Port. afra drops leaves from too much water in particular, but also from lower light levels...
and from too little water. So I think you're seeing a compounding of symptoms.

Misting will not accomplish anything with a Port. afra. Water thoroughly when the leaves
begin to soften slightly. It is not a Jade (Crassula ovata), which can be left quite dry.
It has more consistent moisture requirements.

This plant is vigorous, and it will back-bud readily, as you mentioned. You can prune it
to encourage even more back-budding. But if you don't change the other conditions, the
new growth will continue to fall off.

What is the pot and soil like?

Josh

The medium that it's planted in is rocky, has fine bark, and a mix of small stones for drainage. As it is still pretty young and the medium hasn't really broken down yet, I don't think it needs to be repotted yet. I should clarify that the reason I mist is to dampen the small amount of moss that is in the pot for aesthetic purposes.
I just wanted to make sure about the trimming even though p. afra is a quick grower compared to most bonsai.

I think I found out what's wrong with the little guy. I think my p. afra has a case of thrips palmi. I haven't seen them on the leaves, but when I watered I saw juveniles running around in the soil. I need to figure out what kills thrips.

Can you post a picture of these thrips?
Normally I'd recommend just changing the soil...but your mix sounds very gritty already.

Also, Port. afras don't need humidity or humidity trays.
They prefer/do best in a porous, gritty mix that is watered consistently.
You might have to choose the moss or the Port. afra - decorative moss is typically
added before a bonsai is shown, but grown separately throughout much of the year.
Moss that develops naturally is different, of course...the assumption is that the moss
developed with current watering conditions and, therefore, shouldn't need additional mist.

If the Port. has access to proper sunlight and air circulation, it should
improve dramatically.

Josh

Hey! Thanks for the response. As the juvenile thrips are less than .5 mm and move very quickly through the soil, I can't post a picture of my own, but here's a link to a magnified picture of one.

Here is a link that might be useful: {{gwi:14490}}

Sorry greenman, forgot to respond to the whole message. The moss is very fine, and as far as I know, occurred naturally and was sold with the bonsai that way. It's not sphagnum like in flower arrangements, but rather like a fine carpet of bryophyte moss. I'm pretty sure though, that the moss is dead now. Maybe it didn't like my water, and maybe the bonsai didn't either. Maybe in looking for thrips in the moss, I've killed the moss, which is sad, but not terrible.
The water bowl/humidity tray thingy is part of the pot that it came in, maybe it's a decorative lake thing? I've experimented having it empty and full but you're right, it makes no discernible difference to the moss or tree.
Here's an extreme closeup of the type of moss:

Here is a link that might be useful: {{gwi:14492}}

can anyone give me styling/pruning tips please? i think this tree has great potential as a bonsai and i dont want to ruin it!

WOW what a lovely tree and certainly lots of bonsai potential. Eagerly waiting for the gurus to respond...!