Construction workers' flags and trees

I’ve noticed that at many construction sites, the structures are topped by a flag and a tree. I understand the flag, but what about the tree? Or are they just saying isn’t it cool to have a tree this high?

According to this page, the tree is for good luck.

Additional: the tree is apparently an ironworker’s tradition from Norway, originating around 1898.

Another cite.

This kind of ceremony looks related to the German Richtfest (a ceremony marking the completion of the roof beams, with the house being topped by a crown like the one lifted by crane in this picture, a speech by the senior carpenter/building foreman and drinks all round). This topping-out ceremonies might well be common to a lot of countries (like Norway, as mentioned) because it’s a point where part of the building crafts’ work is done.