It's really not. If you want a citation for "growing trees takes CO2 out of the atmosphere", a high school bio book will suffice -- the carbon mass of the tree is carbon extracted from the atmosphere (and dead plant mass in soil, but the carbon there came from the atmosphere too).
If you want a citation for growing trees for timber and pulp as an effective CO2 sink, the citation is right in the grandfather post (an international climate change study by the IPCC including researchers from over a dozen countries, how much more authoritative can we get?), or just look up the paper and timber industries on wikipedia.
