There seems to be a modern trend towards guerilla gardening. Bristol seems to be keeping up with the times.
The fine tree in the first photo appeared in the middle (yes - in the middle) of the junction of Clevedon Terrace and St Matthew's Road, in Kingsdown. I took this photo yesterday (Christmas Eve) at about 6:00 pm and, apparently the tree had been there the day before. The tree is quite tall and has decorations and lights.
A little research has found an article about this on The Kingsdown Conservation Group website.
The second photo shows the roundabout at the bottom of Redland Hill with its own Christmas tree. The tree is pretty small, only just taller than the bollards. It has what appear to be battery powered - see third photo where the battery boxes are clearly visible - blue LED lights (so 2008). I saw a group of people, apparently celebrating around the tree, yesterday in the late afternoon. Perhaps they were commissioning it.