I am in a forest 35 kilometres west of London, meeting someone who is trying to make young trees old. 鈥淥ld trees form hugely important habitats,鈥� says , a mycologist at Cardiff University, UK. 鈥淏ut in Europe in general, including in Britain, we don鈥檛 have all that many now.鈥� She is one of a handful of scientists around the world trying to do something about it.
Boddy takes me over to look at聽a聽gnarly oak, which is roughly 300聽years old. 鈥淵ou can see all these lower branches here that are starting to decay and rot,鈥� she says. 鈥淭hat鈥檚 a sign that it is a veteran.鈥�
If we could look inside the trunk, she says, we would see massive amounts of decay from fungi gradually eating the tree from within. This is called , and it is a perfectly natural process that happens to all trees in the end. It has only been studied in earnest for about a decade and its importance to forest ecosystems turns out to be immense.
Advertisement
鈥淭he centre of the tree is no longer functional to the tree,鈥� Boddy explains. 鈥淭he outside-ish regions of the trunk conduct water from the roots up to the leaves. That鈥檚 the sapwood. But in the inner regions, the heartwood, there are no longer any living cells.聽Water is not conducted there.聽So fungi can start to develop in those central regions because the wood starts to dry out.鈥�
鈥淭he fungi are just feeding on dead stuff, so it鈥檚 wrong to call them parasites,鈥� says Boddy.
Indeed, the rotting core of a tree is immensely valuable as a habitat, not just for the fungi themselves聽鈥� many of which are heart rot specialists聽鈥� but also for myriad other species. In the UK, around 1800 invertebrate species depend on rotten wood, says Boddy. Across the world, heart rot supports immense numbers of聽species, including about 1000聽birds and hundreds of other vertebrates, such as bats. The fungi are also important contributors to聽the recycling of nutrients upon which聽all ecosystems depend.
The problem is that the veterans are dying out and there aren鈥檛 enough middle-aged trees to replace them. Forestry practices over the past few centuries have taken out trees long before they reach the heart rot stage 鈥� which is聽around 300聽years old for an oak and 120 for a beech 鈥� so there is a huge gap between the ancients and the next generation. Heart rot fungi and the species that depend on them are in danger.
For now, the UK is well-endowed with old trees due to royal hunting forests, such as the New Forest.
鈥淚 think we鈥檝e probably got as many veteran trees in Britain as in the whole of the rest of Europe put together,鈥� says Boddy. But they won鈥檛 last forever. 鈥淓ventually, they will just die of old age, and in聽lots of places humans are thinking 鈥榯hese are inconvenient, we want to build here 鈥� chop them down!鈥�,鈥� she says.
Boddy takes me to see an ancient oak on its last legs. The聽heart is visible through an immense hollow and has entirely rotted away. The tree is basically a聽cylinder of narrow sapwood full of dark brown, nutrient-dense mulch. When the tree eventually dies, the聽fungi will need to find a聽new home, of which there are too聽few.
The answer, possibly, is to 鈥渧eteranise鈥� young trees. 鈥淲e try and make young trees older before their time,鈥� says Boddy. Elsewhere in Europe, veteranisation usually means tearing off branches to let the fungi in, setting fire to the base of the trunk or damaging the bark with sledgehammers. 鈥淭here is evidence that habitat is being created, but of course we don鈥檛 know whether it really mimics the聽heart rot habitat,鈥� says Boddy.
So, here in the UK and also in North America, she and others have taken a more targeted approach, deliberately infecting young trees with heart rot fungi. This is done by chainsawing four cuts in an oblong shape near the base of the聽trunk, then pushing the saw in聽sideways to pop out a block of聽wood about the size of a brick.
The block is replaced with a聽similar-sized block that has been聽dried out and exposed to heart rot fungi in the lab. Once these have been established in the聽wood, the聽block can be put into聽the聽tree and tamped in with聽fungus-rich sawdust, inoculating its heartwood. The聽hope is that the聽fungi will then聽start the rotting聽process much earlier than would have occurred naturally.
Boddy takes me to a section of the forest where her experiment is ongoing. The landowner has designated it a perpetual forest, meaning it will never be felled. The beeches here are about 60 years old, far too young to have heart rot. But Boddy wants to be a rotter to them.
A few years ago, she and her team chose 60 beech trees and cut blocks out of them. They left some with a hole, while others had the wood block replaced untouched, and the rest were inoculated with one of four heart rot fungi. Uncut trees serve as controls. You can see the blocks inside the inoculated trees, though many are now being encroached by new bark growing around the scar.
There is some evidence that聽it聽is聽working. A technique called sonic tomography, which transmits sound waves through the trunk to map its density in three dimensions, can peer inside a tree without damaging it. Rotten heartwood is less dense than sapwood, so the heart rot regions show up on the tomograph. Boddy聽and her colleagues also take聽drill cores and DNA sequence them to discover which fungi are present. Both techniques have revealed some success.
But there is undisputable evidence that inoculation works. We head over to a tree that lost a huge branch in a storm this time last year. The falling bough hit a neighbouring beech and felled it. As luck would have it, the toppled tree had been inoculated with a heart rot fungus about four years previously. 鈥淏ecause it had come down, we were allowed to saw it up,鈥� says Boddy.
The tree was probed with聽sound聽waves after it fell and聽showed hints of heart rot close to the inoculation site. But聽the chopped-up trunk told a different story: the heart rot had spread several metres up the trunk. 鈥淐ertainly, the tree was changing,鈥� says Boddy.
That bodes well for the future聽of聽heart rot fungi and the聽valuable habitats they create, but we won鈥檛 know for decades whether it really works.
鈥淲e鈥檒l have to leave this for 20 or 30 years and find out what happens,鈥� says Boddy. At that point, some of the really ancient trees will have died. We can only hope that, by then, there will also be a new generation of veteranised trees ready to fill their venerable roots.
