Bonsai are typically root and top pruned every year to maintain the size and shape - think of it as living sculpture. Yours looks like it has pretty much already been developed into its basic shape, with both the top and roots needing to be worked on in order to uphold the design - again there was an image of what the plant was going to be when it was trained initially. And the upkeep on into the open ended future will be that which keeps the original concept intact. With the tray being the final container, that you will have to use root pruning and soil replacement on an annual basis to keep able to house the specimen indefinitely.

Thanks a lot for your reply.

I believe that the species is Sageretia theezans, correct me if I am wrong. Do you recommend me to prune the roots that are coming out of the bottom hole of the pot or repot in a slightly bigger container?

Again the usual intention would be that the existing container would be used indefinitely, with the size and shape of the tree being kept generally the same through the years by snipping both its top and bottom (roots) back as needed.

Try searching the web for likely phrases like "bonsai care and maintenance" for more about this.

With any luck, Al (tapla), our resident container gardening and bonsai guru, will drop in and offer his advice. But generally, the advice provided by Embothrium is pretty much right on the money.