If you are walking through an old growth forest and you see a Blue 'S' painted or spray painted onto a tree trunk, it means you have entered a tree spiked area
The Blue S is a symbol tree spikers use to warn that it is hazardous to cut down trees in the area or that the surrounding trees will be difficult to process due to embedded contaminants.
Normally the marking occurs in conjunction with notification to newspapers and logging companies with interests in the area will already know, but you might want to call them yourself if you feel like being a pro-active member of society