I would make an Alpha Channel of the tree pic, then using a combination of Levels and Dodge and Burn Tools (Dodge-Highlights and Burn-Shadows), make a high contrast version keeping detail in the branches. Then invert the alpha channel and click the Load as Selection button.
Hopefully your original has fewer JPG artifacts than your posted image.
Or try "refine edge" and see some tutorials on selections; especially for hair which is even worse than trees.
It certainly looks an ideal candidate for an alpha mask, but out of interest, what was the original for this image? I ask as there is a lot of noise in the sky, and being grey scale, it doesn't respond well to noise reduction. You are still going to get a cracking mask out of it, and I don't suppose total accuracy is crucial, but in a situation where accuracy was important, I'd start with a full bit depth image and make it as clean as possible before making the mask.