Leaf Rainbows

Flowering dogwood, (Cornus florida)


Sugar Maple, (Acer sacharum)




Well, the wonderful days of changing colors have slipped into the past, again. The greens bowed to yellows, the yellows to orange, and the oranges laid carpet down for the late red arrivals.
We’ve been doin a lot of playing with leaves and colors this year. One of the things the kids have enjoyed is making rainbows with leaves, varying colors within the same tree. It’s the first time I’ve really been “in tune” with the succession of colors, and the first thing that struck me was the order of transition. Greens, to yellows, to oranges, to reds…generalizing of course. Each species of tree follows this path, some ending mid-way, and some skipping along. Some jump directly into the browns. But, in no case does the order of the color transition change. For example, reds do not precede oranges (if oranges are in the sequence of change for the species). And oranges do not come prior to yellow (if yellow is in the sequence for that particular species).
Here-in lies a mystery. It’s clear that, as with all living things, selection has resulted in a particular set of behaviors or phenological displays. And in trees the leaves change colors as chemicals within the leaves change their level of dominance, or as they decompose. The chlorophyll breaks down exposing a new dominate color, a shift from greens (chlorophyll) to yellows (xanthophyll) to oranges (carotenoids). These expose the next, … the reds and purples (anthocyanins, lycopene). If the tree is high in tannin, shades of yellowish-brown, mustard, or brown follow green. Trees high in sugar produce large amounts of red (as glucose gets trapped in the leaf late in the season and decomposes).
  • What has selected for this pattern of exchange? : chlorophyll, xanthophyll, carotenoids and tannins, and anthocyanins and lycopene.
  • Are insects the culprit?
  • Why that order? Why does the sequence in colors produced over time just so happen to match the light spectrum?….coincidence?…or did the qualities of light have an influence in the selection process that resulted in this order of change: green to yellowish-green , yellowish-green to greenish-yellow, greenish-yellow to yellow, yellow to orangish-yellow, orangish-yellow to yellowish-orange, yellowish-orange to orange, oranges to reds, etc., etc.

Notice that a leaf never goes the other way in the sequence of color change. Red never changes to orange, orange does not change to yellow. While it is well understood why chemicals create color variations at particular times, I have found no good explanation of why the specific order in chemical unveiling and decomposition presents colors in a way that mimics the light spectrum, except for coincidence. Anyone?

By the way, it appears that anthocyanins (reds) do not appear in Europe! It is theorized that insects have played a major role in selecting for this phenomenon.