks. More trees mean less free-floating CO2 and this slows the warming of atmosphere. Trees do a great job of this. Roughly speaking, a 40 year-old tree has absorbed about one ton of CO2. A 2001 study lists the following species of trees as being especially good at storing and absorbing CO2: the common horse-chestnut, black walnut, American sweetgum, Ponderosa pine, red pine, white pine, London plane, Hispaniolan pine, Douglas fir, scarlet oak, red oak, Virginia live oak, and bald cypress (12). Do you notice something? That’s right, there’s a very worrying overlap with the list of species we’ve just been talking about. This is not good news.
Australia treats the spread of tree diseases as an issue of Biosecurity. Their government website states, “Imported live plant material can introduce foreign plant pests and diseases that could be harmful to Australia’s environment, agriculture and economy. To protect against this risk, the Department of Agriculture, Water and the Environment sets conditions for the import of all live plant material to Australia.” That they mean business is flagged right at the top of the website, where this message is highlighted: “Notice to import industry: changes to import requirements to protect against Xylella. The Australian Government is strengthening the import requirements for a number of plant species to safeguard Australia against the bacterial plant pathogen Xylella (Xylella fastidiosa)”(13).
Australia is clearly prepared to go to great lengths to prevent the spread of Xyella and other deadly tree diseases. The question is, why aren’t we?
- (1) https://www.gov.uk/guidance/find-a-specific-tree-pest-or-disease
- (2) http://www.wildengland.com/rook/
- (3) https://www.theashproject.org.uk/ash-trees/
- (4) https://www.forestresearch.gov.uk/tools-and-resources/pest-and-disease-resources/ash-dieback-hymenoscyphus-fraxineus/
- (5) https://ashdieback.co.uk
- (6) https://www.forestresearch.gov.uk/research/living-ash-project-lap/
- (7) https://www.woodlandtrust.org.uk/trees-woods-and-wildlife/tree-pests-and-diseases/key-tree-pests-and-diseases/ash-dieback/
- (8) The one exception, of course, is that of the Triffids, in John Wyndham’s post-apocalyptic science fiction novel, The Day of the Triffids (1951). Once plants started walking, humans were in deep trouble.
- (9) https://www.agriculture.gov.au/import/goods/plant-products/how-to-import-plants
- (10) https://www.forestresearch.gov.uk/tools-and-resources/pest-and-disease-resources/xylella-xylella-fastidiosa/
- (11) Let me be clear, there are many other contenders for this title, including the humble yew tree, an example of which, growing in a churchyard in Wales, is thought to be somewhere between 4,000 and 5,000 years old. https://en.wikipedia.org/wiki/Llangernyw_Yew
- (12) https://www.thoughtco.com/which-trees-offset-global-warming-1204209
- (13) https://www.agriculture.gov.au/import/goods/plant-products/how-to-import-plants