Its put up by the Norwegian Embassy as a thank you for WWII (they also do one for the UK I believe) It's been a fake tree with a different theme for as long as I can remember.

It's a gift from Norway to celebrate Norwegian-US Relations.

Fun Fact: The current King of Norway, Harald V, spent the first five years of his life in Washington, D.C. with his mother and sisters while Norway was occupied by Nazi Germany during WWII. He speaks English with a slight American accent as a result.

It might be a Norwegian pine but it's an evergreen tree from Norway that is a gift to DC that they send every year.