Consider the Humble Leaf
Crafty Green Strategists, Autumn Leaf Color and Climate Change
This was not my intended topic for this biweekly series of essays on weather, climate and climate change, but as I sit at my desk watching the leaves fall, it seems the right time to talk about the seasonal show that deciduous trees and shrubs put on as the growing season ends. The Fall season and the coloring of leaves are treasured experiences here in New England, and I suspect in most places where trees grow; a pleasurable if ephemeral exclamation point to Summer to be savored because we all know what comes next.
In that brief exuberance, when the forests transition from dark green to bright yellow and red – or even dark brown - news and travel websites are full of predictions as to when color will “peak.” The timing can vary widely year-to-year. Delay your trip by a week and you could miss it. I remember one Fall day heading to the White Mountains here in New Hampshire with family members to see the foliage, and being turned back by an unexpected blizzard!
Artists and poets derive inspiration from the Autumn blitz, but a scientist can as well. Color and shape have much to do with my ponderings about the season, but while watching the leaves fall, I tend to think about things like this:
Why do some trees lose their leaves each Autumn and some hold them through the Winter?
Why do leaves come in such a wide variety of sizes and shapes?
Why do they turn different colors – or any color at all – before they fall?
What else is going on as the colors brighten from dark green to reds and yellows?
What determines when the color will happen?
And, yes, given the nature of this site and these essays, how might climate change affect the timing and the color?
I will confess that the topic of leaf strategies is one of my favorites. As a new graduate student, I had the pleasure of reading a book called The Adaptive Geometry of Trees by Henry Horn. This book was one of those concise, insightful, inspiring books that helped with my research but also showed that a good science book could also be entertaining and enjoyable.
In the book, Horn describes trees as “Crafty Green Strategists.” He did not intend by this to attribute intelligence or forethought to these green giants, but used it as metaphorical shorthand for the longer and more precise scientific version: “through the process of natural selection, tree species have evolved a variety of traits and physiological mechanisms to enhance viability and reproductive success into the next generation.” More accurate perhaps, but I think “Strategists” conveys the concept more effectively!
So why does “the humble leaf” show such a wide range of sizes, shapes and longevities? If all leaves have one primary purpose, to gain carbon and chemical energy through photosynthesis, then why do they exhibit such a variety of strategies? Strategies imply tradeoffs – how to balance gains and losses – how to balance different limitations. How does that apply to leaves?
While the primary purpose of all leaves is photosynthesis, an unavoidable consequence of being open to the air to gain carbon dioxide is the loss of water from within the leaf. So plants are balancing carbon gain against water loss and water stress.
Leaves are displayed, at least in forests, in a range of light intensities, from fully exposed at the top of the canopy, to quite dark at the bottom. How should leaves change to optimize carbon gain in these different light environments?
The sun’s energy drives photosynthesis but heats the leaf as well. Excess heat can damage leaves or at least reduce net carbon gain. How are leaves “designed” to reduce heat load?
Part of the energy captured by photosynthesis will go into buds that will become new leaves next year, but more than sugars are needed to do that. Deciduous leaves are about 2% nitrogen, with lesser amounts of other elements. Evergreen leaves are generally closer to 1% nitrogen – but more on that in a minute. In both cases, the availability of nitrogen can limit how many leaves can be produced in a year.
The amount of nitrogen packed into a leaf, then, becomes part of that “crafty strategy.” The more nitrogen, the greener the leaf (more chlorophyll, the molecule used to capture that solar energy). So why not just “stuff” the leaf with as much nitrogen as possible? You can guess that there are strategies and trade-offs involved.
More nitrogen means more chlorophyll but also more of the nitrogen-rich biochemical machinery (enzymes) required for the complex reactions that happen in a living leaf. There is a carbon cost (respiration) to maintaining this system, and the more nitrogen-rich, the higher that cost.
So leaves with more nitrogen have the potential to gain more carbon through photosynthesis if conditions are favorable, but also will lose more carbon if conditions are not favorable. For example, if there is not enough light or enough water to make use of all the chlorophyll packed into the leaf, then the extra cost of building and maintaining that chlorophyll, and the enzymes that go with it, will mean more carbon is lost to maintenance than is gained through photosynthesis. Not a sustainable situation!
Then how do those green strategists design and display leaves to make as much sugar as possible, given the availability of nitrogen and light and water, and the impacts of heat balance and temperature? There are many adaptations to light and shade, to rich soils and poor ones. For simplicity, let’s demonstrate the concept of strategies using just two: Sun leaves versus shade leaves and evergreen versus deciduous.
Leaves in full sun tend to be smaller, thicker and more deeply lobed. Why? Thickness has an advantage for carbon gain. Thicker leaves have more layers of chlorophyll in them, and can capture more sunlight. They will also tend to have more nitrogen per unit area because the photosynthetic machinery is packed more tightly and more deeply.
Shade leaves are larger and thinner, spreading their nitrogen and chlorophyll over a greater area to optimize capture of the diminished sunlight deep in a forest canopy. This means less nitrogen per unit area of leaf. Investment of carbon and nitrogen are optimized for each environment by leaf size and shape.
Differences affect heat balance as well. Leaves lose heat through a boundary layer of still air surrounding them. The larger the leaf, the larger the boundary layer. Smaller and more deeply lobed leaves will transfer heat more effectively to an edge, and then through a thinner boundary layer. They will lose heat to the atmosphere more efficiently and stay cooler in full sun – all in comparison with larger, thinner shade leaves.
The natural world abounds in exceptions to these general trends, and here is one I like to use in class: The leaf of the quaking aspen tree is not small or lobed, yet aspens are most successful in hot, dry, disturbed sites, especially in the western U.S. Instead of small, lobed leaves, the aspen leaf has evolved so that the stem or petiole is flattened at a 90 degree angle to the leaf blade. This unique structure causes the leaf to twist or “tremble” in even the slightest breeze, increasing the rate of heat loss from the leaves, and providing an advantage on those hot, dry sites.
Leaf longevity is our second strategy. Why do some trees in the temperate zone lose their leaves every Fall (deciduous), while some are evergreen (holding their leaves - needles mostly) through one or more Winters?
Trees retain leaves for more than one year as a means of optimizing growth in stressful environments. For example, on a poor soil there may not be enough nitrogen available in a year to build enough leaf mass to capture the available sunlight. One way to increase leaf mass is to dilute the nitrogen, and evergreens in the temperate zone tend to have about half as much nitrogen (~1% by weight) as deciduous leaves. Another is to hang onto the foliage that you can build in one year for several years. Effectively, evergreens accumulate nitrogen capital in the canopy by holding leaves for multiple years.
Carbon balance is involved as well. Needles with lower nitrogen content will photosynthesize at slower rates, and so may not “repay” the carbon cost of building that leaf in it’s first year, but only by retaining each needle for more than one year.
There are additional costs to building those tougher needles that will survive more than one year. They need to be able to resist winter damage, and also more than one year of insect or browser attacks. Another cost and another tradeoff!
In contrast, on nutrient-rich soils where nitrogen is plentiful, the competitive edge will go to trees that can build flimsier leaves with high nitrogen content that will capture enough carbon to repay the costs of their production in the first year. Not needing to invest carbon in toughening up the leaves, as evergreens need to do, deciduous trees can put that extra carbon into next year’s buds instead, expand and display those leaves for one growing season only, and then let them go.
Letting the leaves go brings us back, finally, to Autumn colors in the woods. Why do those maple leaves light up, to the delight of the leaf-peepers, and what else happens as the colors brighten from dark green to reds and yellows?
There is a lot going on in those leaves as they change color. Knowing that nitrogen limits photosynthesis, you might think that trees have figured out (evolved) a way to minimize the loss of the nitrogen invested in those leaves before they fall – and they have! The whole process of shutting down in foliage is called senescence, and during senescence, as much as half of the nitrogen in a leaf is transferred back (retranslocated) into twigs and branches to be used again next year. Clever, no?
Reclaiming nitrogen happens in parallel with the color change. The greenness of leaves is due to chlorophyll. While the leaf is active, this chlorophyll masks a number of other pigments that add to the total absorbance of sunlight. Leaves look green to us because chlorophyll is best at absorbing red and blue light, while reflecting more green light.
In the living leaf, those other pigments, with names like anthocyanins, and carotenoids, play complex roles, often linked to responses to stress. In appearance, they tend to be red, orange and yellow. When leaves senesce, chlorophyl is lost as nitrogen is retranslocated, while the other pigments tend to remain. So that bright leaf color in the Fall is not due to the production of new pigments, but the unveiling of these other existing pigments due to the loss of chlorophyll.
Then the final question: When does senescence happen and why? Here is a theory. Leaves are all about optimizing carbon gain, right? So leaves should be shed when the environment no longer supports net carbon gain by an existing leaf. In New Hampshire, where I sit, senescence was often triggered by damage to leaves caused by an early first frost. Many trees were affected simultaneously, and forests would “light up” all at once.
So now we get to a possible climate link, related to one additional leaf trait. Deciduous leaves “age” across a growing season, becoming less adept at capturing carbon as the season progresses. As Summer passes into Fall, aging leaves combined with shorter and cooler days can reduce net carbon gain. When carbon gain by leaves no longer exceeds the carbon costs of keeping that leaf alive, trees should be shedding those leaves.
Autumns have gotten longer up here. Historically, the average date of first frost in southern New Hampshire has been around October 6. This year, as of October 25, the lowest temperature recorded at the nearest weather station is 39 Fahrenheit. The forecast shows nothing below 40 Fahrenheit for the rest of the month. One site and one year do not constitute a trend, but organizations tasked with predicting things like growing season and first frost have noticed the same trend across regions and years.
The classic plant zone maps produced by the U.S. Department of Agriculture, for example, have recently been redrawn, with zones moving north as the climate warms. You may notice that some weather sites no longer use the traditional 1950-1980 average temperatures as a baseline in predicting if tomorrow will be cooler or warmer than average. Using that older baseline would say that most days now are above average! The baseline period now in use is often 1980-2010. Other baseline periods can be found.
Driven by these same changes in climate, leaf color display has become more extended. It seems that more leaves are reaching senescence not due to that first frost, but to leaf aging, shorter daylengths, and maybe seasonal drought. As these vary from site to site and species to species, trees senesce at different times. Autumn color may be less intense on any given day, but may also be spread over a longer period of time.
How will the autumnal display change as the climate continues to warm? Over the next couple of decades, species composition of the woods is not likely to change that much, and the different responses to end of season stresses like leaf age, drought, daylength, and others might further extend and dilute the season. By the end of the century, predictions are for some significant shifts in species composition. If you live in New England and prefer the reds of maples to the browns of oaks, you (or your children and grandchildren) may not be happy. On the other hand, perceptions of Fall and leaf color are likely to shift over time as well.
As with every other aspect of the natural world, Autumn color will change as the Earth warms. Will we miss those intense, short displays of color? That question is really one of personal perceptions. I think I will miss the intensity, but there is something to be said for longer, warmer Autumns as well.
In a warmer world, the timing of that “leaf peeping” expedition may not be so crucial, but the intensity of the experience on any given day may be diminished.
Sources
The image of Autumn in New England woods is from:
https://commons.wikimedia.org/wiki/File:New_hampshire_colors.jpg
and was posted from
https://www.flickr.com/photos/75681523@N00/80045338
The full citation for Henry Horn’s book is:
Horn, Henry. 1971. The Adaptive Geometry of Trees. Princeton University Press. Monographs in Population Biology, Volume 3. 146pp
Background on foliage color
https://www.fs.usda.gov/visit/fall-colors/science-of-fall-colors
https://www.esf.edu/pubprog/brochure/leaves/leaves.htm
Sun and shade leaves of oak are from:
https://www.minnesotawildflowers.info/tree/northern-red-oak
and for maple are from the book by Horn cited above
The image of aspen leaves is from
https://landscapeplants.oregonstate.edu/plants/populus-tremuloides
Changes in growing seasons, first frost and baseline temperatures are here
http://www.nrcc.cornell.edu/services/blog/2009/10/06_first_frost/index.html
https://www.arborday.org/media/mapchanges.cfm
https://www.ncei.noaa.gov/products/land-based-station/us-climate-normals
Change in the timing of Fall foliage
https://www.washingtonpost.com/weather/2021/10/06/fall-foliage-leaves-climate-change/
Change in species composition into the future. This USDA Forest Service site has interactive maps of predicted future distribution of all major tree species