BMW wood trim typically has an aluminum backing to it, then the wood then is covered with the plastic or whatever material. The cracks (I have seen em on our e36, e34, e39, and e53.) I have also seen it in countless other cars, just not ours.

I think it the plastic material that is cracking, not the wood, and I have lived in colder climates and warmer climates and noticed it can happen in either. One thing that seems to be common is that it happens faster on the pieces that are more exposed to direct sunlight, ie: light through the windshield.

While living in Florida and using a sunshade I have noticed that we do not tend to get the cracks as quickly.

It may be more a Sunlight/UV issue than overall temperature.

I can't tell you if it can be replaced under warranty, as it seems to take quite a while to happen (e39 has 180,000miles and just got a crack on the radio cover).