Every year at the end of october, I drive about 100 miles to an annual bridge tournament. This year, many of the trees I noticed while driving still had green foliage. On previous trips, the majority of the folliage had changed to fall colors. I assume that the difference is due to a warmer than usual autumn this year. This makes me wonder if for some trees the color change is triggered by lower temperature (those that did not change) while for others it is triggered by time of year (those that changed as usual). Perhaps the trees that did not change color yet merely have a different temperature threshold than the trees which did change color.
I think you got it right, or at least close. Hmm I think it also has to do with lightlevels, but I'm not sure.
http://en.wikipedia.org/wiki/Fall_colors Deciduous plants are believed to shed their leaves in autumn primarily because the high costs involved in their maintenance would outweigh the benefits from photosynthesis during the winter period of low light availability and cold temperatures.[1] However, there is no reason why leaf fall should necessarily be preceded by the production of vivid autumn colors, and the function of the color change is still uncertain. Autumn colors (especially red) are not just due to the breakdown of chlorophyll; in fact anthocyanins (red-purple) are actively produced in autumn. What use is the production of pigments in leaves that are about to fall? A number of hypotheses have been proposed, including photoprotection, coevolution and allelopathy: [edit] Photoprotection According to the photoprotection theory, anthocyanins protects the leaf against the harmful effects of light at low temperatures[2][3]. It is true that the leaves are about to fall and therefore it is not of extreme importance for the tree to protect them. Photo-oxidation and photo-inhibition, however, especially at low temperatures, make the process of reabsorbing nutrients less efficient. By shielding the leaf with anthocyanins, according to the photoprotection theory, the tree manages to reabsorb nutrients (especially nitrogen) more efficiently. [edit] Coevolution According to the coevolution theory [4], the colors are warning signals towards insects that use the trees as a host for the winter, for example aphids. If the colors are linked to the amount of chemical defenses against insects, then the insects will avoid red leaves and increase their fitness; at the same time trees with red leaves will have an advantage because they reduce their parasite load. The coevolution theory of autumn colors was born as a branch of evolutionary signalling theory. It is a general feature of biological signals that, when a signal is costly to produce, it is usually honest - that is it reveals the true quality of the signaller, because it does not pay for a low quality individual to cheat. Autumn colors might be a signal if they are costly to produce, or they could be an index, which is maintained because it is impossible to fake (because the autumn pigments are produced by the same biochemical pathway that produces the chemical defenses against the insects). Although it is not certain that aphids have red receptors, there is some evidence that they preferentially avoid trees with red leaves. This is what the coevolution theory predicts at the intraspecific level (more insects on dull leaves). It is also known that tree species with bright leaves have more specialist aphid pests than do trees lacking bright leaves[5], which is the interspecific prediction of the theory (autumn colors are useful only in those species coevolving with insect pests in autumn). The coevolution hypothesis has been subjected to criticism.[6] The change of leaf colors prior to fall have also been suggested as adaptations that may help to undermine the camouflage of herbivores.[7] See also: Deciduous and Plant defense against herbivory Many plants with berries attract birds with especially visible berry and/or leaf color, particularly bright red. The birds get a meal while the shrub, vine or typically small tree gets undigested seeds carried off and deposited with the birds' manure. Poison Ivy is particularly notable for having bright red foliage drawing birds to its off-white seeds (which are edible for birds, but not most mammals). [edit] Allelopathy Researchers at New York's Colgate University have found evidence that the brilliant red colors of maple leaves is created by a separate processes then those in chlorophyll breakdown. At the very time when the tree is struggling to cope with the energy demands of a changing and challenging season maple trees are involved in an additional metabolic expenditure to create anthocyanins. These anthocyanins, which create the visual red hues, have been found to aid in interspecific competition by stunting the growth of nearby saplings in what is known as allelopathy. (Frey & Eldridge, 2005)
The sugar maple grove in the park near my new house - first autumn here - did not turn typically bright red, as I have come to expect from sugar maples. The trees in it turned yellow, a little brighter than the ironwwoods in the same general area. Trees vary in the pattern of the turning, and where it seems to be controlled: Some (most oaks) turn leaf by leaf, with even adjacent leaves on the same twig turning days or weeks apart. Some turn branch by branch, or outer to inner on each branch. Most maples seem to turn as a whole tree, or in large blocks of foliage. I ahve seen boulevard and meadow-edge maples turn in almost straight-edged blocks, with the first block normally facing the southern sky - that is, the demarcation a slanted, almost straight, boundary line between completely red and completely green leaves, the angle of the slant matching (roughly) the latitude of the tree, the red leaves on the sky-facing block. It's hard to imagine a single causal factor that would account for these patterns. Sunlight and sky-exposure seem to be important factors. I notice the sky-exposure is not mentioned in Wiki - leaves open to the sky are much more likely to ice up, early in the year before the benefits of photosynthesisis or storage retrieval are reduced. If there is some kind of anti - icing benefit to these chemicals, that might be a factor.
Essentially, the process of color change in leaves comes down to chemical and environmental factors. The green color we all see in leaves most of the growing season is chlorophyll. Chlorophyll is the key ingredient in the food-making process of photosynthesis. Hidden behind the green pigment are two yellow pigments called carotene and xanthophylls. Chlorophyll is unstable and is regularly being broken down and resynthesized. Carotene and xanthophylls are produced as the leaf unfolds and are stable, remaining in the leaves until they turn brown. The red color we see is caused by pigments called anthocyanins which are produced late in the growing season. These red pigments or anthocyanins are water soluble and are located in the leaf’s upper layer. The anthocyanins depend upon high sugar and tannin concentrations which are produced best by bright sunny days and cool nights. Because the anthocyanins are located in the upper layer of leaf cells, it can cover the other pigments either partially or completely. http://www.theheartofnewengland.com/LifeinNewEngland-Autumn-Leaf-Color.html
