There is a cap on the number of Forests you can clear at any given Age. Also, the cost of clearing a Forest and the time required increases with every Forest you clear.
You can get a 10% refund upon clearing a Forest by researching chapter 1 of the ExplorationTechnology at the Library.
The time to clear the forest could be reduced by 30% at maximum by researching the Expansion Time of Emperor Moctezumain the University.
The cost to clear the forest could be reduced by 30% at maximum by researching the Expansion Cost of Empress Wu Zetian in the University.
The number of workers needed to clear the forest can be reduced from 3 to 2 by researching Empress Wu Zetian in the University.
After the 45th forest section is cleared, additional forest sections do not provide any additional XP.
Tip: Leaving one forest section uncleared keeps all animals/debris (including spawned trees) in that single section rather than all around the base. For this reason, it's a common strategy to clear all but one forest section.