That trees are the major tools to remove CO₂ from the air is clear, but how many new trees need to be planted to compensate all human CO₂ pollution (6 gigatons / year?)? We should work with standard tree and count a tree in the tropics double since they absorb year round. If yearly net uptake of growing trees is 20kg CO₂ in temperate zones, global average might be 30kg/year. This means that "only" 6 × 10¹² kg ÷ 30 kg = 200 × 10⁹ = 200 bn trees are needed. I say "only" because this means only 10% extra forest, while I was counting on a doubling of global tree cover is needed (and possible).
Trees follow the logistics curve for growth. More complicated, the number of trees descreases as they get larger. E.g. after a forest fire, lodgepole pine sprout at about 10 per square foot. In 10 years there is about 1 per square yard. At 30 years it's one per 6 foot square.
The fastest mass growth rate is when a given area is mostly at the 1" diameter stem. Some variation by species. Google biofuels for more info on this.
A mature forest is carbon neutral. Over the course of a year the process of decomposition equals the process of sequesterization. Most tropical rain forests are carbon neutral.
Some numbers: An immature aspen forest (3" to 6" trees)increases biomass at roughly a metric tonne per year per acre. At GT of carbon is about 1.5 GT of cellulose & lignan. So it would take about 1.5 billion acres of aspen forest per GT of carbon, or 9 billion acres for your 6 GT.
About 14 million square miles, or about 4-5 times the area of the continental U.S.
Now my numbers are from the Canadian Praire, latitude 54. Growing season about 115 days. While longer growing seasons will help, remember that a lot of those days here have 18 hours of sun, so it doesn't help all that much. But you might gain, say 30% that way.