Note: It seems I was a bit premature when I posted this in September. Autumn was still a long way off in many places then, and it wasn’t even very advanced here. It’s looking a lot more autumny now, so here it is again. rjb
Leaves use the same technique as artists do to mix their colors. It comes down to what colors of light are absorbed and which are reflected. When most of the pigment reflects green light, we see green. When enough green pigment goes away, we get to see the other ones. There are also yellows, reds, blues and browns in most leaves, their amounts depending on the species of plant and local conditions.
When Fall comes and deciduous trees drop their leaves there’s an orchestration of events going on. The tree draws moisture and sugars out of the leaf, for storage in the roots. At the same time it seals off the branch and weakens the base of the leaf’s stem. Soon the leaf is holding on by just the fibrous veins that used to flow with the tree’s juices.
While all this is going on the chemistry in the leaf is changing. Chlorophyl, the green pigment that spent the summer converting water, carbon dioxide and sunlight into sugar, is no longer being replenished and is breaking down and fading away. Now the yellow pigments can be seen. They are carotenoids, which have been converting some of the green light not used by chlorophyl, but which reflect light in the yellow range.
The red color seen in some leaves is due to another pigment which is actually produced as the leaves die. Stray sugars combine with colorless flavonols and sunlight to produce a pigment that ranges from red to blue. The amount of red seen in leaves is dependent on the species of tree and the weather. Cool nights and sunny days stimulate the production of the blue-red anthocyanin pigments, and thus encourage the reddest leaves.
The yellows and reds also decay and fade away, leaving tans and browns. The most common brown pigment is tannin, and by the time the leaves hit the ground they are almost exclusively brown.
The color in autumn leaves depends on what pigments the plant produces, and in what ratios. Then there’s the different rates at which the various pigments decay. And finally there’s the effect of weather on the production and decay of the pigments. Different types of trees turn different colors, and trees of the same type can differ, depending on their local conditions.
Nature has been mixing pigments all summer. It’s time for the show.