Ah yeah, that's actually not so bizarre when you know the reasons behind it. Still extremely wrong but understandable at least.
So yew trees are weird. They are extremely long lived with basically no known upper limit to their age. They do this by simply being extremely good at not dying like other trees do.
When a normal tree gets to an old age what usually happens is a fungus gets into their heartwood and takes hold. Their internal, dead wood rots away and they hollow out, lose structural support and collapse. Depending on the species this process can take decades or a good few centuries or so.
While yew trees do hollow out in this way they simply keep going afterwards. A ring shaped yew tree with most of its trunk missing is actually just middle aged and the most ancient yews get even weirder than that.
Wikipedia has this image of a Scottish yew where the start of this hollowing process can be seen. To be clear - for most tree species this would already have been fatal.
The thing is seeing a very old yew in this condition looks wrong to a tree surgeon, it's like the tree is constantly on the verge of death. So, if it's a well loved tree you try and do what you can to stop it from falling apart entirely.
A hundred years ago people tried all sorts of things like chaining up branches and also, yes, plugging the hollowed trunk with concrete. We know better nowadays.
Funnily enough there are even yews that survived this treatment and are still alive today.
This is a picture of the Tisbury yew in 1998 from the Ancient Yew Group, barely a minute ago from the tree's perspective.
Yews are fascinating plants with roots in European culture as ancient as the trees themselves. A few individual specimen trees are even estimated to be around five thousand years old - literally prehistoric in age.
Oh also they do weird things with sex as well sometimes. One of the oldest UK trees, the Fortingall yew appears to partially be turning from male to female on one side. It'll be interesting to see what becomes of it in the next few centuries of its life.
Sorry if this is all stuff you already know, I couldn't resist a chance to infodump about one of my favourite species.
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