A few other things to avoid sitting on or against:

In cattle country, what are euphemistically known as "cow pies."

Trees with lots of fresh pitch in their lower extremities. It is possible to remove it from clothing (saturate with cooking oil first and then use hot water and lots of strong detergent), but it's a lot of work!

In semi-desert country, ant hills at the base of the tree. The little critters resent having their passage blocked, and their bites are painful.

Look carefully before sitting!