Here's why:

* Heat of vaporization: This is the amount of energy required to change a substance from a liquid to a gas. Water has a very high heat of vaporization.

* Transpiration: When trees transpire, they draw water up from their roots and release it as water vapor into the air.

* Energy absorption: To change from liquid water to water vapor, the water molecules must absorb a significant amount of heat energy from the surrounding environment. This energy comes from the air, leaves, and even the tree itself.

* Cooling effect: As the water vaporizes, it takes this heat energy with it, causing the surrounding environment to cool down.

In summary, the high heat of vaporization of water allows trees to effectively absorb heat from their surroundings during transpiration, leading to a cooling effect.