Undergrowth
Fungal blooms, vines, roots, and short bushes cover much of the ground in a forest. Undergrowth counts as difficult terrain (see page 257), provides concealment (20% miss chance), and increases the DCs of Acrobatics and Stealth checks by 2. Squares with undergrowth are often clustered together. Undergrowth and trees aren’t mutually exclusive; it’s common for a 5-foot square to have both a tree and undergrowth.
Source
Core Rulebook pg. 397
