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Deliberately ageing trees to save them

SCIENCE

Some trees are ancient. Take the Big Belly Oak in Savernake Forest in south-west England. It’s around 1,100 years old, and almost 11 metres in girth. Now gnarled and knobbly, its trunk is strapped up with a metal girdle to stop it falling apart. As impressive as it is from a distance, it’s what’s going on inside that is more intriguing.

Oak polypore fungi and stag beetle larvae feast on the dead heartwood, adult stag beetles sup the sugary liquid from the "sap runs", the living layers of wood that transport water and minerals throughout the tree. Hover flies lay eggs in water-filled rot holes, rat-tailed maggots devour leaf litter and violet click beetles eat up wood mould rich with faeces and other remains, accumulating over a century. Knothole moss and pox lichen cling to the bark in rainwater channels. Barbastelle bats hibernate in crevices and under loose bark. Woodpeckers and nuthatch enlarge holes for nesting, while owls, kestrels, marsh tit and tree-creeper move in to ready-made cavities.


These rich pockets of life are a secret world, a diverse habitat teeming with insects, fungi, lichen, birds and bats. These ancient forests provide essential food and shelter for more than 2,000 of Britains invertebrate species. In Savernake Forest alone, these trees are home to nearly 120 species of lichen, more than 500 species of fungi, and other important wildlife such as the elusive white-letter hairstreak butterflies.


However, these micro-worlds are, one by one, at risk as the ancient trees of today are dying and there aren’t enough ready to replace them.


Worldwide, more than a third of primary forests were cut down between 1900 and 2015, mostly as woodlands were cleared for farming and housing, and trees were harvested for wood. The way we manage forests has changed, too, explains Paul Rutter, woodland advisor for Plantlife and project officer at Ancients of the Future. As agriculture has become more intensified, fields have been made larger with more and more hedgerows and trees squeezed out. Traditional forest management practices have largely been replaced by plantation forestry and whole-tree extraction. Ancient trees are becoming smothered by overcrowded canopies, saplings, shrubs and brambles. Add in tree diseases and a changing climate, and there are now fewer trees surviving — or being allowed to grow — into old age.


So now, the race is on to save them and their unique worlds. And the Ancients of the Future has an unusual way of going about it: they are deliberately speeding up the ageing process for some trees!


Trees reach their ancient (or senescent) phase of life at different ages. During this time, the trunk hollows, holes and cavities appear and deadwood reaches above the living canopy. It can take up to 300 years before heart-rot — the decay at the centre of an ageing tree — is established enough that insects can start moving in and laying their larvae, Rutter tells Future Planet. “It becomes a complex ecosystem. The ancient trees we have today, ones that are 300 to 900 years old — perhaps older — support an incredibly wide range of species.”


A recent study, for instance, found that oaks native to Great Britain support 2,300 other species, of which 326 are completely dependent on them. In autumn, mammals such as squirrels, badgers and deer feed on the acorns. The leaves fall to the ground and form a rich leaf mould where invertebrates including stag beetles and fungi such as the oakbug milkcap thrive. The resident insects, in turn, are a vital food source for many birds and bats.


Alice Parfitt, conservation officer at Buglife and an Ancients of the Future project officer, says, “Invertebrates that rely on these habitats provide all sorts of services such as pollination or processing the decay of materials. The really rare invertebrates we’re looking at in this project — we don’t know what they do. We don’t know enough about them.”


Stephanie Skipp, a PhD student at University of East London, is investigating beetles that live in decaying wood. “Beetles perform an overwhelming number of services throughout different ecosystems,” she says, “and the presence of deadwood beetles is vital for maintaining woodland health.”


Many deadwood beetle species recycle the nutrients of woodlands. Working alongside fungi, bacteria and invertebrates, they break down dead wood and return the nutrients to the soil. The larvae of some deadwood beetles are predators to other insects and help restrict the numbers of potential pests. Others are pollinators. “With current trends towards general invertebrate decline, we need to support as many pollinators as possible,” says Skipp.

“For centuries, trees have been pollarded — cut and allowed to regrow. This encourages new growth and was used to produce fodder for livestock and timber,” says Rutter. “The trees grew hollow inside and we’ve now found they are rich habitats for some very demanding species of beetle and other insects. Veteranisation is based on this idea.”


Veteranisation is the practice of damaging younger trees to initiate decay sooner than would occur naturally. It’s hoped that habitats usually seen in older trees will begin to develop much earlier. Veteranisation is not new, explains Rutter, but it’s not well documented and only recently has research been initiated to monitor its progress.


An international trial, begun in 2012 and set over 20 sites in Sweden, England and Norway, is evaluating the veteranisation of almost 1,000 oak trees. The methods applied include creating woodpecker-like holes, breaking or ringbarking lower branches or the trunk to mimic damage from animals such as deer or horses, and creating nest boxes for birds and bats. The project doesn’t end until 2037, so results are only partial. “The signs are very promising,” says Rutter. “Most of the trees are responding well, healing and continuing to grow. Birds, bats and insects have all been found living in the artificially created niches.”


Ancients of the Future has been trialling these same methods on beech and oak trees. Rutter says after two years cavities are starting to appear. “Normally, you’d have to wait for a lightning strike or a limb falling off for the decaying process to start. That can take hundreds of years. These are vigorous, young trees and niches are already beginning to develop.”


The violet click beetle, found at just three sites in Britain, is the main target of Skipp’s work. They require wet wood mould at the base of beech trees. Skipp has been installing beetle boxes, which are designed to mimic hollows that form at the base of ancient trees. The boxes have an entrance at ground level and are filled with decaying wood, similar to the nutrient-rich wood mould that you might find naturally. Parfitt adds, “Invertebrates need deadwood in different tree species and in different forms. So, it’s important to veteranise the trees in different ways.” To that end, scientists have been exploring another method: inoculating young trees with fungi could accelerate the ageing process even more.


Neither plant nor animal, fungi are in a class of their own and are found all over the world, but, still, we know so little about them. As of today, 148,000 species have been identified but scientists believe more than 90% of species remain unknown. Fungi decompose dead material into the building blocks of new soil and can also break down living material too, including trees. They are the main drivers of wood decay, and a crucial resource for many invertebrates is a living tree with columns of fungal decay in the heartwood.

Heart-rot fungi only move in when trees are mature, feeding on the dead wood at the centre of the tree. When holes begin to form, the wood softens then insects and other species such as woodpeckers are able to excavate it further. Over time, a hollow forms and the cavity floor is lined with wood mould, a rich soil-like mulch. “Heart-rot species are key,” says Rutter. “These fungi are able to break down the lignin, the very hard part of the wood that is normally incredibly indigestible. Many heart-rot fungi happily eat the central dead wood without harming the living tissue on the outside and can co-exist with a tree for 600 or 700 years. We want a tree to live a long time so the habitats can continue for as long as possible.”


To mimic this process in younger trees, Ancients of the Future is growing heart-rot fungi on blocks of wood in the lab, inserting the blocks into holes cut in young trees and recovering them with bark. They are left that way for a few years, then the blocks are removed to see if the fungi have taken hold inside the tree.


The project is not the first to try this technique; fungi inoculation has been trialled in North America where researchers found it could reduce decay time from 100 years to just three when used in combination with traditional methods.


Lynne Boddy, professor of fungal ecology at Cardiff University in Wales, explains why this new method of veteranisation may work better than previous methods: “We’re putting the fungi we want where we want it, rather than just hoping it turns up.” She says the hollowing of ancient trees by fungal decay, previously seen as detrimental, is a natural part of the ageing process and can even prolong the lives of trees, feeding them nutrients from the inside.


Boddy’s team has been using a new, minimally damaging DNA sampling technique to analyse the inoculated wood. It means researchers are able to take a much smaller sample from the tree and get much more detailed information. “Fungi are all around us,” she notes. “Inside every tree trunk, every leaf, every stem, every bit of plant that’s decaying on the floor, in the soil beneath our feet. But we can’t see them, which makes studying them very difficult. Now that we can extract DNA we can see exactly what’s there.”


Boddy is optimistic that fungal inoculation could help speed up the ageing process, and therefore bridge the gap between the ancient trees of today and those of the future. But it’s still too soon to tell. We still know very little about heart-rot — how fungi get inside a tree in the first place, how their communities change over time or how that affects decay.


Apart from the science, there are some more immediate steps. The Ancient Tree Forum is offering landowners advice on how to care for their ancient trees: putting up barriers to protect them from livestock; clearing nearby vegetation competing for light; creating a root protection zone; and propping up heavy limbs or bracing ageing trunks. And the Woodland Trust wants all ancient trees to receive full legal protection to prevent further loss, and for the government to enforce urban development policies that prevent encroachment on ancient woodlands.


“We need to think beyond our own lifetimes and look after the trees we’ve got now, to give them a chance to grow into ancients,” says Rutter. “Trees are fragile, complex chemical factories and major hubs for biodiversity. Without them, many species won’t survive.”


BOTTOM Doing controlled damage to a young tree could help it become a thriving home for more wildlife

PHOTO Matt Wainhouse

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