。
There wasn't a single tree in sight.
Literal
Tree [topic-は] one-counter [also-も] could-not-see.
一本も…ない ('not even a single [long thing]') is the standard 'not even one' construction using a counter + も + negative. The counter 本 is used for long, thin objects, which covers trees. Strong universal negation: 'not a single one' rather than just 'none.'