MGA Faculty Perspective: The Science Behind Fall Leaf Colors

Author: News Bureau
Posted: Wednesday, October 16, 2024 12:00 AM
Categories: School of Health and Natural Sciences | Faculty/Staff | Pressroom

Cochran, GA

In October, many of us in the Southeast begin to look forward to what we hope is a vibrant season of fall leaf color. We asked MGA’s Dr. Sharon Standridge, associate professor of biology, for a primer on the science behind the seasonal changing color of leaves.

Ah, fall! Cooler weather, pumpkin spice everything (seriously Spam???) and the changing of the leaves are all things to love (or hate depending on your view of pumpkin spice and yard work) about this time of year!

Many plants in temperate terrestrial zones (see the USDA Plant Hardiness Map: https://planthardiness.ars.usda.gov/) shed their leaves in fall as a survival response to cooler weather. This helps the plants conserve resources and shed unprotected tissues that would freeze and die during the colder winter months. The process of dropping leaves is known as leaf abscission. During leaf abscission, water, sugar and other materials are retrieved from the leaves and taken up into the main stem of the plant body.

One layer of cells where the leaf is attached to the stem starts to breakdown the material that “glues” the plant cells together. This material is composed of mainly pectin which people have long used to make jams and jellies and is the reason why if you find a leaf that has not completely fallen off, the layer of cells that are being broken down will feel slimy to the touch. Another layer of cells on the stem side of the leaf will start to add a waterproof waxy material called suberin. The layer of cells that get broken down allow the leaf to “fall” or drop from the plant while the cells coated with suberin make a nice protective cap that keeps out any potential pathogens and prevents loss of water like a “band-aid” over the tissue where the leaf has separated from the stem.

Another “cool” part of the leaf abscission process is the loss of the pigment known as chlorophyll. During the warmer times of the year, chlorophyll is continually broken down and replaced but as nights get longer and temperatures get cooler, the leaf cells stop replacing chlorophyll. When enough chlorophyll disappears from the leaf cells, other pigments like carotenoids and anthocyanins that are usually masked by the chlorophyll become visible.

While chlorophyll gives plants their usual green color, carotenoids give plant parts yellow, orange or brown colors (think corn and carrots) and anthocyanins impart red or blue colors like in cranberries, red apples and blueberries. Chlorophyll and carotenoids are present in the internal membrane of a plant cell’s chloroplast (organelle in plant cells that does photosynthesis) as both types of pigments are lipid-soluble molecules (prefer to associate with fat-based molecules in the cell) while the water-soluble anthocyanins are usually found stored in a plant cell’s large central vacuole and are not usually present year-round in the leaf cells like chlorophyll and carotenoids. Once the sap (water/sugar) from the leaf starts returning to the stem during abscission, the leaf cells will then begin making the anthocyanin pigments which will give some leaves their characteristic red/purple Fall colors.

In Georgia forests, native dogwood leaves will turn a reddish-purple color, oaks will turn a deep red or brown, sugar maples will turn a nice orange-red and red maples will of course turn a bright red but can also turn yellow or yellow-green. Ginkgo tree leaves will turn a very bright yellow and even though they are not a species native to Georgia (they are actually native to China), because of their stunning fall leaf displays many have been planted in yards, on city streets and in parks for us to enjoy.

Not all plants have the same amount of each pigment (carotenoids and anthocyanins) in their leaves so that is why we see a range of different colors in fall - from yellow to red to orange and brown in different species.

So why do we see some years producing a really bright stunning fall leaf display and other years turn out to be not so stunning?

The type of weather during spring, summer and fall can all affect the amount of sugar produced which in turn impacts the amount of anthocyanins produced in the fall. Remember the amount of carotenoids is not affected as those pigments are always present in the chloroplast so the yellow/orange colors are not as affected by a change in weather. Usually, the best conditions for a really good fall leaf display would be a warm, wet spring followed by a not too hot or dry summer and a warm, sunny fall with cool but not cold (freezing) nights.

The main reason for such conditions producing a really good fall leaf display is the high production of sugar that occurs with a warm, wet spring and a drought-free summer. The more sugar the leaf cells have come fall and the more sunlight there is in fall (fewer cloudy days), means greater production of the red/blue anthocyanins which in turn lead to deeper red/purple leaf colors. Conditions that may lead to a less spectacular fall display would be a cooler than normal spring (later start to warmer weather), a severe summer drought or warmer than normal fall temperatures. This is mainly because of delayed leaf budding that would occur with a shorter spring growing season, less water for growth in a really hot and dry summer and higher than normal temperatures (warmer nights) in fall that would prevent leaves from starting the process of abscission (and result in less anthocyanins) prior to freezing winter temperatures.

This year we may have a moderately nice display despite the drought conditions we experienced in summer and early fall (not to mention Hurricane Helene’s impact). So go outside, take a hike and check out the fall colors of nature while you enjoy some sweater weather and pumpkin-spiced Spam. Take a picture or two of yourself with a fall leaf and post it to the Georgia State Parks Instagram account (https://www.instagram.com/gastateparks/). Make sure to tag your leaf pics as #GaLeafWatch and #GaStateParks and you may have your photos featured on the Georgia State Parks Leaf Watch webpage: https://gastateparks.org/leafwatch.