in the recent patches, planting, harvesting, replanting, harvesting, does NOTHING. The 10 shield forest harvest is a one time deal - don't waste your time.
Pollution is caused by pollution causing improvements (e.g. coal plant, factory) or by excess city population. (over 12)
These are reduced by recycling and mass transit, respectively.
Number of shields has NOTHING to do with pollution - you can have a 50 shield city at pop 12 without factory etc and there will be NO pollution.
The only extremely indirect connection is a forest may build a factory faster - but that is extremely tenuous.