And remember, too, that the little juniper probably just left a greenhouse somewhere so it isn't used to winter temps. Trees are slower to respond to changes than we are, and the tree would probably need to be given time to acclimate.
First off, take those rocks off the top. They aren't helpful for the tree to grow. If they are glued on, that's going to prevent water from getting to the roots and oxygen also.
Another point to consider is that these little junipers are often just cuttings, not well rooted, when they are sold in massive quantities to the public. They have not been treated or grown in a way that makes them healthy. The soil is very likely poor and slow draining, the roots may be unestablished, and the tree could be on it's way to bonsai heaven before anyone can tell. (That's often why they use junipers.... like Christmas trees, they stay green for a long time before they show signs of problems)
If your little tree doesn't make it (and it's a gamble) but you want to learn about bonsai, stick around and we will help you to choose good stock, train it, keep it healthy, and enjoy a very relaxing and rewarding hobby!!
Besides making some wonderful friends.
Many of us started with these junipers, and we now have serious collections of trees in training....
Joanie