Dang man! This is one great Begonia! How come I didn't add it sooner?? Each week it has more flowers so I was strongly tempted to wait another week to share a photo of it. But that's what I've been doing for the past several months...
It's not an epiphyte (that I know of) but it sure is happy on the tree. It's growing on the shady side of the tree...more than one story up. I created a pocket for it using coco fiber basket liner. The pocket is stuffed with a couple handfulls of New Zealand Sphagnum.
It's relatively drought tolerant. Recently I've been watering every other night. However, during our last hotspell I missed a watering and the next day the boliviensis was slightly wilty. But it perked up just fine after I watered that night.
Growing immediately beneath the boliviensis is an orchid... Cyrtochilum serratum. The serratum is doing really well and has beautiful pseudobulbs...but it has yet to bloom. Even though it has the opportunity to crawl towards the sunny side of the tree, it seems to just want to stay on the shady side. Maybe the recent winters haven't been cold enough to encourage it to bloom.
Beneath the orchid is an Echeveria that grew from seeds that I sowed on the tree last year. You can see another one growing in the root basket of the Ansellia africana (http://www.flickr.com/photos/epiphyte78/7586375116/in/photostream). I had no idea that Echeverias were so easy to grow from seed.