Friday, September 15, 2006

Why Do Leaves Change Color in the Fall?

Every fall we are treated to the grand spectacle here in the northeastern US. It is the fall foliage show in which leaves change color from a bright summer green to varying shades of yellow, orange, red, purple, and brown before falling off. The shedding of leaves occurs only in certain types of trees known as deciduous trees.

Many of us wonder why and how this happens. The story lies in the chemistry occurring in the leaves. Leaves are the food factories in trees. Leaves produce sugars in spring and summer when sunlight falls on the leaves as water and carbon dioxide are combined in the presence of a chemical called chlorophyll.

Chlorophyll is a green-colored chemical (also known as a pigment) that reflects the green portion of sunlight while absorbing other colors. This is what gives leaves their normally green color in spring and summer. Leaves contain several other pigments which are yellow, orange, red and brown in color, but the chlorophyll overshadows the rest.

During spring and summer, when days are long and bright, leaves are able to produce a lot of sugars; they send these sugars to the roots and the stem. But as fall sets in, days start getting shorter and the temperature begins to drop. This sends a signal to the tree that it has to prepare to shed its leaves and manage the winter on stored food (known as dormancy).

The tree cuts off the supply of nutrients and water to the leaves by the growth of a layer at the leaf-stalk connection. This event is called abscission. Once abscission sets in, the chlorophyll in the leaf breaks down quickly and loses its color. It is during this period (fall) that we see leaves of trees like birch, beech, cottonwood, hickory, willow, etc., turning yellow. Others, such as maples, sweetgum, and sumac, turn red. Purple is seen in dogwoods, some species of ash and some maples. Several oaks turn brown.

Some factors that control the colors include the temperature, the humidity (wetness), and the amount of sunlight during fall. Bright, sunny, cool days and chilly nights (without frost) create the brightest colors. The leaves exposed to bright sunlight might turn red, while those on a shady side may turn yellow. Wet weather usually leads to decrease in the brightness of the colors.

There are several places in US that have excellent fall foliage displays: New England, New York, Pennsylvania, even some parts of Idaho and Texas. But note that mountainous trees such as conifers (cedars, firs, pines, spruces, etc.,) do not change color and remain green throughout the year. Individual leaves on conifers sometimes stay on for three to four years.

Little Lion Experiment 1:

We will try to do some experiments to separate out the different colored chemicals in leaves (both green and fall leaves). Note that you must have adult supervision during these experiments because use of some items requires extra care and prior knowledge of safety. The process of separation of colors using chemicals is known as chromatography.

Items Needed:

  • Rubbing alcohol (ask your parent)
  • Hot tap water (ask your parent). Be careful not to hurt yourself.
  • A coffee mug
  • A large bowl or deep container that can hold the coffee mug
  • A pencil
  • Scissors (be careful while using)
  • Clear plastic wrap
  • Coffee filter paper
  • Green leaves and some yellow or red leaves
  1. You will begin by cutting 2-3 green leaves in small pieces with the scissors.
  2. Then you can crush them in the coffee mug and you can use a spoon to further mash them gently. Add some rubbing alcohol to the coffee mug--just enough to cover the leaves in the bottom.
  3. Place the mug inside the large soup bowl or container and pour some hot tap water outside the mug so that it can get warm. [NOTE: this is the ONLY safe way to heat; do not try any other method. And keep the set-up away from all kinds of stoves]
  4. Let the leaves soak in the warm rubbing alcohol for an hour or more.
  5. Put plastic wrap over the mouth of the mug to slow evaporation of the alcohol.
  6. If the water in the bowl gets cold, take the mug out, and empty the bowl and fill with new hot water. Replace mug back in position.
  7. Wait until the liquid in the cup gets dark, showing that pigments are dissolved in it.
  8. Using scissors, cut the filter paper (if you don't have filter paper, use paper towels) into one or two strips about 4 inches long by 1 inch wide.
  9. Put the pencil down across the mouth of the cup and drape the filter paper across the pencil so that one bottom end touches the bottom of the cup through the liquid.
  10. Let this stand for about 30-40 mins. What happens?

This works best if you seal the coffee cup with plastic wrap so that the alcohol does not evaporate. Sometimes you might have to soak for over 2 hrs depending on the leaves.

The pigments will move at different rates through the paper, and if you wait for 30 or 45 minutes, you will see them separate. The paper strips can be dried and you can keep them as a record. Next you can try this with yellow/red leaves. It is also a good idea to write down the name of the leaf (tree) with each strip if that is possible/known.

Little Lion Experiment 2:

This is a very simple way to preserve bright colored leaves:

  1. Gather pretty leaves from trees.
  2. Wash them gently to remove dust, and dry them with paper towels.
  3. Place each leaf between two wax paper sheets. Keeping a cloth on top, ask an adult to run a warm iron over to press the wax paper to the leaf.
  4. You can then cut the paper along the edges of the leaf and preserve it.