Tree Life: The Science Behind Fall Colors

As autumn quickly approaches, Wisconsin’s landscape is getting ready to put on its final show: a display of vibrant fall colors. You might even already have plans to visit your favorite pumpkin patch, nature preserve, or view the seasons changing right here in the Garden! But why do some trees drop their leaves earlier than others? What determines whether a forest becomes fiery orange or soft ochre? Let’s explore some common fall questions together!

How do trees survive the winter without leaves?

You’ve probably heard the terms deciduous and evergreen before. They’re both two distinct ways of living life as a tree, and each strategy has its costs and benefits.

Deciduous trees drop their leaves. This means that each spring, they spend massive amounts of energy creating large leaves to soak up all of the summer’s sun. These leaves are delicate, release loads of precious water into the atmosphere through evapotranspiration, and require tons of energy to maintain. When conditions become unfavorable, they’ve got to go. Still, deciduous leaves soak up so much energy from the sun during the warmer months that the tree is able to coast through the winter, and still has enough energy to bud each spring.¹ Shedding leaves also means that deciduous trees are less likely to be damaged by the harsh winds and heavy snow of winter.⁴

Evergreen trees are in it for the long haul. They often have leaves that take less resources to create, are less costly to maintain, and are hardy enough to survive the winter. However, they come with drawbacks. Evergreen leaves, like pine needles, normally can’t capture as much sunlight energy as deciduous leaves with large surface areas, like catalpa leaves. Still, they’re able to make the most of dwindling winter sunlight, and can photosynthesize to sustain themselves year-round.¹

Don’t be fooled! Not all conifers are evergreens. Some conifers, like Wisconsin’s native tamarack trees, are actually deciduous and drop their needles in the fall.²

Why do trees shed their leaves?

Trade-offs are a constant in the life of a tree. While deciduous trees need leaves to photosynthesize, sometimes their costs outweigh their benefits. As the days get shorter going into fall, trees aren’t able to photosynthesize as much because of the decreasing day length. If a tree continues to maintain its leaves, it risks spending more energy on their upkeep than it’s receiving through the photosynthesis they carry out. When a deciduous tree senses that it’s photoperiod, the amount of time it is receiving sunlight, is declining significantly, it causes a series of hormonal changes in the tree that cause abscission, the shedding of its leaves.³

These hormonal changes can also be triggered by other stressors. If a deciduous tree is experiencing a drought period, dangerously cold temperatures, or a number of other threats, it may drop its leaves as well.³

Why do trees change color?

The rich greens that trees display in summer are caused by chlorophyll which is created and used by the tree for photosynthesis. This strong green often prevents other colors that exist within the leaf’s cells from showing.⁵ As a tree prepares for abscission, the production of chlorophyll is slowly halted.³ This allows colorful carotenoids and anthocyanin to shine. Carotenoids cause yellow, orange, and brown hues while anthocyanin tends to bring a punch of red or purple. The amount of each in a tree specie’s cells determines its fall hue. Sugar maples, for example, tend to be a bright vermillion while black maples turn golden. Oaks, on the other hand, range from red to brown.⁵

When are peak colors in Green Bay?

Right now, we’re expecting peak fall colors towards the end of October⁶, but clearly there’s a lot at play! Plan your trip to the Garden today, and keep an eye on the fall color forecast so you don’t miss out.

Sources

1. Baldocchi, D., Ma, S., Rambal, S., Misson, L., Ourcival, J.-M., Limousin, J.-M., Pereira, J., & Papale, D. (2010). On the differential advantages of Evergreenness and deciduousness in Mediterranean Oak Woodlands: A flux perspective. Ecological Applications, 20(6), 1583–1597. https://doi.org/10.1890/08-2047
2. Field Edventures. (n.d.). Tamarack or Eastern Larch. EEK! Environmental Education for Kids. https://www.eekwi.org/plants/tamarack-or-eastern-larch#:~:text=The%20tamarack%20is%20the%20only,%2C%20soft%2C%20and%20flexible%20needles
3. Garver, K. (2023, April 7). What Causes a Tree to Enter and Exit Dormancy?. Penn State Extension. https://extension.psu.edu/what-causes-a-tree-to-enter-and-exit-dormancy
4. gov.scot. (2020, October 30). Why Do Trees Lose Their Leaves?. Forestry and Land Scotland. https://forestryandland.gov.scot/blog/trees-in-autumn#:~:text=There%20are%20several%20reasons%20why,in%20winter%20storms%20and%20gales
5. U.S. Department of Agriculture. (n.d.). Science of Fall Colors. Forest Service. https://www.fs.usda.gov/visit/fall-colors/science-of-fall-colors
6. Wisconsin Department of Tourism. (n.d.). Wisconsin Fall Color Report. TravelWisconsin. https://www.travelwisconsin.com/fall-color-report