The oak is a keystone species.  Just as building an arch, the stability of the arch is gained from the positioning of the central stone - the keystone, so in many woodlands oak trees are what holds the woodland together; offering both stability and diversity.  There are two types of oak : The pedunculate oak, which ‘likes’ richer and somewhat heavier soils.  It can be distinguished from the sessile oak as Its acorns grow on long stalks [called peduncles]. The sessile oak, which can grow and survive on poorer and somewhat acidic soils, so can be found in upland areas.  Its acorns sit more or less directly on a twig. Both these oak trees contribute to many woodlands across the UK, and have done so for thousands of years.  Some have great age and are veteran trees. Their life can be between 300 and 800 years.    An oak woodland often represents the climax community in lowland Britain, which has considerable stability.  Because of their long lifespan and great biomass, they also represent significant stores of carbon.  Their extensive canopy also moderates the temperature and humidity of the woodland beneath, allowing for the growth of shade tolerate species such as hazel and holly. The trees also support an amazing diversity of life,  some two thousand species are known to find a ‘home’ in oak trees.  Feeding on the leaves or flowers are hundreds of species of insects - notably moths and butterflies.  The leaves provide food for many species of caterpillars, which in turn are a food source for birds, like the tits and warblers.  The branches of the trees provide nesting and roosting sites for many bird species.  The acorns of the oaks are a rich source of nutrients and energy not only for squirrels, but also mice, deer and other mammals.  Birds, like the jay, also take them.  Jays and squirrels create ‘caches of acorns’, that is they bury them as a store for later.  However, many are forgotten so they effectively distribute the acorns over a wider area and ‘plant’ future oak trees. The bark of the oak is rough and corrugated offering specialised niches for lichens and mosses.  When an oak dies , it is then home to a variety of fungi and beetles, which feed upon the rotting wood (saproxylic beetles).   The roots of an oak penetrate deep into the soil, which helps improve drainage, and the roots establish mycorrhizal connections with soil fungi.  These connections are symbiotic in nature, in that both partners benefit from the association.   The tree gets mineral nutrients from the fungi, and, in return, diverts some of its carbohydrate material to the fungi.  The breakdown of the oak leaves in autumn returns vital nutrients to the soil and provides food to a variety of decomposer organisms, from bacteria to earthworms.  Thus, oaks are a vital part of many woodland food chains.   Sadly, our oaks face a number of threats.  In elizabethan times, the main threat was that of being cut down and used in ship building.  At the height of Nelson’s navy in the late eighteenth century / early nineteenth century it took some 4,000 oak trees, to build a single 100-gun ship, and the navy had some 300 ships!  More recently, there have been problems with acute oak decline and infestation by the oak processionary moth.  The caterpillars of this moth feed on oak trees and large populations can strip an oak tree bare.  This leaves the tree vulnerable to other pests, and stresses such as drought.    Incidentally, contact with the hairs of the older caterpillars of this moth can cause itching, skin rashes and eye irritations, plus sore throats and breathing difficulties in us and other animals.   

I was born in the 1950s, a time when black smudges began to appear mysteriously on the trunks of sycamores in Britain. The culprit: sooty bark disease, a fungal infection caused by Cryptostroma corticale. Back then, I had no idea my life would unfold alongside a slow but steady parade of arboreal afflictions. But looking back now, I can measure the years not just in milestones and birthdays—but in the trees we lost along the way. Sooty bark disease doesn’t get the headlines these days, but it was a grim marker of post-war environmental change. Sycamores, long naturalised in Britain, would suddenly wilt and die, the bark flaking away to reveal a sinister black fungus. We didn’t yet understand how much stress—particularly from the hot, dry summers of the 1950s—played into its spread. It was an early sign: a warning that trees are far more vulnerable than they seem. Then came the true giant of tree diseases: Dutch elm disease. It began making headlines in Britain in the late 1960s and ravaged the landscape through the 1970s and '80s. Caused by a fungus (Ophiostoma novo-ulmi) spread by elm bark beetles, this pandemic decimated the native elm population. It’s estimated that over 25 million elms were killed in the UK alone. I remember the shift in the landscape. Once-common elm-lined avenues and hedgerows simply disappeared. As a child, I’d climbed elms in the park; as a young adult, I watched them vanish almost overnight. Dutch elm disease wasn’t just a biological tragedy—it was a cultural one. It marked a turning point, an awakening to the vulnerability of our treescapes. The decades ticked by. Chestnut trees became a familiar sight in my children’s drawings. But by the 2000s, I noticed the conkers looked smaller, sadder. Bleeding canker of horse chestnut, caused by Pseudomonas syringae pv. aesculi, began spreading rapidly across the UK. It causes a sticky, rust-coloured ooze from the bark and often leads to dieback and death. The disease didn't just affect the health of the trees; it diminished a cultural icon—conker tournaments and autumn walks lost something in its slow assault. Around the same time, sudden oak death (Phytophthora ramorum) emerged, though it affects more than just oaks. First identified in the US in the 1990s, it reached the UK in the early 2000s, causing widespread concern in woodlands and nurseries. It targets a range of species including rhododendrons, larches, and beech. The name alone—sudden oak death—carried a dramatic finality. Then came perhaps the most alarming of all in my later years: ash dieback, or Hymenoscyphus fraxineus. First identified in Poland in the 1990s, it reached the UK in 2012. It’s a true scourge, expected to kill up to 80% of the UK’s ash trees. These aren’t just forest trees—they line our roads, dominate hedgerows, shade our back gardens. Their decline feels intimate. Walking in ash woodland today is like passing through a ghost forest. The signs are unmistakable: leaf loss, crown dieback, diamond-shaped lesions. I’ve watched entire copses hollow out over just a few seasons. The cost is measured not only in timber or beauty, but in ecological networks—over 1,000 species depend on ash. And let’s not forget the oak processionary moth, which first arrived in the UK via imported oak trees in 2005. While not a disease in the fungal sense, it’s a threat nonetheless. Its caterpillars strip leaves and their tiny hairs can trigger allergic reactions in humans and animals. Forestry teams now issue warnings during their seasonal outbreaks. Oaks have stood proud for centuries, but even they are not safe anymore. There are others: sweet chestnut blight, plane wilt, the pine processionary moth, and new strains of Phytophthora that attack multiple species. The list gets longer, not shorter. So what’s going on? Part of the answer is globalisation. Trees, soil, and ornamental plants now travel easily between continents, bringing pathogens with them. Climate change plays its role too—stressed trees are more vulnerable, and warmer conditions allow pests and diseases to thrive. And while tree diseases aren’t new, our ecosystems today are more fragmented and less resilient. [caption id="attachment_8120" align="alignleft" width="400"] Leaf miner in Horse Chestnut[/caption]   What strikes me most, looking back, is how predictable this pattern has become. Every decade or so, a new name enters our vocabulary. Each time, we scramble to learn its symptoms, its vectors, its likely victims. And each time, the outcome is similar: loss, adaptation, then a wary lull before the next wave. Measuring my life in tree diseases might sound grim—but it’s also grounding. Trees are long-lived beings; their suffering unfolds slowly, deliberately. Watching them struggle is a reminder that the natural world is neither invincible nor immune to human action. The next great tree crisis is likely already on its way. But perhaps with better biosecurity, international cooperation, and public awareness, we can at least slow the tide. For me, though, the trees I’ve known—and the diseases that marked their passing—will always be a living calendar. A record of change, and of resilience.    

Recently I wanted to find out more about Phytophthora ramorum so I went to a seminar that was organised by Natur  (The Welsh Institute for Countryside and Conservation Management) held in north Wales. Seminar on Phytophthora ramorum The seminar was specifically relevant for woodland owners, contractors and countryside workers. These groups are most likely to come into contact with this disease and therefore be well placed to identify it before it becomes established in a new location. There is also the danger that these groups could become vectors spreading the disease between sites. Read more...