The fourth part contains trees that have a low APTI but are indicators of pollution levels.
(The fourth part; contains; trees that have a low APTI but are indicators of pollution levels) (trees; have; a low APTI) (trees; are; indicators of pollution levels)
The fourth part trees trees
contains have are
trees that have a low APTI but are indicators of pollution levels
a low APTI
indicators of pollution levels