Here is a delightful video by the Woodland Trust on how to recognise veteran and ancient trees! Read more from their guide here.
Ancient woodlands, when left to their own devices, develop a healthy variety in their age structure -- this being the variation in ages from tree to tree. This variety in age is crucial to the health of a woodland as differently-aged trees support different wildlife that inhabit that woodland. For instance, young shrubby trees are favoured by browsing deer and songbirds, and form a large part of the understory layer. Meanwhile, at the other end of the scale, ancient trees provide deadwood microhabitats and form the tallest part of the canopy. The older a tree gets, the more it supports.
A major problem the UK faces is that a lot of our woodland does not contain this variety in age structure, and this can be for a lot of reasons. The UK houses a lot of secondary succession woodlands. These are woodlands that re-colonised a site that was previously cleared for human use. For instance, old quarries or pit heaps. The effect of this is that the age structure of the woodland has a hard limit, sometimes of only a couple hundred years. As well as this, a large amount of our woodlands (such as Hamsterley Forest, pictured to the left) are ex-plantation, or actively used as plantations for timber. This means that when they come of a certain age they will be felled, removing their habitat value. Plantation sites in particular are awful for conservation value as pine woodland isn't our natural woodland type, and therefore serves very little use to our wildlife. I would love to write a page on the details of coniferous plantation habitat value in the future as there are so many layers to the topic.
By their strict definition a veteran tree has high biodiversity and cultural/heritage value It is also usually very old for its species -- all ancient trees are veteran trees, but not all veteran trees are ancient. This is very important! VERY few trees reach the ancient life stage.
An ancient tree is defined strictly by its age, relative to its species. Examples of this include a 800-1,000-year-old oak tree, a 500-year-old elm, a 200-year-old ash, or a 300-year-old beech!
A veteran tree is usually relatively old, such as a 400-year-old oak. The veteran ranking is gained through, crucially, its cultural/historic/biodiversity value. So long as it has these values, the age range can skew toward a younger tree.
Why keep a Veteran Tree?
Scientific value -- True ancient veteran trees are rare to come by, and can have a great deal of scientific significance. For instance, there are many rare organisms that can only be found on veteran trees.
Cultural significance -- The Betty Kenny tree, pictured to the right here, is the supposed origin of the popular nursery rhyme Rock-A-Bye Baby, where a mother would lull her children to sleep in the hollowed-out branches of the yew tree. Other examples of trees with cultural significance include the Sycamore gap in Hadrian's wall, famous for its unique location (and bolstered by its appearance in the 1991 film Robin Hood: Prince of Thieves), or the famous Robin Hood's Oak or "Major Oak" in Sherwood forest, fabled to be where he and his merry men slept. Cultural significance can even come purely from the fact that the tree has been there for such a long time it has local emotional significance; imagine standing under a tree and knowing that, for centuries, your family has been seeing the exact same tree!
Historic value -- Some ancient woodland trees can be used to figure out a lot of information about the history of the area. One example of this is popular coppice tree species. If you find an ancient tree that has many trunks clustered in one place, it's likely that the tree was coppiced historically. This is an ancient method of woodland management. Another way that a tree can have great historic value is in its tree rings. Dendrochronology (tree-ring dating) is the scientific method of dating the rings found within trees to the year they were grown based on the relative amount of rings surrounding them. You can tell some fascinating things about the history of the tree's life from its tree rings, tying into its uniqueness. You can see evidence of burns, changes in climate affecting amount of growth e.g. droughts, and all sorts!
Uniqueness and beauty -- When a tree is so old to become a veteran, it will have seen a lot of history. A lot of this history will show up in the tree itself. There are many, many unique instances of veteran trees -- for instance, a veteran oak tree I saw had a birch tree completely fused into it's centre growing straight up out of the gnarly branches. Another ancient tree we saw had a glass orb fused into its base. It's incredibly memorable and one of a kind. All sorts of damage, scarring, strange growth, or simply fascinating features can be found in ancient or veteran trees!
Biodiversity -- This topic will be covered in greater depth later in the article, but for now, let me make it very clear that veteran trees have a huge amount of value in regards to biodiversity. The tree can become a microhabitat that contains multitudes of more increasingly niche microhabitats within it, some of which are relied upon by very rare species of fauna. A veteran tree is almost it's own world!
Veteranisation of a Tree
As so much of our woodland is made up of secondary succession or ex-plantation sites, woodland management needs to account for this when trying to make the habitat as valuable as possible in terms of conservation. But how can you replicate this varying age structure?
Enter: Veteranisation!
Veteranisation is the act of artificially aging up a tree by giving it the characteristics of true veteran trees. This largely revolves around making the tree really messed up, such as by giving it diseases, rot, wounds, etc.. What you're aiming for in veteranisation is to encourage rot and decay at a faster rate.
Here are some examples of how you can veteranise a tree!
Habitat Creation
Tearing cuts and stripping slices of bark can replicate damage to trunks from squirrels and deer, and are a good way to encourage infection of the tree.
Ring-barking is the act of cutting the cambium layer around the circumference of the tree in order to kill off all growth above the ring. You can use this method to create entire standing deadwood trees when done on the main trunk, but in veteranisation you want the tree to mostly be alive. So, in that case, you would ring bark major branches away from the main trunk!
Creating cavities by ripping off branches and cutting holes is another way to encourage further infection. The rougher the injuries, the better!
Encouraging MORE Infection
Plug spawn or injection of fungal spores into trees is a great way to encourage heart rot in a younger tree that might not yet be infected by heart rot fungi. Plug spawning is a popular method of growing mushrooms to cultivate, but works very well on standing and living trees too!
Creating large open wounds and gaping tears on a tree leaves a lot of vulnerability to infection.
Mimicing a lightning strike also functions to encourage rot and decay.
Features of a Veteran Tree
In this portion, I will go over the niche microhabitats and special features of your average veteran tree. Some species lend themselves more to specific features, but all the same these are broadly applicable. The amount of microhabitats found on a tree can denote its veteran status.
Natural water pools
The "crotch" of a tree is the depression where branches join to the tree's base, or to other major branches. During wetter seasons, rainfall can cause water to accumulate in the crotch, creating a mini-pond! This can become a major habitat for a variety of insects and mosses. There are some particular species of fly that specifically use these mini-ponds in order to breed!
Holes for wildlife
Naturally-forming holes in veteran trees, caused by decay such as heart rot that hollows out the inside of the tree, are excellent habitats for nesting birds and bat roosts. A lot of veteran trees are afforded legal protection due to being inhabitated by bats or birds, protected under the Wildlife and Countryside Act.
The extremely rare Barbastelle bat, found only in southern England and Wales, exclusively roosts in tree crevices. They are very elusive, and only have a single pup every time they breed. Look at him!!
Deadwood in canopy
As trees grow, their canopy can become lower to support their aging frames, especially as rot starts to really settle in, resulting in a "stag-head" effect where topmost branches die and jut out of the canopy. These are a very unique niche habitat that supports a highly-specialised range of invertebrates, fungi, and birds. Temperature extremes in the summer will "bake" these exposed branches, which makes them a bit more difficult for invertebrates to burrow into -- however, this hard deadwood is a crucial habitat for solitary bees and wasps that nest in the abandoned holes of wood-boring (or xylophagous) beetles!
Crevices in bark
This element is more common on trees with thicker bark such as elm, lime, and oak. It's a brilliant habitat for insects, which in turn is a great food source for birds such as woodpeckers, who feed primarily on wood-boring beetles!
Fungal fruiting bodies
Veteran trees are an excellent habitat for a wide variety of fungi, particularly due to the variety of habitats within such as living material, deadwood, roots, and water accumulations. There are many different species of fungi that are found in different living states of tree, and a tree can collect a variety of colonies overtime!
These fungi contribute to the gradual decay and decomposition of the tree's core.
A major problem the UK faces is that a lot of our woodland does not contain this variety in age structure, and this can be for a lot of reasons. The UK houses a lot of 
