Will Global Climate Change Affect Fall Colors?

grandfather mountain with fall foliage


Recently the Washington Post reported that previous estimates of global warming may all be greatly underestimated, according to a new United Nations Science Compendium. Robert Correll, chair of the Climate Action Initiative, which helped write the report, suggests that even if countries adopt all of the climate policies now being debated in Copenhagen, global temperatures will still rise by 6.3oF. This is twice what the Nobel Prize-winning International Panel on Climate Change in 2007 predicted would cause catastrophic changes to civilization worldwide. This is scary stuff and suggests that to keep the world from overheating, we'll have to initiate even tougher greenhouse emission policies than those currently proposed.

Of course, if civilization itself is threatened by such global warming, few people will be sitting around wondering what it might do to fall foliage displays by trees. There will be more important things to contend with if that happens. But for the next few decades at least, civilizations will persist, and we will have the luxury of pondering how global change might affect fall color. Although it may seem a trivial aspect of our lives, fall color affects us materially and spiritually: it is one of those intangibles that make life worth living, that adds, literally, color to our lives. So, if global climate change is going to alter this aspect of our lives, we ought to know about it up front.

Along those lines then, how could global climate change affect fall color? To paraphrase the Bard, let me count the ways: (1) higher temperatures, (2) altered timing and/or amounts of precipitation, (3) changes in humidity, (4) changes in cloud cover and light striking the trees, (5) increases in the length of the growing season and displacement of the timing of leaf out and leaf fall, (6) higher levels of nitrogen inputs to ecosystems from agricultural practices such as fertilizing and hog production, (7) acidic deposition that causes nutrients to leach out of the soil, (8) migration of trees farther north to escape the heat, (9) extirpation of trees that can't migrate for one reason or another, and finally, (10) changes in competition due to greater pest loads or invasive exotic species.

This reads a lot like a Letterman Top 10 list for how to screw up future fall foliage displays! In fact, it is, but as I detail below, not nearly as funny. As you can see, lots of factors could disrupt fall color displays, which will make predictions difficult, but nonetheless, hardheaded scientist that I am, I will briefly run through a variety of scenarios we might expect to see, especially in the next two decades.

First, let's group several of these factors together because they will have the same effects. For example, higher temperatures, increased precipitation, increased cloud cover and higher nitrogen should all act together to mute fall colors. To understand why this is so, we need to know a little basic plant physiology. Don't worry, I'll keep the jargon down to a bare minimum, and you won't have to go back to school to read further down this column!

Trees cue in on two main environmental factors with respect to fall foliage color: daylength and temperature. As the days get shorter in August and September, trees sense this and begin anorderly process of leaf senescence, usually involving the loss of chlorophyll, the green pigment that plants use to capture light for photosynthesis, and the synthesis of anthocyanins, which give leaves their red color. Some leaves don't make anthocyanins, and when the chlorophyll degrades, it reveals underlying pigments, such as carotenoids and xanthophylls which are responsible for our orange and yellow colored leaves. Trees use daylength as their main cue because it is a good proxy for coming cold weather. Year after year, daylength will come with great regularity at the same point in earth's orbit, reliably signaling the inevitable onset of colder weather. But temperatures can vary greatly at any time of the year due to variations in the weather (like this year when temperatures were unusually cool all summer) and so are less reliable cues for the coming of fall. Trees use temperature only as a secondary cue; if the fall is cool they hasten the development of their fall color, while if it is warm, they procrastinate and delay it.

If global warming results in warmer fall temperatures, either during the day or night, this would tend to delay fall colors to later in the season. Will it mute the colors? It might, especially if the disconnect between daylength and temperature becomes extreme, thereby confusing trees and disrupting the synchrony of color development, such that some trees attain peak color later while others are less susceptible to the temperature cues.

Warming will also extend the potential growing season during times when light levels are low due to the lower zenith angle of the sun in winter and because shorter days allow less time for trees to carry out photosynthesis. This would result in less photosynthesis relative to respiration, which would lower their sugar reserves, and sugars are necessary to stimulate synthesis of anthocyanins, which give leaves their bright red fall color. Higher precipitation would lower the intensity of fall color, not because it washes out the colors (a wives' tale) but rather, because the concomitant cloud cover and low light levels reduce photosynthesis.

But why would increased nitrogen reduce fall colors? As I noted in a column last year, trees produce anthocyanins to help withdraw nutrients back into their twigs so they can use them for next year's leaves. The fewer nutrients a tree has, the more precious they are, and so the more necessary it would be to protect their withdrawal system, which is what the anthocyanins do. So, trees that are slightly nutrient stressed, particularly those with low nitrogen, often have enhanced red color. Paul Schaberg and his associates in the U.S. Forest Service in Vermont, have shown this with sugar maple. The key here is that when trees are stressed by low nitrogen, they tend to accumulate sugars in their leaves, and high sugar content is a trigger for anthocyanin production. So, in a future high nitrogen world, trees will be less stressed, and as a result, perhaps less red too. Conversely, if acidic deposition continues to leach nutrients from our soils, then this might counteract the additional nitrogen coming in from farming activities. This could result in a stand-off between leaching and deposition, and perhaps no change in fall red coloration.

The biggest unknown will be how global change will affect tree distributions. We have already seen hardwood trees migrate upslope nearly 40 m in Vermont in just the past half century, where mean temperatures have risen 1.5oC. For trees on flatter terrain, warmer temperatures will force them to migrate north. How far north is still an active area of investigation. Louis Iverson and his colleagues at the U.S. Forest Service in Ohio have generated maps of the present and predicted future distributions of our major tree species. Sugar maple, which is a dominant in the forests of the northeast, and in particular, New England, is predicted to migrate north into Canada, leaving few individuals in the U.S., and possibly decimating maple syrup production in Vermont, New Hampshire and Maine. If this species leaves the U.S. for our northern neighbor, what will give us our brilliant red colors in the fall in New England? Such a migration will alter the composition of our forests forever, and a new color balance will have to emerge. Whether another species steps in to take the place of the sugar maple is not something that can be determined at this point in time. Even if a tree species can migrate (and by migrate, we mean produce seeds that are transported north each year – trees of course, don't uproot themselves and march along with the animals, despite what you might have seen in the Lord of the Rings movies!), it might not find its new habitat suitable for survival. Trees tend to be picky about their soil quality, and if a tree moves northward and the new soils are unacceptable, that tree may not survive.

A more dramatic change could occur if global warming allows exotic pest species to invade our forests, or, permits native species to burgeon. Right now, we are seeing the deleterious impacts of the exotic hemlock woolly adelgid, which is killing off our beloved hemlocks, including our own native Carolina hemlock. Large blotches of dead, gray hemlock trunks dot the Smokies and surrounding hillsides, showing the destructive power of an insect only slightly larger than the period on this page. In the upper mid-west, the Asian longhorn beetle is killing hardwoods and causing great concern as it spreads ominously east and south. Out west, millions of spruce and pine have been killed by native bark beetles that normally would be kept in check by cold winters, but which now persist due to warming, allowing their populations to build up to unprecedented sizes.

To sum up, I predict that global climate change will generally exert more deleterious effects than beneficial ones. This is yet another motive to try and reduce the impacts of global climate change. Although less brilliant fall foliage displays may not rank high on the list of concerns about global change, those muted colors could be the canary in the mine shaft telling us that these shifts could be markers for more subtle, and potentially more consequential changes in our world. A tree stressed is a symptom that something larger is wrong with our world. Our civilizations depend on healthy, functioning ecosystems for services such as clean water, erosion control, and clean air, not to mention food, fiber and shelter. If you don't think that's true, just look at what happened to the Mayan civilization in central America, or the xx civilization that built Ankgor Wat in Cambodia. Those peoples destroyed the ecosystems around them and now, where are they?

So, let's you and I work together to try and reverse global climate change, not just for the sake of fall color, but for the future of our planet.

Thanks for reading. Here's a short list of relevant literature should you want to read further about these subjects:

DeLucia, E.H., C.L. Casteel, P.D. Nabity and B.F. O'Neill. 2008. Insects take a bigger bite out of plants in a warmer, higher carbon dioxide world. Proceedings of the National Academy of Sciences 105:1781-1782.
[Although this report focuses on soybeans (think of them as just shorter trees!) it does show how warming can alter plant-insect interactions]

Iverson, L. Atlas of tree migrations.
[This map is also available on the web at: www.nrs.fs.fed.us/atlas . This is the most detailed report of how global warming could cause trees to migrate to new habitats.]

McMullen, C.P. and J. Jabbour. 2009. Climate Change Science Compendium 2009. United Nations Environmental Programme, Nairobi, EarthPrint.
[This report is available on the web for free download at: http://www.unep.org/compendium2009/. I highly recommend reading it!]

Beckage, B., B. Osborne, D. Gavin, C. Pucko, T. Siccama and T. Perkins. 2008. A rapid upward shift of a forest ecotone during 40 years of warming in the Green Mountains of Vermont. Proceedings of the National Academy of Sciences 105:4197-4202.
[This is an important paper. It shows how far upslope hardwood trees have migrated in just the past 40 years due to global warming, and more importantly, portends what may yet happen in the southern Appalachians, where warming has not yet been detected.]

Schaberg,P. A. van den Berg, P. Murakami, J. Shane and J. Donnelly. 2003. Factors influencing red expression in autumn foliage of sugar maple trees. Tree Physiology 23:325-333.
[This is the paper that shows that sugar maple trees with less nitrogen have more red coloration.]