[southerndrift]

You can also look at this the other way round:

By buying a Christmas tree, you create a market for those trees which makes people plant them. This leads to several generations of trees growing at any time, so that there is a net-reduction of CO2.

[notabee]

To make the reduction permanent however, the ultimate fate of all those Christmas trees is important. That market also entails more (fuel burning) economic activity surrounding Christmas and transportation of those trees. If they are disposed of in such a way as to be decomposed by microbes, much of the carbon returns to the environment. It's very hard to store carbon in a more permanent way than deep underground, which was where it was before we foolishly started pulling it all up and burning it.