I don't know how modded trees work. Is its absorption going to increase as it matures or not at all?

Yes the trees have a positive effect on the pollution and bigger trees filter more than smaller ones.

I can confirm pollution is reduced. I finally learned how to debug it. From the F4 menu showing pollution values and there's 3 numbers. I'm not 100% on what they mean but it makes sense if i think of:
Current pollution / Trees and entities / Terrain. So seeing value like 4000 / -0.100 / -0.007 at chunks with trees looks right. Second number is 0 elsewhere.

It's not as effective as Air Filter mod but something. And it's a good secondary wall against biters... Planted in mass with the field. Or maybe i'm abusing it in a way it's not meant to :p