Answer:
Let's first consider the difference between a subject and a theme. A subject is a briefly expressed topic - in this case, nature. A theme, on the other hand, is an opinion or an angle on that particular subject. It is usually expressed as a phrase or a sentence. In this case, the central theme would be the power of nature over humans and their petty interests. (Other themes could be nature vs. humanity or humankind; human intrusion on nature never comes without consequences, etc.)
Step-by-step explanation:
The power of nature is introduced in the very first sentence. The Carpathian forest on a winter night seems terrifying, and Ulrich's vigilance confirms it. Even though he has a rifle and is waiting for Georg to appear, we can't help but feel that he is an essentially helpless and fragile creature.
Later on, as the two enemies are standing face to face and quarreling over the land, nature reasserts its power as the giant branch topples them over and disables them. That's what brings them together - but not for long. Even as they reconcile, a pack of hungry wolves appears. As if the nature announces: I don't care about your greed or conflict or destructive hatred for one another - or even your newly acquired respect; both of you are interlopers on this piece of land which was never yours to begin with.