In Hawaii trees are dying at an alarming rate due to an unknown and uncharacterised disease.
Since 2010 66 million trees have been killed in the Sierra Nevadas due to an invasive pathogen called Sudden Oak Death.
In Montana Bark beetles and mountain pine beetles are killing trees at a rate 10 times higher than normal.
These are a few but not exhaustive examples. Want a better summary of the trials facing our american forests? Read about it over at the Guardian.