For anyone who has ever been in Scandinavia in the fall (Finland, Norway, Sweden, and nearby countries) you would have been impressed by the fall color display of the trees, just as we are here in North America. But one thing would be missing that we take for granted in the United States and Canada, and that is the color red. Trees in northern Europe are dominated by yellow and orange fall color, and relatively few turn red. In fact, only four northern European tree species have red fall color: Prunus padus (bird cherry) , Prunus spinosa (blackthorn), Sorbus aucuparia (European mountain ash) and Acer platinoides (Norway maple) while in all the rest of Europe there are just 24 species that turn red. Here on this side of the pond there are at least 89 species that have red leaves in the fall, such as sugar maple, mountain ash, sumac, scarlet oak, dogwood, sweetgum, and sourwood, just to name a few. In East Asia, the number of trees with red fall color is above 150.
So, why should there be this difference in fall color between the New and Old World and could there be a scientific explanation? These were the questions that researchers Simcha Lev-Yadun from Israel, and Jarmo Holopainen, from Finland, addressed in a recent article published in the scientific journal New Phytologist (see Lev-Yadun and Holopainen 2009 at end of this essay). They suggest that this asymmetry in red fall color is the result of the glaciers that have intermittently covered the northern temperate zone.
How could fall color displays in trees be related to the glaciers? Yev-Ladun and Holopainen reason as follows. Beginning nearly 35 million years ago, in the Tertiary, the northern hemisphere was subjected to repeated episodes of cooling and warming. During the cool times, glaciers covered much of the land in the northern hemisphere. Plants and animals had to migrate southwards to avoid the ice and cold temperatures. In North America (and East Asia) the mountains are aligned primarily north-south, so species could migrate on the west and east sides to more southerly climes. We know this because when scientists analyze soil cores extracted from lakes and bogs in the south, they find pollen grains of species that today grow only in the north, such as red spruce and jack pine.
However, in Europe during periods of glaciation, the southerly march of species would have been prevented by the fact that the mountains (the Alps) run predominantly east-west, creating a barrier to migration. Thus, many species would have been trapped up against the mountains and extirpated. As but one example, England has only 12 native tree species, the rest having been wiped out by glaciations and others prevented from migrating over from mainland Europe after England became separated by the English Channel. In contrast, Great Smoky Mountains National Park, which is only a fraction the size of England, has 125 species of trees!
Ok, you say, I believe that the glaciations caused trees to go extinct in Europe more often than in North America, but why would this lead to the preferential elimination of those species with red fall color? To answer this, we go refer back to two influential papers published in 2000 and 2001 by Marco Archetti, William Hamilton and Samuel Brown, who together developed a theory that red fall color serves as a warning to insect pests. In essence, their hypothesis states that some insects, aphids mainly, avoid brightly colored (i.e., red!) trees, because the bright color was a signal that this tree was well defended chemically against predators, and therefore the aphid mothers should not lay their eggs on them. Aphid mothers tend to seek out trees in the fall to lay eggs on, and if they laid them on a very red tree (i.e, well protected chemically), their offspring would not survive as well as those whose mother placed them on a less well defended tree. This theory is analogous to the one that states that brilliant colors on tropical tree frogs warn predators away because they are loaded with toxic compounds that could cause their death if ingested.
As you might suspect, this hypothesis has generated much debate; not because it is an evolutionarily based hypothesis (we have no quarrel with evolutionary theory in this camp!), but rather, because the evidence in support of it has been hard to obtain, and also because it ignores physiological theories for why trees make these red pigments (which by the way are called anthocyanins). Ecophysiologists have postulated that trees produce red foliage in the fall to protect the leaves against the ravages of high light (see my earlier column on this subject).
Well, if red trees are acting to warn insects not to lay eggs on them, then why are there so few in Europe compared to North America? Yev-Ladun and Holopainen argue that not only were tree species extirpated during the glaciations, but so were their insect pests. And because the Alps run east-west, they not only caused species to go extinct during the glaciations, but they also prevented those species that did make it south from migrating back north when the glaciers retreated about 16,000 years ago. And what makes this theory so intriguing is the additional fact that some aphid species are particularly sensitive to cold, so many species may have been eliminated during the glaciations.
With the elimination of these insect pests, so to came the elimination of those evolutionary selective pressures to produce red leaves. You see, anthocyanins are actively synthesized while leaves senesce in the fall, as opposed to the passive presence of the yellow and orange pigments (xanthophylls and carotenoids). These latter pigments are present all summer, but you don't see them because they are masked by the more abundant chlorophyll. When these leaves senesce in the fall the chlorophyll degrades, the pigments are unmasked, and you get yellow or orange leaves.
But to make a red leaf the anthocyanins have to be synthesized de novo, which takes time and energy. That means resources have to be diverted from other uses such as making sugars or growing new roots. And that, in turn, suggests there must strong selective pressures to do so, or otherwise these species would be outcompeted by other ones (Darwin's Theory of Natural Selection writ large!). Thus, in Northern Europe, those tree species that survived would not be under any selective pressures to make anthocyanins because their insect pests were absent, and therefore would have only yellow or orange leaves.
In North America, those insect pests forcing trees to synthesize anthocyanins could ride the wave of species down the east and west sides of the Appalachians, and again northward when the glaciers retreated. Therefore, the co-evolutionary relationships between insect pest and host were never broken, and the selective pressures to produce red leaves never relaxed. And so, we have lots of red fall color in the New World, and appreciably less in Northern Europe. Once again, we see the power of Darwin's Theory of Natural Selection to explain patterns in nature that no other theory can.
I think Yev-Ladun and Holopainen have produced an interesting paper on why trees in Europe have mostly yellow leaves, while those in North America have relatively more red leaves. It should generate even more discussion about the potential adaptive value of fall color in trees.
In my next column, I will discuss whether global climate change might impact fall color and what we can expect in the coming decades. Stay tuned, and Happy Fall Color Looking!
To learn more, you can read these articles:
Archetti, M. 2000. The origin of autumn colors by coevolution. Journal of Theoretical Biology 205:625-630.
[The first published paper on the coevolutionary hypothesis for fall colors.]
Hamilton, W.D. and S.P. Brown. 2001. Autumn tree colours as a handicap signal. Proceedings of the Royal Society of London Series B 268:1489-1493.
[This is the paper that started the great debate about whether trees are signaling insects about their defensive status – it is controversial, but has stimulated over 80 subsequent research papers, which is what good science can do! Hamilton was one of the giants of evolutionary research in the past 50 years, but died tragically at age 63 after contracting malaria from a study he was doing.]
Lee, D.W. 2002. Anthocyanins in autumn leaf senescence. Advances in Botanical Research 37:147-165.
[One of the first scientific investigations of fall color from a systematic viewpoint. David Lee is one of the world's experts on plants that express colors other than green. For a truly wonderful experience, read his book Nature's Palette: The Science of Plant Color, University of Chicago Press, 2007]
Lee, D.W. and K.S. Gould. 2002. Why leaves turn red. American Scientist 90:524-531.
[A wonderful, and colorful, article about why leaves turn red. Easy reading, but highly informative.]
Schaeffer, H.M. and D.M. Wilkinson. 2004. Red leaves, insect and coevolution: a red herring? Trends in Ecology and Evolution 19:616-618.
[Not everyone agrees with Archetti, Hamilton and Brown about why trees turn color in the fall. This is a dissenting viewpoint, well worth reading.]
Yev-Ladun, S. and J.K. Holopainen. 2009. Why red-dominated autumn leaves in America and yellow-dominated autumn leaves in Northern Europe? New Phytologist 183:506-512.
[I based this essay mainly on the above paper, which is an easy read for anyone.]