Why Do Leaves Change Color in Autumn?
The Arrival of Autumn
One of the signs that summer is ending is green leaves changing to amazing shades of red, gold, and brown. This doesn’t happen to all trees and the depth and intensity of the colors differs each year. So why do only some trees make this change and what else affects the resultant display? In brief the answer is the type of tree, its location and the weather.
Trees can be either evergreen or deciduous. Evergreen trees are like their name; their leaves remain green the entire year. Deciduous trees are the ones that create beautiful color displays that the fall season is known for. Their show appears just before the tree drops its leaves and lets them fall; hence the alternative name of the fall for the season of Autumn.
Oaks turn red, brown, or russet; hickories, golden bronze; aspen and yellow-poplar, golden yellow; dogwood, purplish red; beech, light tan; and sourwood and black tupelo, crimson. Maples differ species by species-red maple turns brilliant scarlet; sugar maple, orange-red; and black maple, glowing yellow.— www.na.fs.fed.us
Why Are Leaves Green?
There are three different pigments in leaves. These are caused by chemical compounds called chlorophylls (green), carotenoids (yellow-orange) and anthocyanins (red). For much of the year green is the only leaf color you will notice on a tree as chlorophylls mask the others. Chlorophylls are essential for photosynthesis and are located near the surface of the leaves. Photosynthesis is the process trees use to convert sunlight into energy (sugars) for growth.
As daylight hours reduce, deciduous trees conserve their resources for better times ahead. They stop producing new shoots and shut down their sugar-producing operation. The chlorophylls are no longer required and they breakdown exposing the underlying yellow, orange and red pigments of the remaining chemical compounds.
Why Do Leaves Change Color in Fall?
Are you a leaf peeper (someone who chases the fall leaf colors)?
Factors Affecting Timing and Hue of Leaf Color Changes
There is still much discussion in the scientific community about the precise reasons why some autumn displays are spectacular while others disappoint. Some of the many factors that may be involved include shorter days, cooler temperatures, intensity of sunlight, amount of rainfall, latitude and altitude.
It is generally agreed that the major factors influencing the timing and coloration of tree’s foliage are temperature and moisture. It has been observed that lots of warm, sunny days in tandem with cool, crisp but not freezing nights, result in some of the most memorable leaf color displays. A good reference book on the biological processes that result in leaf color change is "" by Bell and Lindsay. It covers over 100 species of trees found in the eastern United States. Fall Color and Woodland Harvests
Experiment to Show Pigment Colors in Leaves
The following experiment can be done as a home science project. It’s a great way to see the hidden colors of autumn. Kids can do this experiment for themselves, but an adult should be close by to supervise!
- A few green leaves from several trees. Try using leaves from different species including those that you know will change to red or yellow in the fall like maples (acer) or witch hazel.
- A glass beaker or drinking glass
- A small amount of rubbing alcohol (Isopropyl).
- Some plastic wrap (cling-film)
- Some chromatography or filter paper (you can use coffee filters)
- A small stake (about 6 inches long) like a pen or pencil.
Test one type of leaf at a time so that you can compare the result for the different trees at the end of the experiment.
- Into your glass beaker put a few green leaves torn into small pieces. (Remember one species only at a time.)
- Pour a little rubbing alcohol over the torn leaves; just enough to cover them.
- Place a piece of plastic wrap over the top of the beaker to stop the alcohol evaporating.
- To speed the process up put the glass beaker into a bowel of hot (not boiling) water. After about 30 minutes all the chlorophyll will have left the leaves and turned the alcohol green.
- Take the beaker out of the hot water and remove the plastic wrap.
- Cut a strip of filter paper about a half inch wide and tape it to the stake. Place the stake over the beaker so that the filter paper is suspended above the leaf mixture.
- Very gently, let the filter paper strip lightly touch the alcohol and pigment mixture. (Don’t let it drop right in!)
The rubbing alcohol and pigment solution will be slowly drawn up onto the filter paper. Be patient and after about an hour, the green color will separate out into its constituent pigments. This process of pigment separation is known as chromatography.
The shades of green, yellow, orange and red show the presence of chlorophylls, carotenoids and anthocyanins. Make a note of which trees leaves produced which colors and look out for them in the fall.
Leaf Color Chromatography
Citizen Science Projects
Some people spend the fall season chasing the magnificent spectacle of changing leaf colors. Many of these are tourists and are known affectionately as “leaf peepers”. A whole industry has grown up to help peepers locate the jewel-like colors before the leaves drop. The best time and places to see these pigments differs each year depending on the weather. Many National Parks and Forests post daily updates on their websites to assist the public in catching the most spectacular displays.
Tourists and locals may also be interested in the science behind the phenomenon. They can take part in citizen science projects. These are often run by university scientists or an environmental non-profit to log changes in time, speed and intensity of the color that occur between years.
Here are some examples of past and present citizen science projects.
- In US, Pop-Clock was a project run by the University of Maryland Center for Environmental Science to study the impact of climate change on certain trees. As part of the research they asked for volunteer observations of fall color change in their area.
- In UK, The Woodland Trust is currently inviting everyone to go to their website and record signs of Spring and Autumn on their “Nature’s Calendar”. The details logged should include the date, species and location of the first leaf color change in your area of the UK.
Projects like these depend on funding and so they may run for just a few years or for many. If you’re interested in taking part in this type of activity, do an online search for citizen science projects in your area to see what’s happening local to you.