Trees are susceptible to fire until they are 15 to 20 years old and have developed bark thick enough to be fire resistant.
(Trees; are; susceptible to fire; T:until they are 15 to 20 years old and have developed bark thick enough to be fire resistant) (they; are; 15 to 20 years old) (bark thick enough; to be; fire resistant)
Trees they bark thick enough
are are to be
susceptible to fire T:until they are 15 to 20 years old and have developed bark thick enough to be fire resistant
15 to 20 years old
fire resistant