Ugh. The dreaded poplar tree. I don't know about everyone else's experiences, but the only trees I lose around my place are the big poplars. They grow like weeds and seem to blow down very easily. I am no pond or tree expert, but I can tell you I have cut plenty of large fallen poplars over the last twelve years. I think I would take my chances and cut the trees down - even the big ones - before they get bigger. Either way it is a gamble I guess.