Subtle Changes to Autumn Leaf Color
Without an early frost, a longer but less red fall leaf season
The leaves are falling once again outside the window as I write this third annual autumnal essay. Such a beautiful season.
Those cascading leaves often bring to mind an older film, The Bostonians, whose opening scene captures a richly colored forest edge in New England at the height of the fall foliage season, with leaves dropping thickly like a biological blizzard. That image remains etched in my memory nearly 40 years later!
But then autumn and the intensity of mountains in color as summer turns to winter has always captured the imagination of poets and enticed travelers to their favorite places to watch the drama unfold. Perhaps some combination of nostalgia for the summer just passed and a bit of unease about the winter to come heightens the experience.
In New England, where I live, few events define the region as much as the autumnal color explosion – that brief time between the solid green of summer and the coming bareness of winter. Robert Frost was speaking of spring when he wrote “Nothing Gold Can Stay,” but the same sense of fleeting beauty and imminent loss could be applied to autumn as well.
Predicting the timing, intensity and duration of tree color is an annual guessing game with important aesthetic and economic implications. Weather, insects, storm damage can all affect when that peak color will occur.
But one trend is clear: The fall season is starting later and lasting longer.
Scientific measurements capture the season’s late start in an unambiguous way. As one example, continuous measurements of rates of photosynthesis of whole forest stands at the Harvard Forest in central Massachusetts have documented both an earlier start and later end to the physiological growing season. Their numbers show that the season is being extended by an average of half a day each year and now ends more than two weeks later than in 1990.
On the commerce side, one recent article quoted the owners of a popular local pancake house in the White Mountains here in New Hampshire who noted that their busy autumn season now runs two weeks longer than it did in the 1970s. Hotels and resorts are booking leaf-peeping bus tours into late October.
So why is the season later and longer, and has that longer season changed the nature of the display? Does that opening scene in The Bostonians still capture the experience?
In earlier decades, the timing of the first frost was a key to the timing of peak color. A hard frost can trigger the process of senescence, the time when the green of chlorophyll used in photosynthesis is lost, and the other pigments, the reds, oranges, and yellows, are revealed.
Those bright reds may be particularly sensitive to the timing of frost and senescence, but more on that in a minute.
When the season is synchronized by an early frost, it might be quick and brilliant, and might be shortened or wiped out entirely by an early blizzard. We spent one memorable day with family from California trying to reach the White Mountains to see the peak foliage, only to be turned back by an October snowstorm. But that was more than 30 years ago.
An early killing frost can send whole hillsides into the sequence of senescence and color, and then leaf drop, at the same time, as captured in the opening scene from The Bostonians. Synchronous color is what you see in the glossiest images, and what might be the goal of those planning a fall foliage experience. But be quick and flexible, synchronous leaf color and leaf drop mean that same hillside might be bare in a week or two.
But first frosts have been coming later, delaying and extending the season. A few decades back, the expected date of first frost here in southern New Hampshire was around October 5. Summer this year has been extended through September and well into late October, with highs into the 80s on some days, and plenty of rain. October 8 was our first day with the kind of crisp, dry, sunny weather that triggers thoughts of pumpkins and apples and raking leaves.
Crisp but not frosty. As of this writing, we have had only one morning below 40F. This week has included near-record highs in the 70s, and the ten day forecast suggests only a slight chance of reaching 32F before November 7. Color change and leaf drop have been slow and continuous, with little of the bright red color. At least locally, leaf color has been delayed and subdued. Some trees are bare, and some still all green. Many red maples have leaves that are turning at different times, and even at different rates within a leaf.
More than once I have heard people say, “Don’t the trees look tired?”
Surprisingly, the science of leaf color and senescence is not completely settled, and is often buried in media presentations on the arguments, sometimes humorous, sometimes not, about which regions have the most intense and most beautiful displays. Is it the southeast with the subtle brown and yellows of southern oaks and hickories? Or the intermountain west where the aspens leaves turn a vibrant yellow?
New Englanders of course know that their region has the best color (let the arguments begin!). If you like the reds, then there is some support for the case. Several sources note that while 10% of temperate zone species produce the red pigments known as anthocyanins, that number is up to 70% in this region! Red and sugar maples and northern red oaks are listed as prime producers, or at least they were in earlier times.
With the science a bit uncertain, I feel free to speculate as to why autumn leaf display is changing, at least in my corner of the world, and will focus on these questions:
When “should” a deciduous tree let go of its leaves?
And
Is there an advantage in turning red first?
l will go against all scientific protocol here, as I did in an essay on trees as “crafty green strategists,” and write as if trees plan these things. We know they don’t, but it gets tedious to say repeatedly that “natural selection has led to the evolution of adaptive traits that enhance the likelihood of successful reproduction of progeny that will survive to reproduce…etc.”
I hope you will agree….
So when should a tree drop a leaf? When energy gain through photosynthesis is less than what is lost through respiration, the energy used to keep the leaf alive. When the leaf becomes a net drain on energy resources, the tree says, “off you go.” (It is so much fun for me as a scientist to be able to write this way – hope you don’t mind!).
A leaf killed or badly damaged by frost will lose the ability to capture energy and should be shed - as should one that becomes a drain due to drought that limits the amount of photosynthesis by that leaf. But those two sources of stress are quite different.
A hard frost can affect all species equally, simultaneously triggering that wild explosion of color, as well as leaf senescence and leaf drop, across all the trees in a stand. Drought can make some leaves useless as well and can lead to an early, partial senescence, but affects different species on different sites, well, differently, so simultaneity can be lost.
Whatever triggers senescence, leaves are rarely dropped immediately. The period of leaf color coincides with the movement of key nutrients back into the adjacent twigs. As much as half of the nitrogen and phosphorus that was in the green leaf can be saved in this way for use next year. And while that crucial process is happening, the living parts of the leaf need to be kept alive and protected from the harmful effects of the bright sunshine previously absorbed by chlorophyll.
Foreshadowing here the discussion of that bright red color….
But this year, in the absence of frost or drought, why are the tress still dropping those leaves?
Leaves on deciduous trees are programmed to die. They have a life expectancy. They age across the growing season. Rates of important processes like photosynthesis peak early in the growing season and then decline across the summer months. Ozone pollution (“smog”) can accelerate this aging process (as it can for human lungs!). Leaves exposed to ozone experience irreversible declines in their ability to gain carbon and energy through photosynthesis.
You can see where this is headed. As leaves lose the capacity to support the tree, an effect augmented by shorter and cooler days, and perhaps accelerated by exposure to ozone, the tree says “enough – off you go - but first give me back those nutrients!”
So here is my speculative explanation (or hypothesis) for the long and relatively less-red season this year.
The leaves have gotten old (tired?). No drought, no frost, a bit of ozone during the summer, and they have exceeded their useful life span. But they become useless at different rates, for any number of reasons that can vary for each leaf or branch, so some go earlier than others.
They “go” by losing chlorophyll and exposing two other categories of pigments, carotenoids and xanthophylls, that were always present and show as yellow to orange.
What about the reds?
Red coloration in senescing leaves comes mainly from compounds called anthocyanins. These pigments are not produced in leaves before senescence, but are formed in the presence of sunlight from other molecules that accumulate as complex compounds in the leaf are broken down. So bright sunny days after a frost, when the leaf is going through senescence and moving nutrients back into the twigs, might be key.
General descriptions of what creates a vibrant leaf season often mention the importance of sunny days and cool (frosty?) nights.
And what do anthocyanins do for the tree (not the leaves - they will be history soon)? Two things:
1. Anthocyanins are very strong antioxidants that can delay aging.
2. Anthocyanins act as a sunscreen, protecting cells from damaging solar radiation as the tree directs the movement of nutrients back into the twigs.
So those vibrant red colors might be extending the life of damaged leaves and protecting them from excessive sunlight while the important process of recovering nutrients is completed. One study reported that lower nitrogen content in fallen leaves was related to stronger red color.
But if senescence is postponed until days are shorter and darker, and leaves have aged beyond the point where they are physiologically vigorous, then perhaps there is neither the sunlight nor the capacity left in the leaf to produce or make use of the protective functions of those red anthocyanins. Outcome: Less red in the fall leaf display.
Can living trees “reprogram” leaves next year to age at a different rate? I don’t know. If not, are we going to experience more yellow-to-brown autumns as genetically programmed longevity determines leaf drop instead of frost? I don’t have an answer for that either. We will have to wait and see.
On the other hand, autumn color is so varied and its perception so conditioned by our expectations and history, we may just end up enjoying the longer season, even if it is a little less vibrant, or a little less red.
But perhaps that opening scene from The Bostonians might prove to be an important historical record of an earlier era, not just in the social context that Henry James captured in the novel on which the movie is based, but climatically and ecologically as well.
Sources
The image of a vibrantly colored mountainside in New England is from:
https://newengland.com/today/new-england-fall-foliage-2022-forecast/
Photo Credit : Jim Salge
The map of expected peak leaf color is from:
https://smokymountains.com/fall-foliage-map/
This site also has a good explanation of leaf color change
The image of the tower at the Harvard Forest in Petersham, MA, is from:
https://harvardforest.fas.harvard.edu/photos/eddy-flux-tower
and the data on the change in photosynthetically active period is here:
Finzi et al. 2020. Carbon budget of the Harvard Forest Long-Term Ecological Research site: pattern, process, and response to global change. https://doi.org/10.1002/ecm.1423
The graph of changes in first and last frost is from:
https://www.epa.gov/climate-indicators/climate-change-indicators-length-growing-season
This site also has a map of changes in the length of the growing season.
Source for the statement that 70% of trees in New England produce anthocyanins
https://en.wikipedia.org/wiki/Anthocyanin
The graph of the effect of leaf age on photosynthesis is modified from:
https://plantlet.org/factors-of-photosynthesis/
Good sources on the role of anthocyanins in delaying aging and protecting against sun damage include:
https://www.fs.usda.gov/detail/whiteriver/home/?cid=stelprdb5388915
https://harvardforest.fas.harvard.edu/leaves/biological
The graphic of the color of different leaf pigments is from:
https://www.chicagobotanic.org/fall/coloring
This site also has a good explanation of the role of each pigment.
Thanks to Colleen Tlagae for the image of Lake Winnipesaukee.
So much is written on leaf coloration and leaf peeping that it is tiresome...every year as director of the Harvard Forest, I used to get the same questions from an array of reporters who then wrote the same tedious articles.
But, this contribution is terrific -- engaging, innovative, informative and knowledgeably speculative.
Fabulous. And please, keep speaking for the trees. I am sure they appreciate it as much as this reader does.
Best, David