There are more efficient natural carbon sequesters than trees. This is especially true when one considers the slow growth rate of even poplars. What we need is a "crop" that can quickly sequester carbon and also serve as an energy source.

Industrial hemp does both, plus so much more. It can replace most cotton and corn in garment and biodiesel production and still leave enough to supplement cattle feed. Most of the US is able to produce bi-annual harvests of hemp and each will sequester as much carbon as an equal acreage of trees. It needs no artificial fertilizer, its water usage is extremely lower than anything it would replace and very few bugs like it, so we could cut way back on chemicals introduced into our soil.