New forests also seem to emit significant levels of carbon dioxide, rather than only absorbing and storing it. When we plant or replant a tree farm, we turn over the soil and kill off roots and ground-level plants. That vegetation was also storing carbon, and it begins to decompose. In some cases, the dying plant matter emits more carbon dioxide than the newly planted trees extract from the atmosphere.
There has also been research suggesting that old-growth forests are more active than they appear. According to a scientific letter published in the journal Nature in 2008, forests continue to add woody matter — both new branches on existing trees and new, smaller plants — for centuries, sequestering carbon from the atmosphere in the process. The net carbon budget — the amount of carbon sequestered minus the carbon emitted through decomposition of downed plant matter — is more favorable in a forest's 300th year than in its fifth year. Overall, the data seem to suggest that old-growth forests keep more carbon out of the atmosphere than high-turnover tree farms, but there is probably significant variation depending on locale and how foresters manage the stock.
This doesn't mean you should forsake a Christmas tree or turn to an artificial alternative. (Fake Christmas trees often include chemicals that are especially harmful to the environment when discarded and are responsible for more greenhouse gas emissions than natural trees.)
A few special considerations set Christmas tree farms apart from producers of trees grown for paper. Christmas tree farmers typically plant more trees than they harvest, giving the new crop a better chance at out-sequestering the ones they replaced.
Evergreens aren't the best arboreal carbon sequestration tools — that title goes to hardwood trees — so the difference in greenhouse gas emissions between a long-lived evergreen forest and a Christmas tree farm aren't likely to be significant. (Razing a hardwood forest to grow Christmas trees would be a bigger problem, but this is a relatively rare event.)