Martin Frey wrote:

Oh ****. We're all doomed. I'll read the article this evening, but it
does seem odd that trees are now turning lethal when for most of
history there have been so many more of them than there are now.

Smog formation requires both VOCs and NOx; NOx ia largely man made.

It's been known since the 1980s (at least) that trees are a significant,
even dominant, source of VOCs (weighted by reactivity) in some
places (around Atlanta, for example).

The implication of this is that pollution control measures should focus
on NOx emissions from powerplants and engines rather than (just)
VOC emissions. Unfortunately NOx is harder to control.

Paul