A combination of changes in hours of light/dark and temperature triggers these
changes, both the changing of colour and loss of leaves going into winter
(short, bright, cool days) and the resumption of leaf growth in spring. Each
year is different in these climatic features so the timing of colour change
will vary.
As for the intensity of colour: leaves change from green to orange/red as
chlorophyll is lost and carotenoid (yellow/orange) and anthocyanin
(red) pigments are produced or rather they exist in a higher ratio to the
Chlorophyll and thus the leaves change colour. As the chlorophyll decreases,
daylight will promote the synthesis of the other pigments so as to maintain
some photosynthesis. Variation in colour is merely a reflection of variation in
the pigment concentration.
hope this helps
Anita.
In article <B5EA9FF1.BE%jclausz at cc.edu>, jclausz at cc.edu says...
>>Colleagues,
>>I've been asked about the initiation of fall colors, why it comes earlier
>one year or another, and why the intensity of color varies from year to
>year. My recollection is that color is initiated by changes in daylength.
>But what about advances or delays in color and intensity? Thanks for your
>input.
>>John Clausz
>>>---
