Reference to : http://www.backyardgardener.com/plantname/pda_35af.html Fall color is the result of trees or shrubs changing colors according to complex chemical formulas present in their leaves. Depending on how much iron, magnesium, phosphorus, or sodium is in the plant, and the acidity of the chemicals in the leaves, leaves might turn amber, gold, red, orange or just fade from green to brown. Scarlet oaks, red maples and sumacs, for instance, have a slightly acidic sap, which causes the leaves to turn bright red. The leaves of some varieties of ash, growing in areas where limestone is present, will turn a regal purplish-blue. Although many people believe that cooler temperatures are responsible for the color change, the weather has nothing to do with it at all. As the days grow shorter and the nights longer, a chemical clock inside the trees starts up, releasing a hormone which restricts the flow of sap to each leaf. As fall progresses, the sap flow slows and chlorophyll, the chemical that gives the leaves their green color in the spring and summer, disappears. The residual sap becomes more concentrated as it dries, creating the colors of fall.
It's my understanding that chilling influences the intensity of the color (such as cool overnight temps with bright daytime sun, will have the greatest impact in my area). The underlined statement as I understand it, implies that the process starts the same time every year triggered by day length. Environmental factors will speed up or slow down this process and impact the intensity or dullness of the colors. For example, peak color hits at different times every year influenced by weather, but the process of "chemical clock inside the trees starts up, releasing a hormone which restricts the flow of sap to each leaf." So it's fair to say the clock starts at the same time each year, but environmental factors speed up or slow down the clock leading up to peak color. Then of course cool nights and bright sun impact the intensity of the show. Where as warm, wet weather, will result in a duller show. Of course, high winds and heavy rain may completely put an end to the show, before it even starts. I have also heard about different elements in the soil impacting color. But remember color and intensity are two different things. Also, the clock starts at the same time each year, but environmental factors influence how fast the clock spins down. Or at least that's my basic understanding of it anyway. This has been a hot topic in the past and there are very strong opinions from different people on here from different parts of the world. So someone is bound to disagree with me.
Not me: I totally second what you wrote! ;-) If the weather had nothing to do with Autumn colours, and that includes the weeks and days before leaf fall, we would always have the same hues each year, and everyone knows that is just not the case.
The weather has some influence, but not a huge amount. Genetics plays a far greater part, with some species (and clones of those species) colouring far more reliably than other species. Where I am, planted Acer saccharum had coloured up orange-yellow two weeks ago (and is now leafless), while planted Acer platanoides is turning yellow now, and planted Acer pseudoplatanus is still solidly green today, despite all three experiencing identical weather within a few metres of each other.
Glad you think so. All the science stuff aside, I like to think of it like a symphony....(here comes one of my weird analogies...) Where the solar calendar and your position from the equator (or simply put - day length) determines when the song starts in your area. Mother nature is the conductor. She determines and controls (by weather) the tempo, intensity, and climax of the performance. Some years the performance is longer and or more dramatic than others. Other years it is very forgettable or maybe it builds towards climax and dramatically / suddenly comes to an end making us want more. Other years the performance is extraordinary and we are left in awe and amazement! Each tree is like an instrument, some are louder or softer / intensely beautiful or dull, but they all play a role in the overall performance, some playing a bigger role than others.
