That's an interesting thought about the trees. It reminds me of how the Tibetan Plateau is often called the "Third Pole" because it holds so much water.

We tend to forget that no NEW WATER is ever created. All of the water around us and in us is billions of years old. Endlessly recycled and reused.

However, how that water is distributed and stored, fluctuates between the "Great Reservoirs". Which are traditionally thought of as the Ocean (Hydrosphere), the Ice (Cryosphere), and the Air (Atmosphere).

In terms of the Water Cycle the Biosphere is literally a "4th Pole".