Survival value of fall leaf colors

[Magical Realist]

I have for a long time pondered the survival value of leaves changing colors in autumn. It seems planned, as if there is some meant to happen chemical reaction in the leaves making them bright red, yellow, and orange. Surely this can be no more accidental than the colors of flowers or of fruit. My conclusion is that the trees with the brightest and most noticeable leaf color changes attracted animals who consumed or gathered their nuts. I'm assuming the color change coordinates with the ripeness of the nuts. So squirrels would tend to climb the trees with the brightest leaf colors to gather their nuts, which tended to make those trees to get planted more, leading to the survival of the color-changing phenotype. Eventually you have evolved a whole species of trees that changes to bright colors in autumn. Just speculating though..

[Image: autumn_leaves2.jpeg]

[C C]

An animal correlation with the leaves of nut-bearing trees might be a consideration in that specific nook. In general, scientists have ventured theories about the evolutionary utility of bright fall colors versus drab colors of leaves. One is the "signal hypothesis", wherein vibrant colors are supposed to warn / ward-off insects that toxins are present in the leaves of these healthy trees. Another is that molecules sporting the fall pigments act as a sunscreen protecting the leaves from harmful rays that are more destructive when minus the presence of chlorophyll.

Tests and studies haven't given a ringing endorsement to any theory, though the sunscreen suggestion possibly fares better than the rest.

Chlorophyll molecules are constantly replaced during spring and summer as they are damaged by by light exposure. That stops during the fall, when deciduous trees close down their photosynthesis activity and the green wavelengths reflected by the chlorophyll consequently disappear. No longer masked by the latter, other pigments in the leaves then become visible, some of which were already present during the growing season, along with others that are indeed newly manufactured during the autumn period.

The yellow colors are caused by xanthophylls, the orange by carotenoids; and these are the pigments that were there, but hidden, before fall. Anthocyanins and their red / purple colors are the ones introduced during that season of cooling down. Leaves still clinging to a tree late into winter will eventually lose these molecules until only the brown of tannins are left. Certain varieties of trees with drab leaves might never seem to have anything but tannins to begin with, color-wise.