Rain occurs when water vapor in the air condenses and falls back to earth.
But how does that water vapor get in the air in the first place? Half of it gets there by evaporating directly from bodies of water, like lakes and rivers.
The other half is put there by trees. This is because trees act like giant upside-down funnels. First, they absorb water from the soil with their roots. This water is then transported up through the tree to its leaves. From there, the water evaporates out of the leaves and into the air, as part of the process of photosynthesis.
This means that areas with lots of trees have more water vapor in the air than areas with few trees. When there’s more water vapor, rain is more likely to occur. Thus, trees affect precipitation patterns as well as temperatures.
Amazingly, this explains why rainforests are so rainy!