Why are old growth forests considered “healthy” ecosystems?

Firstly, we need to know what a healthy ecosystem is. When an ecosystem is healthy, this would mean that it could supply all the resources that all organisms present in it and with all the resources, there is diversity. Old growth forests are considered healthy because of this reason. These forests remain untouched and had remained in providing the resources needed for the organisms present in it.

We can plant more trees and plants.