Overcame with guilt, I set out to create a most ambitious project. I created a power switch and a simple combinator setup to disable my steam engines - one of the most polluting things I could find - as long as the accumulator still had charge. It was one of my first combinator projects too and I was very happy it worked.0.12.0:
Trees degenerate slowly when exposed to pollution at high levels.
I then placed around 2000 solar panels and accumulators. I'm not kidding. Of course I had the logistic system actually build it, but still. I also added Efficiency Modules - the green ones that reduce energy consumption and so pollution - to most everything. Around a hundred assembling machines - 4 each - even more mining drills - 3 each - and all electric melters - 2 each.
And it worked. I made it through a night without the steam engines ever having to kick in. Pollution slowly dissipated. It was a joyous occassion really.
What was an angry red cloud of pollution and a testament to my failure became a barely visible pink overlay.
But the trees never recovered and I was heartbroken. And so my simple suggestion is:
TLDR: Make trees become healthy again if pollution drops considerably in their area.