@Animal the forest habitat opens in summer period, when the daily temperature is too high (like heatwave period), but not on a regular basis. I believe keepers check meteorological data and vegetation maintenance level daily and decide to open the habtiat or not. It seems that the zoo's female asian elephant herd are able to use the habitat about 1-2 times a week during the summer period (personal prediction based on several visits during summer).
This is such an innovative idea! Many Asian elephant exhibit around the world are often a good size, but most of time are grassy fields with minimal vegetation. This "forest habitat" is a perfect solution to providing the elephants with a proper rainforest environment while preventing any significant destruction to the trees and plants. It would be so cool to see more zoos invest in similar concepts.
@StoppableSan I believe the link below is what you mentioned. This is another interesting example for creating better elephant habitats in zoos.. Thanks for the information! Elephant Vacation Yard