But it's also worth pointing out a lot of this is the result of travel or remote trade. Stopping this sort of thing doesn't look anything like "stop burning so many fossil fuels" or "recycle more", it looks like telling humans they aren't allowed to stray more than a few dozenish miles from where they live. It's not really clear how to prevent this. We have mitigation policies in place, yes, but in the biological world 99.9999% enforcement can be as good as 0% enforcement and that's a hard bar.

Heck, we can't even contain the spread of human diseases all that effectively, and that is something that you'd think we have every motivation to do.

"The American chestnut tree reigned over 200 million acres of eastern woodlands from Maine to Florida, and from the Piedmont plateau in the Carolinas west to the Ohio Valley, until succumbing to a lethal fungus infestation, known as the chestnut blight, during the first half of the 20th century. An estimated 4 billion American chestnuts, up to 1/4 of the hardwood tree population, grew within this range."

http://www.acf.org/range_close.php

They're trying to reintroduce blight resistant strains by cross breading with Chinese trees.

http://www.acf.org/r_r.php

I'm trying to think of specific instances of pathogen based flora destruction, and don't know if I could mention many offhand. In California specifically, the replacement of wild bunchgrasses with European annuals (the "Golden Hills" are very much a modern phenomenon) would be one.

More generally, though, the motto "forest preceed us, deserts follow" has applied to human settlement for millennia. The Cedars of Lebanon and Pines of Rome aren't just allegorical. The former are where the Egyptians secured their lumber (painfully scarce along the Nile), the latter previously surrounded the Eternal City.

I'd be interested to know the history of forests in India, Persia, and China, other great centers of ancient civilisation.