Trees in stages of dormancy are actually still growing and require some nourishment. Remove covers of moss from potted trees. Excess moss will harbor insects and disease. Excess moss holds moisture at dangerously high levels. Removal will encourage better drainage, an increase of beneficial oxygen to the roots and warm the root ball.
Winter is a good time to nursery hop - looking for new material to train and enjoy next year.
Look for plants that did not sell during this past year. They may often be bargains. This is also a good time to add to your rock collection. Look for pieces of rock to place under or around exposed roots or deformed trunks to disguise these defects and add to the natural effects of your bonsai plantings. This time of year is the best to read, study and learn about bonsai.