One of the largest and oldest trees in the Botanical Garden at the University of Oldenburg is sick. You could say that it is losing its battle against parasites due to old age. Klaus Reis, Technical Director of the Botanical Garden, explains why the imposing silver maple will have to be felled sooner or later.
On the side of the mighty trunk facing away from the path, Klaus Reis kneels down on the forest floor. He taps on a somewhat strange-looking piece of what looks like bark at the foot of the tree, which stretches over two roots like a small, hard canopy. Thanks to its brown colour, it looks and sounds like part of the tree. Anyone who "knocks on wood" to prevent misfortune will recognise the sound. In the case of the silver maple, misfortune can no longer be stopped.
The tree has been infested with a fungus for several years and the supposed bark, which the technical director of the Botanical Garden can knock on as if it were wood, is the fruiting body of a lacquer spore. Three years ago, the technical director of the Botanical Garden noticed the well-camouflaged fungus for the first time. Since then, he has known that a battle is raging inside the mighty tree, which is 70 years old.
"Under normal conditions, silver maple trees live to be around 100 years old," explains Prof Dr Dirk Albach, Director of the Botanical Garden. "Their wood is not as resistant as that of oaks, for example." The specimen in the North America section of the garden is therefore already a senior and one that has experienced a lot.
Injuries are gateways for fungi
The fact that the maple has suffered many injuries in its long life can be seen with the naked eye. Bulges have formed around various knotholes as they have grown over and it may have been a branch that was torn off in a storm that caused an injury to the trunk, which then became the unnoticed gateway for the lacquer fungus.
"When such a fungal infestation occurs in one place, it's usually not that bad," explains Reis. While the fungus hollows out the trunk from the inside, so to speak, by breaking down its woody material until only polystyrene-like material remains, the tree continues to grow outwards anyway. "The plant grows", Reis calls it. As long as the tree grows faster than the fungus hollows it out, it leads the fight - and remains stable. However, a tree grows less and less with increasing age.
Sonic tomography is imminent
In the meantime, Reis has noticed more fruiting bodies on the trunk of the silver maple. "The inside of the tree is covered in mycelia," he says. The visible fruiting bodies are just the tip of the proverbial iceberg. The iceberg itself is the thread-like fungal cells that are causing problems on the inside of the tree. Around two years ago, tree climbers tied the three mighty branches of the maple together with strong straps so that stronger winds would spread across the entire top and prevent individual parts of the weakened tree from falling.
A sonic tomography will soon reveal the exact state of the tree's interior. A tree expert will drive small nails into the outer sapwood layer of the silver maple. The nails will be tapped. The sound impulses generated in this way find their way through the tree to the other nails, which pick it up again with small sensors. The impulse changes depending on the condition of the tree. A computer programme can then be used to create an image of the inside of the tree.
Safety takes centre stage
This information plays a role in assessing how long the silver maple will remain in the Botanical Garden. "I am responsible for the stability of the tree," emphasises Reis. If it is seriously jeopardised, the tree will have to be felled. "Sooner or later that will happen," he says.
He doesn't like it either, but safety comes first. Visitors to the Botanical Garden could be injured if the 22-metre-high tree falls uncontrollably. There are also railway tracks and overhead lines in the immediate vicinity. They could also be damaged.
There is no cure
There is no cure for the silver maple. Previous attempts to fill tree cavities with concrete to prevent fungus from getting through are no longer relevant. As with humans, the life of a tree is finite - even if it stands in a Botanical Garden and is regularly inspected by experts. Reis, for example, checks the more than 500 trees in the Botanical Garden every one and a half years so that he can examine them during different seasons. The trained horticulturist and landscape gardener and qualified landscape gardener asks two questions in particular: How healthy does the tree look? And: Is it still stable?
If doubts arise, Reis examines a tree more closely, as he has been doing with the silver maple for the past three years. He knows that its stability is now the main issue - no matter how lush its leaves look. At some point, he will have to make the final decision and order the felling of the old silver maple. "Unfortunately, when you discover such a severe fungal infestation, you know it's the beginning of the end," says Reis.
