The loneliest tree in the world was knocked over by a drunk driver in 1978. The new loneliest tree in the world is very close to the southernmost point of New Zealand.

Firstly, I am defining loneliness as “furthest away from another tree.” I don’t know whether trees have emotions (apparently the science is still out on that one) so I suppose we could say “most isolated tree” instead – but that doesn’t sound nearly as romantic.
For at least forty years, the Tree of Ténéré was the loneliest tree in the world. The final survivor of a stand of acacia trees in the Sahara Desert, the Tree of Ténéré survived by putting down very deep roots: a ridiculous 30 metres or more down under the desert. Once its neighbours died off, this tree was more than 400 kilometres away from any other tree.
Somehow, that didn’t stop a truck driver crashing into the tree in 1973. It sounds like a goofy comedy bit, somehow driving so badly that you hit the one and only tree within hundreds of kilometres. The rumour is that the driver was drunk, but there’s actually surprisingly little detail online about him. I hope, wherever he is, he’s not proud.
Since the demise of the Tree of Ténéré, the new titleholder of loneliest tree is a Sitka spruce tree on Campbell Island. Campbell Island is about 600km south of the major islands of New Zealand, and is uninhabited (by people). The tree was planted over a hundred years ago by a former governor of New Zealand, and it has survived pretty well so far. The nearest trees are on another uninhabited island chain 222km to the north; making it the new loneliest tree in the world.