The Poetry Archive has got one of those tag-clouds so people can find poems by theme. From which we learn that the most popular subject in modern poetry is death. Woohoo. Potentially more cheerfully, death is closely followed by childhood, animals and love. After that come poetry, language and war. And so on. You can see for yourself.
It looks like ‘nature’ would score very high if all the appropriate categories were lumped together.
Not the most statistically rigorous experiment of all time (selection bias, much?), but kind of interesting.