Growers like Mr. Cheek say there is also reason to be optimistic about sales this year, since there has been some good weather on key tree-buying weekends this month and an apparent fondness among 20-somethings to buy real trees, even if they are table-top size.

WSJ: Fir Real? Christmas Trees in Crisis as Americans Flock to Fakes

The head-tracking system, which was developed by the Fraunhofer Heinrich-Hertz Institute in Germany, works by calculating how far away a head is, based on its apparent size (the direction is obvious, of course).

ECONOMIST: Monitor