Why Do Trees Turn Colors in the Fall?

Normal pattern
Normally this happens. Why Do Trees Turn Colors
in the Fall? Blotchy Change
Why not this?

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For those of you living in the temperate parts of the world, each year you see that the trees change from their summer greens into often spectacular colors with the onset of the fall season (upper left). But have you ever seen a tree that is blotchy with sections of it changing while other parts are unchanged? What is it, anyway, that causes a tree to go into its fall colors? What is the trigger? Is it a photoperiod phenomenon, or is it governed by dropping temperature? And finally, does the tree have only one trigger that must be activated causing the whole tree to change at once, or does the tree have many, widely scattered triggers such as that each branch changes as a unit, or each leaf? Perhaps we can discover the answers to these questions!

But first, let's quickly recognize that this in not a quick experiment, so don't consider this if you don't have a couple of months to do your project. What is more is that this must be started about a month prior to when your local trees normally begin to change colors. But the good thing is that the project won't take much work on your part - it will all be on the trees to do the work!

It is suggested that you not use the large trees growing on your property or those of your neighbors. Instead procure several small, potted, two-year old trees from a nursery. Here are some criteria for those trees:

They are all of the same kind
They are of a type that grows wild in your area
They each have several branches so that the branches can be treated differently than the remainder of the tree.
(Optional) You might get "reject" misshapen trees which the nursery might merely give to you without charge.

EXPERIMENTAL DESIGN - IN GENERAL

You should have your trees outdoors, perhaps with the pots buried so that the soil line inside the pot is about that outside the pot. Next, you will have to isolate one or two of the branches of each tree away from the others so that they may be "treated" differently than the remainder of each tree.

You must also recognize that you might not be able to both of the two following "treatments." Do not become frustrated: if you only do one, no matter what your result, you will have something to report. For example, you will be able to report that while photoperiod is known to have an effect, you will be able to say whether or not its effect involves only one main triggering photosensor for the whole tree (no matter what you do, the whole tree changes simultaneously), or if the tree has many different photosensors scattered throughout the branches (you are able to get a blotchy tree as show to the right of the title, above).

Full instructions step-by-step are not given here because you will have to figure out how to make things. Remember: it is your project requiring your thinking. It is not the project of this website's author!

PHOTOPERIOD EXPERIMENT

Let's start with the easier of the two treatments. What you want to do is take a branch of a tree and be able to keep it constant with regard to the day-night light cycle. You know that as summer declines and fall begins, the number of hours of daylight become less and less - the photoperiod decreases. You want the main part of the tree to experience the normal outside decreasing daylength, while your isolated branch has its days augmented by a bright light. Of course, you must be careful that the artificial light does not illuminate the remainder of the tree. An example of what you might do is shown to the right. It is an opaque fence (allow no light to get through the cracks), and probably should be oriented so that the fence is in a north-south direction so that both sides of the fence experience about the same amount of direct sunlight each day. Of course the fence must be made of something that is weather resistant since it will be outdoors for several months of rain and sun. On the fence above the branch you should have one or two all-weather fixtures fitted with very bright light bulbs connected to a 24-hour timer switch. Even better than shown would be fixtures that have reflectors to direct the light downwards.

TEMPERATURE EXPERIMENT

A bit more complicated is this aspect of the research. Here you might want to build a sort of small greenhouse on one side of the fence. The inside of the glass-box can be heated with a heating pad that lies supported on a box just below but not touching the branch. The heating pad can be connected to a thermostat (inexpensive ones can be purchased at a hardware store). Obviously, this aspect of the project will take some construction skills!

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