A closed terrarium environment traps moisture, warmth and generates its own humidity. So terrarium ferns have ample supply of everything they need. Plus, on a whole, ferns are low light terrarium plants. They typically grow on forest floors and under tree canopies, so they're used to bright, indirect sunlight.
Rainforests also help maintain the world’s water cycle. More than 50% of precipitation striking a rainforest is returned to the atmosphere by evapotranspiration, helping regulate healthy rainfall around the planet. Rainforests also store a considerable percentage of the world’s freshwater.