I have no clue at all where that's coming from...
Actually the trees are definitely absorbing pollution: https://www.dropbox.com/s/3hcotji3czwivls/pollution.png?raw=1
i also checked every tree thats in the game after data-final-fixes, but they all have negative values (just like the vanilla trees)
I assume it has something to do with the trees getting destroyed and recreated when they grow
The tree count in the gui also seems off, i tested it with over 100 trees but the pollution gui showed "3"