By Liz Summer, I Start Wondering Columnist
The man stood transfixed outside my door. He gazed rapturously as the flickering golden leaves rustled, producing a crinkling crunchy symphony in the chilling breeze. “The sound!” he said, as if hearing for the very first time. “The sound is beautiful! What type of tree is it?” When informed that the tree producing this delicious noise was a cottonwood, his joy turned to a scowl as he muttered, “Oh, those are such a nuisance!” Shutting out the sound, he carried on with his day.
Today, in mid-December after a mostly warm wet fall, most of the leaves in this part of Texas are gold to light green and still clinging to the tree, and to my ears at least, still a joy to listen to. Surrounding deciduous trees, from many species of oaks to Osage orange, ash, maple, pecan, elm, etc., are also all in various degrees of leaf shedding for the winter. The seasonal variations seem to have led to very different responses not only between different types of trees, but also between individuals of the same type of tree. It is as if it is just not obvious what the best winter strategy is this year.
A Critical Decision
What decisions does a tree have to make? And what basis can it use to make this decision? Surprisingly, there are many. Plants protect and prepare themselves as much as any animal does, just not by moving about. For deciduous trees (trees that lose leaves in the winter), the decision is based on how much energy leaves can generate versus how much energy or risk there is keeping the leaves on. Risk takes several forms – the most obvious being how a full head of large leaves becomes a branch-breaking hazard when frozen.
Less obvious is the source of the real challenge and real purpose to leaves – photosynthesis. In photosynthesis, plants use the energy of the sun to convert a gas – CO2 – into solid material – sugar. The sugar is used to make the parts of the tree.
But light energy is dangerous. Inside the chloroplasts are sheets and sheets of membranes dotted with large complexes, the photosystems. These play a sort of “pass the hot potato” with energy. High-energy electrons must flow flawlessly through the complexes or they escape and literally tear up the membranes and destroy the complexes. On a molecular level, it is a serious game. Complexes must move around to adjust themselves to getting just the right amount of energy – too much and ZAP!! Destroyed systems with dangerous high-energy electrons zapping about!
When the weather gets cold, it is harder to adjust the complexes and easier for high-energy electrons to escape their proper channels of transmission. The trees must decide when it is time to call it the end of the season and start to break down the complexes to scavenge them for useful parts for next year. Chlorophyll, which is the green part of the complex, is broken down. This reveals carotenoid pigments of orange and yellow that were there, but masked by the overwhelming green. To provide an additional barrier against dangerous high-energy electrons, new pigments called anthocyanin might be synthesized, producing purples and deep reds. For some thick-leafed trees, such as many oaks, leaves are simply stripped of all pigments, leaving them a dark brown, which is the color of the thick cell-wall material in these leaves.
Ultimately, red, yellow, purple, orange, brown – each color represents a different strategy the tree uses to protect against the real dangers that would be posed by out-of-control electrons zapping about their cells unchecked. In effect, changing leaf colors represent a pigment-based defensive martial arts that each tree must carefully decide when to fully deploy.