Both trees, as you may know, have big "issues" when used as bonsai. Birch (almost any species) tend to drop branches indiscriminantly after you work years to build them. It's unstable as bonsai material. Still, some people put up with it.
Sycamore is a huge tree with dinner plate sized leaves--which can reduce with some effort, but they're never really tiny. Additionally, the trees don't ramify branching very well and almost always look funny as bonsai.
Brent at Evergreengardenworks.com has a couple of birch species, but read the fine print on them. "Suitable for large bonsai..."
He doesn't sell sycamore and I've not seen that species sold in any bonsai nursery. Best bet for that one is a local nursery...
Very valuble info, thank you. I'd be horrified if that happened. Is that genetic to it's related species ? I did buy a few crabs from Brent in '08 and was lucky to have him as my first online dealer. What an amazing guy. Spent $60 and got 8" of interesting trunks.
Birch are "pioneer" species. That means they grow fast and indiscriminently to pioneer areas that are only marginal for growth of other species. That means they throw out all kinds of growth and readily give it up when conditions deteriorate or they're damaged. That tendency means bad things for bonsai culture. PRuning of any sort can lead the tree to simply drop the branch you've cut back (or trunk) to begin growth elsewhere on the plant. That means if you're pruning branches it can stimulate the plant to kill the entire branch off (trunk chops on larger plants can have significant die back, sometimes to the root crown, on the trunk).
Additionally, the white bark that people want in bonsai is VERY slow to develop if at all on bonsai specimens. You usually have to start with a larger tree that already has it, but that means a BIG trunk chop and risk of trunk die back.
I have about a dozen birch i rescued that i have been chopping at and wiring to see what they can handle i now have 3 that i'm happy with the way they are going. but i have noticed this tendency for branches to die back almost randomly but i am going to perservere as there growth habit is very easy to predict and is very rapid. so ifeel that restyling if any major branches are lost should not be too difficult. I have found that going much more easy and trimming a little at a time and feed between each trimming session appears to cut down dramatically on the die back ratio. But its still early days.