The science behind fall color: Q and A with UW-Green Bay Assistant Prof. Karen Stahlheber

What’s really the science behind fall color? It just so happens that UW-Green Bay has an expert in plant ecology, Assistant Professor Karen Stahlheber. In this Q and A, she shares what she has learned through the years in her own research, as well as that of the plant physiology class that has been measuring pigment concentrations since 2005!

Q: What types of trees turn color and why?
A: Trees turn color in the fall as they recycle nutrients and prepare for the winter. Each species of tree has their own characteristic pattern of autumn color change, which is caused by the type of pigments present in the leaves, and whether or not that species produces new pigment.

Colors like yellow, orange and brown are produced by pigments that are always present in leaves, but are usually masked by the green color of chlorophyll. Around the same date every year (determined by the days shortening in the fall), trees stop producing new chlorophyll in their leaves, and instead focus on breaking down and extracting nutrients for storage before the leaves fall. The loss of the green color unveils yellow and orange pigments, which are not recycled as quickly.

Trees with leaves that turn red or purple actually synthesize new pigments, called anthocyanins, which are not present in the leaves during the rest of the year. The exact reason why some trees do this is not well known. One hypothesis is that the red color acts as a kind of “sunscreen” to protect other components in the leaf that are being broken down and re-absorbed into the stems. Another idea is that the color is a signal to insect pests about plant health, causing insects to choose less vibrant trees for laying their eggs.

Q: Why does the same tree often have multiple colors?
A: Each leaf can be going through the process of dying at a slightly different rate, meaning that some leaves are still green while others are colorful. Sunlight affects how much of the red pigment leaves produce, so the sunny side of a tree canopy may also appear more vibrantly colored than shadier parts.

Q: What provides the intensity of color?
A: Temperature and sunlight can affect intensity of the red coloration. So, a period of dry, cold nights with bright sunny days will bring out the most brilliant colors! This hints at the idea that red pigments provide protection from light for the structures in leaf cells. The intensity of yellow and orange colors mostly depends on the species – some trees will drop leaves while they are still green, other trees like beech turn brown as the yellow pigments are destroyed and the remaining tanning pigments are oxidized.

Q: Do the same trees turn the same color at relatively the same time?
A: Generally, yes. Most studies of phenology (the timing of seasonal events like leaves falling or the emergence of leaves and flowers in the spring) in trees have found that individual trees tend to be consistently “early” or “late” from year to year. The beginning of the autumn for trees is signaled by changes in the length of day, which is always the same each year. This makes the season relatively predictable, with variation provided by secondary cues from temperature that control how fast the season progresses.

Q: Any other interesting facts you would like to share about the science behind color change?
A: We still don’t have a clear sense for how fall will change along with the climate of Wisconsin. Cooler nights in fall are best for brighter colors, so warm nights could make fall leaves of the future look dull. A wet season can make colors duller as well, but drought in late summer can make trees drop their leaves early, effectively ending the show before it begins. On the other hand, increasing carbon dioxide is predicted to intensify colors. The upper division plant physiology class on campus has been measuring pigment concentrations in several tree species on campus since at least 2005, so we have a rich source of data here on campus to begin looking at this question!

Q: What is your best educated-guess, please. When will “peak” be on campus this year?
It’s hard to say because it depends on a lot of factors and can change quickly! (Think of windstorms, cold (or warm) snaps or heavy rain). But I would guess the first week of Oct will be looking really good around campus.