In many areas of the U.S., the autumn trees paint the landscape with shades of warm, glowing reds and oranges. Fall foliage displays are much more common in areas of the U.S. where the seasons have distinct differences and where there are many deciduous trees (ones that drop their leaves in the winter). Shorter days and cooler temperatures contribute to the spectacular display, but why do leaves change color in the fall?
Here's a look at the complex biological and chemical processes that produce and affect fall foliage and a fun family project that can help you make some predictions about your hyperlocal autumn foliage show.
Why Do Leaves Change Color?
Leaves change color due to changes in the length of daylight and temperature from summer to fall.
But, to really understand the science behind this process, we need to understand the purpose of leaves and how they do their work.
Leaf Food Production
Simply put, leaves produce food for the tree. While they serve several purposes, their main purpose is to convert sunlight, carbon dioxide, and water into glucose, which provides food and energy that the tree uses to support its growth. Deeper roots, vigorous new shoots, flowers, and fruit are all possible because leaves are little powerhouses of glucose production. They also help the tree breathe and get rid of excess fluids.
Leaves do all of this through photosynthesis, the process by which green plants produce sugar, seemingly from thin air. In fact, photosynthesis requires four things: water, carbon dioxide, the sun's ultraviolet rays, and chlorophyll, the naturally occurring pigment in plants that makes them green.
Chlorophyll is the most plentiful pigment in leaves. It absorbs energy from the sun and combines it with water and carbon dioxide to produce sugar, which destroys chlorophyll. The glucose is then transported back into the tree through the petiole, which is the correct name for the “stem” of the leaf. The tree uses the glucose to build new roots, leaves, stems, and flowers. It also transports water and nutrients from the roots to the leaves, giving them the energy and ingredients they need to produce chlorophyll.
How Leaves Change Color
As the days get shorter and the number of UV rays lessens, the tree starts shutting down its growth operations and transitioning to its winter preservation mode. One of the first things it does is close off that connection to the individual leaves by changing the abscission layer, a thin-walled layer of cells at the base of the petiole.
The abscission layer consists of tiny tubes that transport water and hormones between the leaf and the tree. In the autumn, those tubes begin to swell and secrete a waxy substance that reduces the flow of nutrients and water to the leaves. Without those ingredients, the leaves can't create new chlorophyll, allowing other pigments in the leaves to become more prominent, producing the range of yellows, oranges, and reds that make such spectacular fall foliage shows.
4 Factors Affecting Leaf Color
While the shorter days tell the trees to start shutting down, it's not the only factor at play in developing the fall color show. If that were the case, all the trees in an area would change to the same colors at about the same time.
However, most people have noticed that there are a few trees that start changing much sooner or that hold onto their leaves longer. Even trees in the same species often change on different timetables and have distinctly different colors. In fact, leaves from the same tree often have strikingly different color patterns — what's up with that? While the reduction of UV energy triggers the foliage process, there are a number of other factors that affect leaf and foliage color.
- Leaf Pigment
Chlorophyll isn't the only pigment present in leaves, even when they're bright green. Leaves also contain carotenoids, which produce orange and yellow colors. The carotenoids are present year-round, but they can't shine as long as there's a lot of chlorophyll around.
In addition, as the last of the chlorophyll produces glucose that is trapped in the leaf by the hardened abscission layer, it forms a new chemical — anthocyanin — which shows up as vibrant reds and deep purples.
While it may seem intuitive, cold weather doesn't actually have a lot to do with the color of autumn foliage. Both summer and autumn weather does have an effect, though. If the trees are stressed during the summer by dry conditions, they may start losing their leaves or changing color early, leading to less color in the fall. A bright, sunny fall can promote brighter foliage by promoting the production of anthocyanins, which need sunlight for their chemical changes to occur.
Since the amount of sunlight directly affects the colors that develop in autumn leaves, trees that are the most exposed to the sun are likely to show richer, more vibrant colors than others. That's why you'll often see the most spectacular colors along the upper ridges of a hill or along its southern exposure.
- Tree Species
Finally, fall foliage color is partly determined by the species of the tree. The Harvard Forest in Massachusetts has been studying and documenting fall foliage over the years. Their project includes lots of information about the major tree species in the northeastern U.S., including close-up photos of leaves from different trees showing the distinctive patterns typical of sugar maples, elms, and other common trees.
Predicting Fall Foliage: A Fun Family Project
Areas that rely on fall tourism have made it a priority to predict peak fall foliage so that they can prepare for the tourist season. The experts use their knowledge of the surrounding forest and the recent weather to help them pinpoint the most likely time for the trees to be displaying their brightest colors.
You can use your home weather station to predict when the leaves in your neighborhood are most likely to change color, and even guess what colors will show up. Here are a few questions that can get you started.
- What kinds of trees are around your neighborhood?
- Which trees get the most and least sun?
- How much rain has fallen in your area this summer?
- How much sunlight is your yard or neighborhood getting? Trees that are in the shadows of tall buildings, for example, may not change colors as dramatically as those that get unobstructed sunlight.
- What are the local meteorologists or weather enthusiasts predicting for your area's autumn weather?
- Which trees turned soonest and latest in your neighborhood in past years?
The information you gather this year can help you make predictions for the future.
Tracking the Changing Fall Colors
AcuRite makes it easy to measure and track all of this information. Any of the AcuRite Olympus Series™ weather stations allow you to track temperature and humidity in real time. The AcuRite Atlas® also measures UV, light intensity, and hours of sunlight and tracks it in My AcuRite®. You can even access this information remotely via the My AcuRite app!
Tracking the autumn foliage is more than just a fun project. The Harvard Forest foliage project notes that autumn colors were different a century ago and will probably change during this century. Many of these changes are due to human activity of one kind or another. The records you and your family keep today may help document these changes as they occur.
A little extra! We always love to hear from our #JrWeatherWatchers. You can download a weather log to track the temperature and other weather facts, including signs of the first autumn leaves. Share your logs and autumn leaf projects online, and be sure to tag us (@acurite) on Facebook, Twitter, and Instagram so we can enjoy them too.