Question

how is a forest able to function as an independent unit

Answer 1

Forests are considered as independent units due two key factors; the ability to replicate on its own and the ability to manufacture its own food.

Through the process of photosynthesis, trees within a forest can manufacture or synthesize food on a daily basis for sustenance. This they do independent of external factors-all that the forest needs, is within itself.

On the other hand, forests also can replicate own their own through such mechanisms as dispersion and cross-pollination.
 Hence, a forest can survive, grow and replicate itself without any external help. 

Answer 2

This is an answer of my book .