I went to a talk by forest ecologist Suzanna Simard last year. One interesting point she made is that the carbon sequestration abilities of a woodland depends on more variables than simply the coverage. If I remember rightly, mature woodlands with a diverse ecology and rich soil environment (with plenty of fungal matter) are much better at capturing CO2. Chopping down an ancient forest cannot simply be compenstated by a huge monoculture plantation somewhere else.