To use Unity's wind, follow these steps:

1. Place the tree and the particle system in the scene.
2. Create a new WindZone. To do so, go to the GameObject | 3D Object | Wind Zone option.
3. The tree will move when you play the game, but the particle system needs to be adjusted.
4. To make the particle system react to wind, open its properties and check the External Forces option.
5. Click on it to unfold it. You can edit the Multiplier field to increase or decrease the influence of the wind.
6. If you're not sure whether your particles react to wind, set their Start Speed to 0.
7. Play the game to see the effect.