[ph0rque]

The easiest thing to do would be to plant trees in areas that have been denuded, instead of expending energy and time (e.g. money) to cut down existing trees.

Assuming there is no more (cheap) space to plant trees, replacing mature trees still accomplishes something: you have a multi-ton tree with ~half the content being carbon. As long as you don't convert the carbon back to CO2, you can use it for economic gain and keep the stored carbon out of the atmosphere: use it to make houses, furniture, bury it so it eventually becomes topsoil (where the carbon stays in the soil), etc.

I'm sure there are people experimenting with various plants that have the highest rates of CO2 removal. Besides kudzu, there's bamboo, and probably other plants. In fact, it would be a neat software algorithm that calculates a group of plants that would remove the maximum amount of CO2 for the given area.