This is the 5198th most frequent Polish word.
pustkowiu
"Wilderness" or "desolation."
The word 'pustkowiu' is used in the locative case (na pustkowiu) to indicate the location where the subject found the tree.
Na pustkowiu znalazłem samotne drzewo.
On the wasteland, I found a solitary tree.
Here, 'pustkowiu' is again in the locative case (na pustkowiu), indicating the setting of the reflections.
Rozmyślałem o życiu na pustkowiu.
I was reflecting on life in the wasteland.
In this sentence, 'pustkowiu' is used locatively to describe where the view appeared.