Actually the tree situation in the US is a bit scary. Way back in the early last century there was the chestnut blight that pretty well wiped out that species (there have been some resistant variations found now making a bit of a comeback). Dutch elm disease has seriously assaulted the American elm which used to line many town streets. And now in addition to the ash problem, there is anthracnose fungus killing off the native dogwoods, woolly adelgid seriously reducing hemlock stands and there are reports of "sudden oak death" in parts of the country. (See evolution at work!) There is also the Asian longhorned beetle, an "import" lurking here and there infesting hardwoods.