why are forests called sentinels of earth?

Answer 1

Cause they keep the rain cycles going.

Answer 2

Because they raise the alarm for many things:
- logged out forest areas lowering numbers of Spotted Owls
- lost habitats in general
- less forest causing wolves and beers moving into residential areas
- Eagles nests in the forest with thin egg shells from DDT
- poor forests growth because there is less fish thus less fish being hauled from the rivers to the forest by bears and thus not feeding the trees
-pine beetle investations in the forest indicate warmer winters
- forest fires indicating drier hotter climate patterns
and so on......

Answer 3

create oxygen