The water level will be 12", the height of the overflow.
We've had somewhat of a change in plans. Due to concerns about the Ash branches rotting out too quickly, and honestly mostly due to my desire to have mosses growing in the tree, I am scrapping the Ash branches.
We will build the tree with an internal irrigation system. I will try to make it as similar in appearance to a mangrove as I can. I will make it out of PVC, 1/4" copper tubing (it's for refrigerator ice makers, so water safe), floral foam, Great Stuff, and two to three shades of tinted concrete painted on.
This tree will be quite a project, but could be pretty darn cool if we can figure it out.
I think concern about the branches rotting is entirely valid, but I'd just suggest you go with bigger branches. It will still decay, but will take much longer, and in the meantime supply nutrients to the plants. If you want to slow the decay even further, you could soak it with thinned epoxy resin or something.
I've got a chunk of driftwood sitting in a pot that I've filled with plants, and it gets watered almost every day, so the wood never quite dries out. There has been some notable decomposition, but the plants look great, and I don't think the structure has been severely compromised.
Heres a pic, from about a year ago (attachment)
anyways, I've seen some great builds done with pvc/great stuff, but as people said, I'd avoid the copper. I think you'll get all the structure you need from the pvc.