My friend Nik, who's lived in Pasadena for several decades, likes to call Pasadena "The Sun's Anvil." Boy, is he right about that. It's not that the weather is so hot here; there's just something about the geography and weather patterns that makes the sun seem closer somehow.
This is the attic belt course trim (the proper term for the trim grouping we've been discussing), over to the far right where it meets the eave. It is quite clear that where the wood is shielded from the sun by the eave for most of the day, the wood is still perfectly sound (I've sanded it lightly to uncover the wood). Moving leftward, as the trim emerges from the protection of the eaves, you can see its condition degenerate. I can't imagine a clearer illustration of the effects of sun on wood.
The sun heats the wood, causing it to expand. It cools off at night, only to be heated again the next day. At the same time, its ultraviolet rays are working on the paint, slowly destroying its elasticity. Soon enough, this one-two punch causes the paint to crack, and then the sun really gets after it, attacking the wood with UV rays directly. Over time, this destroys the lignin, which is what holds wood together and gives it rigidity and strength.
This isn't usually catastrophically destructive with siding, because it only presents one face to the sun, and it is backed with other wood that absorbs some of the heat and draws it away from the surface. The earlywood will slowly weather away, but the denser latewood will remain largely intact. This is why weathered wood has a pronounced grain pattern.
Exposed trim pieces do not fare so well. They get hotter, and the damage goes deeper, eventually reaching all the way through. At that point the wood is quite vulnerable to physical damage from any source—an insect, a squirrel, a falling pine cone—because there is really nothing holding it together but force of habit.