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As the Minnesota Fall Color Map helps leaf peepers track the leaf color progression across the state, St. Olaf College Assistant Professor of Biology and Environmental Studies Jake Grossman answers questions on why we see such a vibrant color change every autumn. 

Why and how do leaves change color at the beginning of the fall season?

To understand why leaves change color in the fall, we need to understand what common pigments — light-reflecting compounds — are found in leaves, why they are there, and when and why they go away. The main players are green chlorophyll, orange-yellow carotenoids, and red anthocyanins.

Chlorophyll is the powerhouse of leaves, allowing them to convert sunlight and carbon dioxide into sugar, which fuels the plant. Chlorophyll –– which absorbs red and blue light and reflects green light –– is so abundant in leaves that its green color dominates our perception of what most leaves look like. 

Leaves are full of chlorophyll from when they emerge in the spring until they become dormant in the fall. Chlorophyll breaks apart as it is used to carry out photosynthesis, so it’s constantly regenerated over the growing season. When it gets chilly and the days grow shorter, leftover chlorophyll is broken down into its components and “recycled” by the plant into other useful forms.

Do all leaves go through the same color-changing process? Why do some turn orange while some are still green?

When chlorophyll dwindles in the fall, we can finally see the background pigments that have been part of the leaf all along: orange-yellow carotenoids. This diverse family of pigments exists to help protect leaves from excess light. On a really sunny day, light energy that a leaf can’t handle via photosynthesis can get shunted to carotenoids, which then disperse this excess energy. That’s a good thing for leaves, which can be “burned” by sunlight that exceeds what their chlorophyll can transform into food. 

Even though plants can increase or decrease the carotenoid content of their leaves, carotenoids are fat-soluble and basically are stuck inside leaves. Leaves can’t break carotenoids down or recycle them like chlorophyll. That means, in the fall, chlorophyll gets drained away. And, in the absence of its green color, we can see the oranges and yellows, which have been there all along.

What about red leaves? Red color generally comes from anthocyanins, a third class of pigments that plants can create to protect themselves from excess solar radiation. Unlike xanthophylls, anthocyanins are water-soluble and can be synthesized and broken down on demand. In fact, they are also produced heavily in new leaves, which sometimes have a reddish tint to them. In the fall, anthocyanins are most visible in regions that have sunny but cold days. Under these conditions, there is a lot of available solar energy, but chlorophyll has either been broken down or is less efficient due to the cold. It has been hypothesized that North American temperate forests have such beautiful fall color in part because many parts of the eastern U.S. and Canada have cold, but sunny, fall climates.

To summarize, fall color happens because green chlorophyll goes away, orange-yellow xanthophylls become more visible, and red anthocyanins get produced!

Do the changing and falling of leaves in autumn have a role in the forest ecosystem?  

Yes — the changing and loss of leaves, which we can call senescence, is one of the main ways that energy and nutrients get recycled within forest ecosystems. Imagine that trees are nothing more than bags of energy and nutrients. This energy is stored within starch, sugar, and proteins,  which are built from nutrients such as nitrogen and phosphorus. Though plants try to reabsorb some of the energy and nutrients in leaves back into their woody tissues, which remain intact all winter, the compounds left in autumn leaves are lost when leaves fall off the plant. 

Dead leaves, or leaf litter, then decompose, and they usually do so close to the plant that shed them. Some litter carbon and nutrients get lost to the atmosphere, some get eaten up by microbes and animals and stored in their bodies, and some get returned to the soil, making it more fertile and richer in organic matter. In this way, senesced leaves maintain soil health and promote the long-term growth of the very plants that produced them in previous years. So this process of leaf senescence can be thought of as a way that plants condition the soil they grow in and “recycle” nutrients!

How does climate change affect trees’ fall foliage? 

This question hasn’t been definitively answered yet! Scientists studying the seasonal timing — or phenology — of fall leaf senescence still don’t know exactly which environmental factors shape leaf senescence across tree species. It seems pretty clear that hours of sunlight in a day and air temperature play a role for most species. For instance, by August in Northfield, many trees have already started responding to shorter days by beginning the processes by which their leaves will senesce over the following two months. Climate change doesn’t affect sunlight hours, so that factor probably will not shift fall color in the foreseeable future. However, we know that our fall days are getting warmer on average. It’s hard to know what impact this will have. Warmer days could prolong summer and keep leaves green, but shorter days may keep this from happening. It’s also possible that warm, clear days will lead to less intense red coloring in fall leaves, since the stress of cold weather plays a role in anthocyanin formation. It’s worth mentioning that summer drought can also kill leaves and simply lead to less autumn foliage on trees. 

How does your work at St. Olaf educate and support the efforts of preserving a healthy environment? 

My students and I study how two global change factors, climate change and biodiversity loss, change the way trees and forests function. For instance, we regularly document the phenology of leaf senescence in the fall and leaf and flower emergence in the spring on a few different species. We also measure how cold hardiness — the temperature to which buds can freeze before they die — and leaf drought tolerance change over the year in environments like Minnesota’s. And we take advantage of big, National Science Foundation–funded forest experiments at the Cedar Creek Ecosystem Science Reserve to study how losing biodiversity in a tree’s neighborhood affects that tree’s well-being.

One of the most interesting things we learned recently is that in a normal, cooler year, red and silver maples flower very early in the spring, then leaves take months to emerge on the same trees. In a warm spring –– the kind that is more common given climate change –– leaf development is sped up and leaves pop out of their buds much sooner. Based on our research, we have found that climate change seems like it is compressing the springtime phenology of these two common maple species! 

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Jake Grossman is an assistant professor of biology and environmental studies at St. Olaf. Grossman teaches in the Biology and Environmental Studies Departments, and mentors student researchers studying the consequences of climate change and biodiversity loss for temperate trees and forests. He is a terrestrial ecologist and plant ecophysiologist, and the faculty supervisor of the Biology Department greenhouse. He also co-manages the Forest and Biodiversity experiment as part of the science team at the NSF-funded, University of Minnesota Cedar Creek Ecosystem Science Reserve LTER.