I posted on a data center topping-out ceremony I attended and got a chance to write on topping-out beam.
Facebook just posted their own pictures of their topping out ceremony on Nov 1.
Part of the ceremony is the tree.
Here is what I found regarding the tree tradition.
An interesting piece of trivia is the symbolism for the Christmas tree.
The custom of decorating the uppermost point of the structure with an evergreen tree is a tradition that predates the structural-steel industry in America by hundreds of years and has old Northern European roots. Although the topping out tree has ancient roots there is no consensus among modern ironworkers as to what exactly the tree symbolizes, or when and how it came to be used by the ironworkers. According to The Ironworker, the union's official publication, "for some the evergreen tree symbolizes that the job went up without a loss of life, while for others it's a good luck charm for the future occupants"(1984:11). Other accounts attribute the tree as signifying simply that "we [ironworkers] did it" (Kodish, 1989:2).
Little scholarship has been published on this custom. Most of what has been published has appeared in newspapers, popular magazines and engineering trade journals. One can get a feel for the age and scope of such tree rituals from James Frazer who discusses tree worship extensively in The Golden Bough. (Indeed, the title of the book itself is an allusion to tree worship.) For example, in Chapter Ten, "Relics of Tree-Worship in Modern Europe," Frazer reports that it was common practice in spring or early summer for the people to go into the woods and cut branches and fasten them to every house (1922:139). Frazer further remarks, "The intention of these customs is to bring home to the village, and to each house, the blessings which the tree-spirit has in its power to bestow" (1922: 139). The evergreen tree's ability to survive the harsh Northern European winter must have made it a powerful life-affirming symbol.