When I found out about the Hardy's Tree I thought: What an amazing way for the nature to express itself!! But when you go to see the Hardy's Tree you also think about Thomas Hardy and see the poetry on the image. Maybe he was trying to make the "sittuation" a little less horrid.
Before writting full time, Thomas Hardy studied architecture and during 1860s the Midland Railway was being built and the original St Pancras Churchyard was on the way of progress. The task to remove the bodies and tombs from the land was passed to Thomas and he spent many hours in Old St. Pancras Churchyard. The headstones around the Ash tree would have being placed there around this time and since them the tree has grown into what we see nowdays.