photoperiodism is probably the primary cue, although some researchers would
argue that this effect is synergistic (in the plant) with responses to water
stress, nutrient dynamics, stored
carbon, etc...the exact reason why leaves change color is still unknown.
like almost everything else ecological, or botanical for that matter, there
is no simple answer.
here in the appalachians we see variation from year to year but all of the
leaves generally change at the same time. there must be an large-scale cue
(like day length) at work here.
i have heard the theory put forth that leaves change color because their
thylakoid membranes begin to degrade (within the chloroplast) which causes
phytopigment changes. i am not a proponent of this theory (no flames pl)
but it seems interesting.
cmb
