Re: What is the evolutionary value of the beautiful colors we see during fall?

First allow me to clear up a common misunderstanding about evolution.  
Everything does not have to be of adaptive value, something that increases 
the reproductive success, to be present. It is sufficient in some cases for 
a character to simply not be a disadvantage, something that will decrease 
the reproductive success of the organism possessing it.

Plants have two types of pigments, those embedded in plastid membranes and 
water soluble pigments in the cell's central vacuole.  Green chlorophyll, 
yellow xanthophylls, and orange-red carotenoids are membrane bound 
pigments found in plastids.  What many people don't understand is that 
leaves are always yellow with xanthophylls and carotenoids, but chlorophyll 
masks these other pigments.  In the fall, chlorophyll synthesis slows and 
ceases in the shorter days and cooler weather, so the yellow-orange 
pigments are unmasked and the leaves turn color as a natural consequence of 
senescence.  For a similar reason, leaves on nutrient starved plants tend 
to look yellow because of low chlorophyll synthesis.

Red anthocyanin pigments are water soluble.  When these colors are 
superimposed on green, the leaves look purple, and when superimposed on 
yellow, the leaves look red-orange depending on the combination.  In the 
fall, deciduous trees drop their leaves, and to do so they must make an 
anatomical zone at the petiole base to disconnect the leaf and seal off the 
tree interior.  Most leaves have lost connection to the tree proper before 
they are dropped, but cells in the leaves of some tree continue to perform 
a little photosynthesis using the dwindling chlorophyll.  This allows a 
little sugar to accumulate, which reacts to produce these water soluble 
pigments.  This is certainly somewhat wasteful, but a necessary part of 
being a deciduous tree.  If the tree waited until the leaf ceased 
functioning, it might get too cold to form and seal its break point with 
the leaf.

So differences in physiology and timing of events leading to leaf 
senescence and leaf fall among different tree species produces the various 
colors of dying leaves.  But these colors are of no particular adaptive 
value for this event.

You may then ask, "Why are these molecules in our visible light spectrum?" 
 Plants of course use visible light because the energy in these wavelengths 
is plentiful, and of course this is why most organs of sight concentrate on 
the visible wavelengths.  Some animals can sense infared and ultraviolet 
beyond the visible spectrum, but in general plants make pigments in the 
visible spectrum for interacting with animals.  Yellow, orange, and 
orange-red flowers and fruits are colored by the same pigments as leaves 
using modified plastids called chromoplasts.  Other flowers and fruits are 
red, blue, or purple because of water-soluble pigments produced by the 
metabolism of sugar just like in dying leaves, but in this case with the 
result of making more conspicuous flowers and fruits.  So in these cases, 
where attraction of animals is involved, the pigments are adaptive, but in 
dying leaves they are not.

I have answered the fall-leaf-color question, or some version of it, more 
times than any other single botanical question.  Hopefully by posting this 
permanently upon the web, it will keep me from having to answer it again.