Hmmm... I've been singing this one for my own pleasure for more years than I care to remember. I'd always assumed that the Easter tree mentioned in the last verse was the first one - ie the Cross, and that the reference to it being red with human blood was reference to the many executions/deaths resulting from religious conflict and disagreement/antagonism down the centuries(eg Protestant v Roman Catholic etc).

Incidentally, I'd always thought that the start of the second line in the third verse was "Four strong nails" (eg for wrists and ankles) .... but then I was learning it from June Tabor's rendition on LP back in the 70's and have never seen the words in print.

I await Dave's comments with interest :o)