Here's some science behind the brilliant colors of Autumn and how the season may adjust due to climate change

NSF's Weather or Not!

Here's some science behind the brilliant colors of Autumn and how the season may adjust due to climate change

National Science Foundation


Why do leaves change color in the fall?

Interviewer: Charlie Heck

Interviewee: Andrew Richardson

Charlie: Fun fact: The typical deciduous forest in New England has about 4 layers of leaves. So, for every one square foot of ground area, there are four square feet of leaf area. Want one more fun fall foliage fact…you got it! In summertime, leaves are green because they contain chlorophyll, the main light-absorbing pigment for photosynthesis. Chlorophyll is very good at absorbing blue and red light, but not quite as good at absorbing green light.

Now, along with fun facts, I have the second half of my interview with Andrew Richardson, about fall colors and how climate change may impact when fall starts in some areas and how long it lasts. Richardson is an associate professor of biology at Harvard University and for him, fall is well under way.

Andrew: In New Hampshire and Northeastern Massachusetts, right now the colors are fantastic. The red maples are brilliant reds and purples. The sugar maples are yellow and orange. The oaks haven't started doing much yet, so there's still-- they're still really green, which means there's probably going to be a fairly long season. But it's looking like it might be one of the better ones in recent years.

Charlie: So, first, I asked Richardson to explain leaf color variations: What’s the science behind all these fall colors?

Andrew: Some species are only ever going to turn yellow. Things like birch, they turn yellow in fall because their chlorophyll is breaking down. And that is unmasking other pigments called carotenoids which are yellow and orange. They were previously hidden by the chlorophyll. So the chlorophyll breaks down. You can see those lighter colored pigments. Other trees like red maple and some of the oaks turn really brilliant reds and purples. And what's going on there is that their chlorophyll's breaking down but they're also synthesizing a new pigment, a new type of pigment-- anthocyanins-- which give the leaves those really incredible bright purples and reds.

Charlie: Then I asked him about the impact of a warming climate on the fall season. Richardson and his colleagues conducted a study a few years ago using long-term records from Harvard Forest. Since 1988, the Harvard Forest has been a Long-Term Ecological Research Site, funded by the National Science Foundation.

Andrew: Our colleague there, John O'Keefe, has been looking at vegetation phenology. So how the seasonal cycle of leaf development and senescence and coloration. He's been going out and looking at the same trees for 25 years and making notes on what each tree is doing every week of the year. And from analyzing those long-term records we can establish relationships between weather and both the timing of leaf color and leaf fall and also how intense those colors were. The start of leaf coloration is triggered by short days and cool weather. But if that was really-- if it was just short days that mattered then there would be no variation from year to year. Our analysis of John's long-term data showed that warmer temperatures could prolong and also prolong the timing of-- or delay the timing of leaf fall and leaf coloration but also prolong the duration of those colors and in some cases increase or decrease their intensity.

Charlie: So what does the future hold for those famous fall colors? Richardson says warmer temperatures will likely delay the season, maybe into November. Second, the colors we’ve come to know and love might get bolder and last longer in New England but in other regions, interactions between rising temperature and reduced summer/fall precipitation may result in more muted displays. Last but certainly not least, over longer time scales we could see a change in species compositions.

Andrew: So if in the long-term if, you know, say, red oak ends up replacing sugar maple across a lot of New England, then we won't have quite as good colors as we do because sugar maple really is one of the most charismatic autumn species around. So the changes in species range could kind of modify any of the predictions that we're doing in our model.

Charlie: That is Andrew Richardson, an associate biology professor at Harvard University. He runs the Phenocam Network.
I’m Charlie Heck, co-editor of Science360’s news service and co-host of the Super Science News Show, at the National Science Foundation.
Phew! OK, now, I’m ready for my next interview. If you have any suggestions I could fall for… get it… fall for… email your cool NSF-funded research idea to, and I will fall-ow up!