What a fun puzzle. Completed the grid without help and changed the 4 letters. Mis-counted a couple of times before finally adding the correct thing to the grid.
Never heard of the situation and found researching it interesting.
Could someone kindly confirm that the unchecked cell in the clue 'Climbing hills to protect eastern tree' is M? I can't parse it but can't think of any other tree other than the one ending in M.
This puzzle was more my standard - challenging (for me!) yet not so tough that solving is a chore. My continued admiration for those of you who regularly solve the difficult ones.