Ash should work well. Stay away from walnut and its relatives. They produce chemicals through the roots to reduce plant growth in the area. I have phals mounted on gum doing well.
looking at the branch you will need to worry about borers and other critters. Some of the OB member bake their wild wood in the oven for awhile. Others soak in stuff but I question whether that will work.
I have very little experience with the water tolerance of ash. That becomes a problem down the line.
Nick
Almost all natural materials used for mounts will disintegrate in time, the idea is to pick one that lasts longest. I suspect it is a big mess when you have to remount a large plant.
Nick
that looks a lot like the popash tree found in the fakahatchee strand, where it spends a lot of time wet and hosting a lot of orchids. The ghost orchid in particular love it. I wish I had one in my yard, it would be INFESTED with orchids.
Nice textured bark...the ash trees around here all have very smooth bark...which dries out too quickly in our dry climate.
My vote would be to leave the branch on the tree if at all possible and mount orchids on it like Swamper mentioned. Well, depends on where you live. If you live in zone 8 or above, I would try to mount a test orchid on it to see how it responds to the wood. You should do that even if decide to take the branch off.