1) Isn’t chlorophyl broken down and reabsorbed by the tree? Maybe this triggers the production of the yellow pigment. And maybe the yellow pigment, has nothing to do directly with photosynthasis and is really a trigger to cause the leaf to detatch from the tree.

2) Maybe the photosynthesis conducted by the yellow pigment isn’t to support the tree, but maybe some process within the leaf itself. Perhaps it aids the leaf in drying out, therefor minimizing the risk of rotten green follage at the base of the tree.

But I rather doubt that whatever evolutionary–or other–mechanisms explain the change of color in leaves would directly explain the retention of leaves.

The latter sounds more like it might have something to do with providing/retaining a certain type of nutrition–or conceivably, parasite or disease prevention–around the base of the tree in the spring which might be lost if the leaves were released to the whim of the winds, snow, and weather in the fall or winter. I guess insulation might also be a possibility–I don’t know if leafless oaks and beeches are possibly more cold-susceptible than, say, maples and birches…

(It’s possible, I suppose, that the reverse is true: that is, that leaves retained over the winter are less likely to contribute to the soil structure or chemistry around the base of the tree, depending again on the affect of wind, weather, etc. This would flip my speculations around, but I would suspect some of the same factors might be involved, but with the “signs” reversed.)

While the “parasite” notion sounds superficially similar to some of what has been discussed regarding leaf color changes, I suspect that diseases or parasites affecting the base of the tree–where I’m assuming the winter-retained leaves preferentially wind up (or preferentially don’t wind up, see the parenthetical paragraph…), with some sort of differential impact on the soil structure or chemistry–would be different from those attacking the trunk and branches.

These are, again, the merest speculations. Scientists who actually know something about trees and leaves would have to pursue these and other notions, filter them through what is known about leaf-detachment mechanisms, the species involved, etc., etc., and then decide which speculations might fruitfully advance to the testible hypothesis stage…

As for the production of red pigments: the trees produce them actively, but there are a number of adaptive explanations that do not include defence against aphdis (e.g., protection of photosynthetic tissue and reducing oxidative stress).

Do the trees really actively make the red pigments or are they a spontaneous chemical reaction that doesn’t require any work on the part of the tree? If the former than this would strongly suggest that there really is some adaptive significance to the fall colors.

What do we know about visual acuity in aphids? Japanese beetles have a strong preference for trees with red or purple leaves. Can aphids distinguish foliar colors? I agree that volatile signals related to color are possible (I have published a number of papers on tree volatiles and insect response).

Secondly, I wonder about timing. Here in Kentucky, most of the peak in fall colors happens when it is too cold for aphids to be dispersing. So are they cuing on something that precedes coloration?

I still maintain my skepticism, but it seems that understanding the cues to which aphids appear to be responding might help us figure out whether this is a real phenomenon.

I did not intend to imply that anthocyanins and carotenoids are not ’sunscreens.’ They most certainly are. But the sunscreen protection is important at other parts of a leaf’s life history, not at the time of death.

Trees throw away leaves like Kleenex, but not until the tree translocates the good stuff (nitrogen) into the perennial parts. As chlorophyll degrades and its nitrogen moves out, the fading green reveals other colors. Leaves always contain carotenoids (yellow) as accessory pigments in the chloroplast. The leaf synthesizes some anthocyanins (reds and purples) de novo, but they only become apparent as the vacuole becomes acidic perimortem.

Trees vary in their genetic capacity for anthocyanin synthesis for reasons that may have little to do with fall colors. Black maple lacks anthocyanins and turns yellow, while sugar maple produces anthocyanins, but vacuole pH only declines in full sun. Hence sugar maples are a mosaic of yellows to orange while black maples are always yellow.

Aphids are primarily seeking nitrogen, excreting most of the tree’s carbohydrates as honeydew. Hard to imagine that they would selectively seek out particular trees or leaves just when nitrogen content is declining most rapidly.

The sunscreen argument assumes that there is something to protect. Since most of the nitrogen is out of the leaf by the time colors appear, and the leaves are in advanced senescence by that time, sun protection doesn’t seem very important.

Trees with an abundance of nitrogen, like black locust, don’t bother to translocate nitrogen, dropping green or greenish leaves. The biochemical state of a leaf when it is finally dropped may have adaptive significance for nutrient cycling. For example, sugar maple leaves with low concentrations of polyphenolics degrade more quickly than the drab brown oak leaves, cycling nutrients more rapidly.

As for the cost argument, the assumption that there is a net cost to color production is unfounded. Caretenoids are constitutive, and anthocyanins reflect modest changes in vacuolar pH.

In the aphid and sunscreen hypotheses, color me skeptical.