How Spring Happens Here, Part One: Old leaves, new leaves

In our mild Pacific Northwest winters, some plants keep a portion of their leaves all through that rainy season. It's as if, in the fall, environmental signals aren't quite harsh enough to push plants into total dormancy. In spring, bursting buds push out fresh growth to contrast with the leaves still left from last year.

(Questions: how much energy is gained from wintered-over leaves? seems introduced plants from harsher climates are most likely to display this pattern, true? is there a pattern (location?) as to which leaves are kept? and, when are these kept leaves finally lost?)