Northern Minnesota phenology report: October 2009
By John Latimer, KAXE Radio, Grand Rapids
The arrival of fall is best announced by the leaves of the trees turning color. For many years it was assumed that these colors were in the leaves from the beginning and that as the tree prepared for winter it stopped producing chlorophyll and the intrinsic colors were revealed. Recent studies have shown that the colors are not in the leaves and must be produced by the tree.
From an evolutionary perspective simply turning color would be a waste of energy because the pigments produced are lost when the leaves fall. There must be a reason why the trees would invest in the color change. Two theories have emerged as possible explanations.
One theory is that the color is a signal to insects to stay away. Many insects approach trees in the fall to lay their eggs and in the spring these eggs will hatch and begin an attack on the tree. There is some evidence that insects avoid the most colorful trees. The trees with the brightest colors will self select because they will experience the least insult from pests. Ultimately these trees should come to dominate the forest. That they don’t is the source of much conjecture.
The problem is that some trees do not turn color in the fall. Rather they just drop their leaves while still green. Scientists that support this protectionist theory contend that there is always going to be some natural variation within the plant community. Research indicates that those trees under the greatest pressure were the ones that evolved to have the brightest colors.
Other scientists have noted, especially here in the far north, that by the time trees begin to turn color the majority of their insect pests have perished. Where then is the pressure to turn color for defense? They contend that the trees turn color to mitigate the effects of sunlight on the leaf as it shuts down.
The entire photosynthetic process must be shut down in the fall. The tree needs to carefully dismantle the chemicals used in the process, nitrogen, and phosphorus among others will be maintained in the tree to be used again in the spring. All this housekeeping requires energy that comes from photosynthesis, yet this is the process that is shutting down.
Leaves cannot use all the energy striking their surfaces in the fall and the excess can cause damage. Anthocyanins, those molecules that cause leaves to turn red, act as a sunscreen allowing the leaf to get its work done without destroying the chemicals the tree is trying to extract.
Scientists produced trees in the laboratory that were unable to produce the colors associated with fall. While these trees prospered in the greenhouse they were unable to ship nutrients to the tree for storage in the fall. This supports the sunscreen theory, but the sunscreen is only present in those trees that produce anthocyanins. The trees that turn yellow manufacture a chemical called carotenoid and this has no effect on sunlight. Currently they are arguing that there is another, as yet undiscovered, chemical doing the screening.
So it goes, the two sides concede that the other may be partly correct. Possibly the leaves turning color may accomplish more than one task. I am just glad that they do turn color in the fall. Whatever the reason it is a time of stunning beauty.
John Latimer is well known throughout northern Minnesota for his phenology work. He appears weekly on KAXE radio in Grand Rapids, and audio and twitter archives are available here. His work is a frequent feature on MyMinnesotaWoods. This article also appeared in the Duluth Senior Reporter. It is printed with the author’s permission.