Unit 4 - The Role of Trees in the Forest Ecosystem

Lesson 2: Water Cycling


As we studied earlier, trees satisfy their internal water needs through transpiration, a process that essentially pumps water from the soil to the leaves.

  • As trees transpire, they release excess water into the air.

  • This water vapour will eventually condense and fall as rain, thereby watering other plants in the forest ecosystem.

  • The amount of water that a single tree releases is impressive. A large broadleaf tree can transpire as much as 900 L of water in a single day!




Now think about how many trees there are in a forest ecosystem.
Can you imagine how much water enters the atmosphere from the transpiration of trees?

Did You Know?

The quantity of water that a tree transpires depends upon several factors: the size and species of the tree, the amount of water in the soil, the relative humidity of the air, the amount of sunlight, the level of carbon dioxide in the air, and the presence of wind.


Trees not only recycle stored water in the soil; they also enhance the ability of a forest ecosystem to capture rainwater in the first place. The leaves and branches of trees intercept rainwater and allow it to slowly drip to the soil below where it seeps into the porous forest soil. In areas that lack trees, rainfall is not captured. Instead, it quickly runs off and often leads to flooding.