Discouraging territorial aggression

Mama_Deb

I'm new to these boards and would love some advice. We've got a new shelter dog, about a year-old lab/border collie/who knows what? mix. He is beginning to growl and bark aggressively at people who approach "his" territory -- house, yard or car, depending on where he is. How can I determine if this is dominance or fear-based, and how can we discourage it? He doesn't show this behavior on walks or with possessions.

Thanks!