From Alpha Centauri Wiki
Forest squares provide one nutrient, two minerals, and one energy. Forest square production can be improved by constructing Tree Farms and Hybrid Forests.
Forests override the rockiness and rainfall of the underlying terrain, though any energy income is added normally. They also cannot co-exist with any resource enhancing terraforming option, though rivers can still be run through them.
Forests provide a 50% defensive bonus and will sometimes expand into adjacent non-rocky squares, even driving out fungus.
Forests can be harvested for 5 minerals by replacing the forest with another terraforming option, such as a Farm.