In 2010 anthropogenic emissions (not including land use change) were approximately 9167 million metric tonnes. 13 lbs is 5.896 kg, so you need 169.6 trees per metric tonne of emissions. Therefore to take up all of the emissions from 2010 you would need 1,554,723,200,000. A mature forest has only about 100 trees per acre, so you would need 15,547,232,000 acres of mature forest. An acre is 4046 square meters or 0.00404686 square km, so this equates to an area of 62,917,471 square miles. The surface area of the land on the planet is 148,940,000 km2, so we could in principle solve the global warming problem by covering a bit of 42% of the Earths landmass with forests.

However, this assumes that the 13 pounds of carbon figure is for mature forests, rather than for growing trees, as Simon W mentions (which would only provide a temporary fix, rather than a permanent solution). There is also the problem that forested land is likely to have a lower albedo than the land surface that it covers, and hence the planet will reflect less sunlight back into space which would lead to some extra warming, so we would also need to compensate for that somehow. I rather doubt 42% of the land surface is fit suitable for forests anyway, the continents have large bands either side of the equator that are generally too arid, for a start, and the regions close to the poles are too cold.

In short, it isn't going to work, even with the most generous assumptions about forest CO2 exchanges (unless of course I have made an arithmetic error, which is definitely a possibility).