Exactly. The World Tree, Yggdrasil is what is looks like. Though the picture is not clear, you can see the green parts. The Legendary Pokemon here, too, seem to be based off Norse Mythology.
This is similar to Sinnoh's Mount Coronet, the entrance to other worlds.
That is, Dialga, Palkia and Giratina's Dimensions.
I am pretty sure that Xerneas and Yveltal are based on two beings from Norse Mythology. Yggdrasil has a bird on the top, which is blind. Notice that Yveltal has blue eyes with white pupils... which makes it look blinder than an usual Pokemon...
The trunk is home to four deer called Dáinn, Dvalin, Duneyrr and Duraþrór. It is said that their horns shape the upper branches of Yggdrasil, so that they can also feel like living on the top, they are probably jealous... See that Xerneas's horns are shaped like branches... These deers also had gems of the colours purple, blue, red and yellow, which Xerneas has on its horns...
Also, there is a Dragon around the World Tree... this might be the third legendary...