"For the real Christmas tree, cultivation (planting, fertilizing, watering, etc.) is the largest contributor of environmental impacts, with one exception. The end-of-life phase of the real Christmas tree results in the largest contribution of greenhouse gas emissions in the real Christmas tree’s life cycle. This difference is, in part, due to modeling decisions concerning the handling of carbon sequestration in the cultivation phase and carbon release in the end-of-life stage.This calculation that an artificial tree used for more than five years has less environmental effect than a series of one-year natural trees is consistent with previous research. My discussion of studies done a few years ago, including the previous time that the American Christmas Tree Association commissioned a life-cycle analysis on this question, is here.
"For the artificial tree, the raw materials used in manufacturing, specifically polyvinylchloride followed by steel sheets, comprises the largest source of impacts in the artificial tree. Among the various life cycle phases, raw materials and transportation are seen to have largest impacts. Raw materials are primarily responsible for greenhouse gas emissions, eutrophication of water sources and use of non-renewable energy. Transportation mainly causes acidification of water, air and soil and smog in the atmosphere.
"Given the quantification of environmental impacts across both of the trees’ life cycles, a comparative assertion shows the breakeven point between the two trees is 4.7 years. That is to say an artificial tree purchased and used for at least 4.7 years demonstrates a lower contribution to environmental impact than 4.7 real Christmas trees purchased over 4.7 years."
Of course, any study like this is subject to questions and concerns. Are you buying a new tree stand each year, or reusing the same one? Does the life-cycle calculation for a natural tree take into account carbon that remains stored in the root system under the tree after the tree is cut? Bert Clegg offers a nice discussion at "The Conversation" website under the title "Don’t stress about what kind of Christmas tree to buy, but reuse artificial trees and compost natural ones" (December 11, 2018). But the WAP report and Clegg both note, if you are someone who is worried about environmental effects of your choice of tree, your own behavior has a substantial effect.
For example, if you make a special trip and drive a long way to shop for either a real or an artificial tree, that is a meaningful addition to the environmental costs in either case. If you pick up either kind of tree as part of a trip you would have made anyway--say, a commute home from work or a shopping trip that would have happened regardless, then the environmental costs is lower. (The WAP study estimates that consumers make an additional trip to pick up either kind of tree.)
When it comes to disposal, Clegg argues that if a natural tree is chipped or composted, the carbon content remains largely in the soil, but if it is burned or send to landfill, the effect on greenhouse gases is much worse. For an artificial tree, the longer you use the tree--and thus postpone disposal--the better for the environment. In addition, many cities offer facilities for recycling artificial trees, rather than just adding them to landfill, at the end of their lives.
I confess that I come from a real tree family and have always had real trees. But I do live in a part of the country where real trees are easy to grow. For the country as a whole, the National Christmas Tree Association (the trade association for real Christmas trees) finds that about 45% of the trees purchased in 2017 were artificial ones. In the holiday spirit of putting aside small differences, I take no position here on the great debates over real vs. artificial trees. Many holiday celebrations in many places have aspects of environmental wastefulness--it's part of what makes them holidays, after all. The challenge is to celebrate heartily, whole-heartedly, and also in moderation. In the broader context of how Christmas is often celebrated, Christmas trees are only a modest indulgence.