That's what my friend Google says as well, but that wouldn't make much sense in terms of behavior assessment. Typically we would say that the the dog is wary (zurückhaltende) or a resource guarder (Ressourcenverteidigung).

Ah, just saw the rest of your comments. Yes, it's a dog, so guarding behavior as in watchfulness, barking, territorialism makes sense.

Many thanks, Tom!

There's a reason why I stay on my side of the Rostigraben...