— It's that time of year when even environmentalists committed to saving trees proudly display a massive tree carcass in the living room, bejeweled and topped with a star. American cities are rarely greener than during Christmastime, when every other street corner can seem to be occupied by a tree peddler.

Christmas trees play into a wider debate among environmentalists: Are tree farms better or worse at carbon sequestration than untouched forests?

The pro-tree-farm argument goes like this: When you plant a tree, it goes from seedling to full-grown plant by rapidly extracting carbon from the atmosphere, including carbon that humans have emitted by burning fossil fuels and raising cattle. (When a climatologist looks at a tree, he sees a leafy pillar of solidified greenhouse gases.) Once the tree reaches maturity, though, it slows its consumption of carbon. By way of comparison, think of the appetites of a growing teenager and a senior citizen. When you're done growing, you stop consuming as many calories. The best move, according to some tree-farm advocates, is to replace the mature tree with a new sapling and start the growth process over again.

Tree farmers have been making this claim for more than two decades, but many climate experts think it's bunk. The most obvious objection to the theory is: What becomes of the trees once they're cut? According to research out of Oregon in the 1990s, 58 percent of felled trees are used for paper, mulch, firewood or other short-term purposes. In those cases, the tree's sequestered carbon quickly reenters the atmosphere after decomposing or burning. The remaining 42 percent is used in ways that keep the wood intact more than five years, such as homebuilding and furniture production. Even in those cases, though, the carbon doesn't stay sequestered forever.

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.)

If you're concerned about the impacts of your tannenbaum on global climate, consider renting a living tree that spends two to three weeks in your home over the holidays, then summers at business parks or other locales. If you're looking for a long-term relationship with a single tree, some companies will bring back the same tree year after year. You should start with something small, through. The trees grow between two and three inches per year, and your living-room ceiling probably doesn't.

In other cases, rented trees are permanently retired to a nice farm or city planter after a single Christmas with a family. Before you decide to rent, be aware that you might not get a classic Christmas variety such as the Douglas fir or Scotch pine. Many companies offer less traditional species including the small-leaf tristania. You should also seek out a local farm, minimizing the gas burned on the way from the farm to your home.