Mike McDonald, business manager for Ironworkers Local Union 27, says, “topping out the steel” is an “ancient” tradition.

The final piece of steel is placed in the construction of Dry Fork Station, in what is called “topping out the steel.”
Doug Ramsey, Basin Electric construction manager at Dry Fork Station, said placing a Christmas tree and an American flag on the last piece of steel is a tradition meant to bring good luck. While the initial reason a Christmas tree is used in the ceremony is not clear, Ramsey said he believes it traces back to Norway, when a tree was placed at the top of a ship when construction was complete.
Check out another photo from the ceremony by clicking on this link: Trip to the top at Dry Fork Station

I wonder where they got the Christmas tree in June?
Great question! I wonder now, too. I’m gonna try to find out.
Don’t ask! Won’t tell!
Wyoming has a lot of pine trees everywhere. That’s really the only tree that grows there.
My husband was the lead connector for this project hanging iron all day everyday.
Kristin, Thanks for the comment! The thing with pine trees is I’ve seen them used in topping out ceremonies in North Dakota, too. I don’t think I ever did come up with a good answer on why it’s done, other than tradition.
That’s neat to know about your husband. Who did/does he work for, and is he building another power plant now?