You're lucky she's not aggressive AT ALL, because this is the exact foundation for fear biters. This dog needs to get out all of the time, and on leash with you showing her the world is not such a scary place. Tied to a tree watching life is no way to live. Get her out and make her deal with all of this. Take her everywhere, and ignore her when she cowers. If something spooks her walk her back and forth by it 20 times with a relaxed matter of fact demeanor. When she stops trying to avoid, praise the crap out of her and move on to the next thing. Enlist your big family to help you at these events, and tell them to completely ignore her at first. Walk her through all the people on leash over and over and over and over and over... well you get the idea. She is scared and she needs you to show her how to act. The tree is not teaching her anything but let her fester in her own fear. Everntually, then you can move on to your family feeding her treats.