Ash Dieback

A fungal disease (Hymenoscyphus fraxineus, "Chalara") that blocks the tree's water transport and kills branches from the top down. It arrived in the UK in 2012 and is now everywhere. Across North Wales most mature ash is showing some level of dieback, and around 80% of affected trees will eventually die.

Healthy ash is one of the most flexible hardwoods, it bends and absorbs shock. Dieback ash loses that flexibility and goes brittle and unpredictable. Limbs can drop without warning, sometimes in completely still conditions. That's the opposite of how most failing trees behave, you can't read it by watching the weather. It also makes the tree riskier to climb, so advanced cases are often best worked from a cherry picker (which needs IPAF qualification to operate, something I have???).

monitor (fine if minimal and away from anything that matters), crown reduction (can buy 2–3 years on an earlier-stage tree, not a cure), removal (the usual end point once it's significantly affected and near anything that could be damaged), and replacement. I'll always suggest a suitable native to plant instead.

If a tree on your land fails and injures someone or damages property, you can be liable if you knew, or should have known, about the risk. Ash dieback is now well enough documented that "I didn't realise" isn't a defence. If your ash overhangs a road, path, neighbour or your own home, it's worth getting it inspected, an assessment costs a fraction of a failure.