I have a row of 4 poplar trees in my front yard, and the 2 closest to the street are dead or dying. One is all the way gone, the other has just a couple of branches with leaves left. I'm wondering how important it is to remove the dead trees - is there a significant threat of them falling and should this be done right away? Is there a disease common to poplars that I should be worried about the other 2 trees? Let me know if there's any other information I can provide that would be helpful. Thank you!