Their research is severely flawed.
Here are a list of problems off the top of my head:
1. Many forests are replaced with genetically altered "versions" of the trees that were cut down.
2. Many forests are replaced with 95+% of the same breed of tree - they cut down a mix of douglas fir, alder, cedar, and hemlock, and they replace it with highly concentrated forests full of douglas fir only.
3. How do they know that the pollution they're measuring isn't from the tons of exhaust all the logging equipment puts out, or the oil that wa dripped on the ground and now the trees are eating, or the dead animals that were caused by all the logging in the first place? Note they said "newly planted" forests, which, around here in WA anyway, generally means it was just clearcut and replanted.
Even they admit that it's the man-made forests that are causing the problems, not individuals planting trees in their gardens.