Every autumn, we eagerly await the succession of fall color that ripples through our urban forest from late October through December. Shorter days and changing weather combine to produce an awe-inspiring show that beckons us outdoors to enjoy fall. Have you ever wondered about the science behind the season? Here is a brief guide to this beloved autumn phenomenon.

What is fall color?  

Fall color occurs in deciduous trees that lose their leaves each year just before they go dormant for the winter. In colder climates, losing their leaves protects them from freezes.

From spring through early fall, leaves produce a green pigment called chlorophyll that allows them to produce sugars from sunlight in the process called photosynthesis.  

Shorter days cue the tree to go into dormancy and begin shedding its leaves. Veins that transport water and nutrients to and from leaves begin to close, which halts chlorophyll production and traps sugars within the leaves. Gradually, leaves lose their green coloration as chlorophyll fades to make way for a rainbow of beloved autumnal shades.  

Fall colors are combinations of pigments called carotenoids and anthocyanins, along with fading green chlorophyll. Carotenoids produce shades of yellow, orange, and brown as found in Ginkgo, Chinese pistache, sycamores, and trident maples.

Red, scarlet, and purple coloration comes from anthocyanins, recognizable in species such as tupelo, red maple, Shumard oaks, and Liquidambar.

If the names of these pigments sound familiar, it’s because they’re common throughout the plant kingdom, including in produce that comes in a rainbow of colors. They give the same coloration to fruits and veggies and provide a myriad of health benefits, although we don’t recommend eating tree leaves for nutrition’s sake.  

Why does fall color vary every year? 

The development of fall color is an intricate chemical dance between the tree and its climate. We can never fully predict the timing or intensity of the display because there are many factors that all affect how the tree responds.  

We do know that the perfect recipe for fall color involves cool nights and warm sunny days as we transition into fall. Spring and summer weather can also play a role. 

This year, early fall color and leaf drop occurred in some trees that were stressed by drought and pests. Many elms in midtown shed their leaves in late September after struggling with severe infestations of elm leaf beetle. Other trees showed early fall color due to inadequate water during this dry year. Drought-related fall color was especially noticeable in trees in low- and no-water landscapes.

Why doesn’t my tree have any fall color?  

Some trees are evergreen and do not lose their leaves during the winter, so they do not show fall color. This includes most conifers (trees with scaly or needle-like leaves such as pines, coastal redwoods, and cedars) and some broadleaf species such as camphors, live oaks, bay laurels, and strawberry trees.  

Additionally, some deciduous trees such as elms and sycamores sometimes don’t produce as much pigment in their leaves and go quickly from green to faded brown.  

Now that you know how the magic of fall color unfolds, you might want to check out some self-guided tree tours to impress a date with this knowledge or find a fall stunner on our list of recommended trees to plant in your yard.