The world has a lot of land, though. Before humans came along, there were roughly 6 trillion trees in the world. Now there are about 3 trillion, mainly due to forests being cut down for agriculture, wood, etc. Getting back the other 3 trillion trees would sequester an additional 75 gigatons of CO2 per year using your figure of 25kg/tree/year. Human activity emits 45gt/year according to his article.