well... eh.. have you seen woods with darker or lighter areas ?
so what are the fact here?
If you are seriously interested in understanding more about wood surface changes thru photo degeneration I would suggest doing some home study. Do an internet search on the subject (US Forest service and other research studies). Many, many books cover the subject such as Hoadley's "Understanding Wood" and "The Encyclopedia of Wood" Library of Congress Catalog # 88-39719 for another. In short order you will know way more about the subject than you wanted

