[fangorn]

In, or rather around, passive houses they are meant to block light during summer. That's why it's recommended to plant trees that will shelter your house from the sun thus lowering temperature inside (and your cooling energy requirements, if you've got air-con, etc.). During winter they shed their leaves and let the sun in, providing the opposite effect. Of course that's in climate zones that actually have summers and winters. Granted, you don't need entire forest for that.

Naturally, that works for most kinds of houses, not just passive ones. It's just especially important for those, if you want to achieve their stated (very low) energy requirements levels.

[sethammons]

When I lived in southern California, shade trees on my house was imperative. When they were trimmed and the shade lost, the inside house temp went up about ten degrees.