One of the significant characteristics of old growth forests is that
we can better appreciate things that are older
some organisms only survive in the complex ecosystems of old growth forests
taller trees have more leaves for photosynthesis
they are home to animals that are older

Typically, forests exists in order to balance the ecosystem and also serves as the habitat of a variety of organisms. In this case, one of the significant characteristics of old growth forests is that, some organisms can only survive in the complex ecosystems of old growth forests. Answer for this would be the second option.

Maybe haha