Fall is here and leaves across Wisconsin are starting to turn hues of orange, yellow and red. But you might be wondering — why do some leaves change color in the fall?
To get to the bottom of fall colors, Lake Effect’s Xcaret Nuñez speaks with Erica Young, a professor of biological sciences at the University of Wisconsin–Milwaukee.
Young explains changing leaves starts with environmental cues, like days getting shorter and chillier.
This interview has been edited for length and clarity.
I don't know about you but fall is my favorite season, and that really has to do with the change of colors in the leaves. But before we get into why leaves change colors, can you help explain why leaves are green?
Yeah, I also love fall! Although, obviously the plants are doing their job over the whole of the summer, and they need that green chlorophyll pigment to help them do that. So this helps them harvest light energy, which they convert into chemical energy, which they can then store and ultimately produce sugars, which they can use as food. But [sugars] can also be stored in the plant and be a food for other organisms that might eat that plant. So that chlorophyll is about harvesting solar energy and basically driving everything on this planet. So chlorophyll is pretty important.
That is a big role! So what happens to chlorophyll in the fall, when the sun isn't out as much like it is in the summer?
The chlorophyll is in the leaves during the summer, and there are other pigments in there as well. But as the plants sense that the days are getting shorter and the temperature is going down, particularly at night, when we get kind of little chills, the plants start getting ready for winter. So one of the adaptations to coping and surviving in the winter is to not have the delicate parts, the leaves, on the plant during the winter. So during fall, when they're getting those signals of the shortening days and the cooler nights, they start activating processes to get ready for winter.
Chlorophyll is a pigment that has nitrogen in it, which is an important resource for plants. So instead of just losing that chlorophyll when the leaves fall off the tree, they withdraw that chlorophyll slowly from the leaves and store that nutrients in the rest of the plant and its roots during winter, so that they can use it again in spring. This involves breaking down the chlorophyll molecules and translocating those nutrients out of the leaf back into the plant, so at that point, the leaves stop being quite so green. But we can see the other pigments that were always there over the summer, but were often masked by that bright, strong green color of the chlorophyll.
So we see other pigments, like carotenoids, which are the yellows and oranges. Some plants also start making other pigments, called anthocyanins, which they make during the fall to protect their leaves during that process, while they're recovering nutrients — we call this false senescence. And so these anthocyanins are more of the reds and purple colors that you can see in some of the plant species.
In addition to chillier nights and shorter days, what other environmental conditions make leaves change color?
So then there's also things like drought or water deprivation. And remember, we had a very dry period in September, and this can stress the plants out a little bit, and in some cases, that can mean that they start the fall senescence process a little quicker or a little earlier.
There are other environmental stresses, like nutrient deficiency, which might mean that they can't hang on to their leaves as long. There could also be things like microbial pathogens, or insect pests that might stress the plant on an individual plant-by-plant basis over the summer, which might cause them to start sooner in their fall senescence process.
So [leaves] are integrating a lot of these different factors. And this can vary both species to species, but also in a microhabitat. You know, one plant that's growing in your garden, versus one that's growing down the street may be experiencing slightly different conditions, and this may influence how quickly they start fall senescence. But there could also be genetic differences — so different genotypes may have earlier fall senescence or may have brighter colors just because of the genetic makeup of that plant.
Is climate change playing a role in when or where leaves change colors during the fall?
Yeah, great question. So as I said, plants are integrating different signals: the day length, and that day length is obviously not changing. You know, 300, 3,000, 3 million years ago, the day length at this time of year was similar, but the temperature is changing. So some plants are going to be a little bit more sensitive to temperature, and some are going to be much more strongly driven in their autumn senescence by day length.
So those [plants] that are a little bit more temperature sensitive may be waiting longer, so to speak, in fall before they start instigating that fall senescence. And the color change that we see, it may actually mean that across a landscape, some trees will be driven by the day length, and so they'll start to change at the same time as they did last year or last decade, whereas some trees may be waiting longer. So you may actually get a sort of a stretching out of the autumn senescence period that you see on a landscape level, as different trees are integrating signals in slightly different ways.
Already, you can see around Milwaukee there are trees that are losing leaves. The trees are gold or those red and purple colors, whereas many trees just look like they did in August. So you can see there that the variability, and I think as you go further north, colder temperatures help to compress the time scale over which the trees are going to start autumn senescence, which is why we tend to go further north to get the, en masse, beautiful, fall color experience.
So it seems that longer summers, or those longer summer temperatures, could delay fall colors in Wisconsin.
Yes, absolutely. The other thing with climate change is that it’s not just climate change, but it’s other environmental change factors that are associated with it — like insect pests and pathogens. Climate change stress may make plants in general and trees more susceptible to those kinds of stresses, and that might also influence how quickly or when trees start that fall senescence process.
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