I walk around Portland constantly looking at lichens and I've never seen this one before. I'm struggling with an ID, any suggestions would be greatly appreciated.

Found on the bark of a deciduous tree (American Elm?) in the middle of Southeast Portland, OR. The color is more green, and less pale/yellow than it looks in these photos. Definitely has a holdfast. About 10-12 mm in diameter. K-. 

Edited to add: the medulla is white, and the cross section of one of the branches is kind of slightly flattened. 

This is my first post to the forum. Hi fellow lichen-obsessed humans!

There isn't a central cord, it's more bushy. Although the inside of the branches and holdfast are white. I just uploaded a few more photos.

When I first saw it I thought it might be Letharia because of the color and structure. But the location and substrate don't match. 

It doesn't have differentiated upper and lower surfaces and the branching is three dimensional.

I've personally never seen a sample like this in Portland, and I walk a few hours everyday, always on the lookout for lichens. I haven't seen a new-to-me lichen in Portland for a long time. 

I'll look into mesomorpha, thank you.

I thought it was Letharia too, at first! I just collected my first Letharia samples from Wallowa lake, so I've got them on my mind.