For a while now, in the economy of New England in the northeastern US, a peculiar industry has been gaining ground: the autumn. In recent years, in states like Massachusetts, Vermont or New Hampshire, the activity of leaf peeping has become popular, consisting of a type of tourism that visits the countryside to enjoy the fall colours. Boston Magazine estimated in 2010 that the leaf-peepers leave behind about 375 million dollars in Vermont each year. Every October, Massachusetts receives two and a half million visitors, thirty percent of whom travel from places around the world just to contemplate the autumn.
What does the fall in New England have? The answer is a dramatic gradation of tones, as described by the New York poet Walt Whitman, comprising “red, yellow, brownish-grey, purple, and light and dark greens.” Anyone who has checked for himself is able to certify that Whitman did not sin by an excess of imagination, in spite of the fact that in Europe we are accustomed to late autumn colours which could be described with many shades, but always starting from yellow. The phenomenon that in the autumn of the Americas and the Far East the reds predominate, as opposed to the European yellows, has been known since ancient times. However, understanding why this might be is not so immediate, let alone for what purpose.
But we must begin, as in all history, by going back to the origin. In spring the deciduous trees and shrubs begin to produce leaves, the power generators of plants. The plant uses the sun and the nutrients of the favorable season to invest a lot of effort in making leaves and stuffing them full of chlorophyll, the green pigment capable of converting sunlight and carbon dioxide into energy and organic compounds. Along with chlorophyll are found some other yellow and orange pigments, called carotenoids, such as those giving colour to carrots, but their tones are masked by the green.
When autumn arrives, the plant must draw in its sails and prepare to withstand the rigors of winter in a state of minimal activity. The leaves are no longer needed, so the plant stops producing chlorophyll and recycles all the valuable nutrients from its solar panels into its wooden body. The leaf dies, but first the absence of chlorophyll exposes the yellow colour of the carotenoids. When the leaf dries and falls from the tree, it is nothing more than an empty shell with nothing usable left.
However, in the trees of Asia and the Americas something unusual happens. With the arrival of fall, the trees begin to produce another pigment called anthocyanin, with the colour red. This explains why there is a difference in colours. Another thing is to understand why the plant invests so much effort in producing a new pigment when the leaf is about to be discarded.
Thus far we have talked about facts. The question that now arises is what biological sense does anthocyanin production have and why do Europeans trees do without it. Regarding the former, one theory suggests that the colour red deters insects. Aphids, or plant lice, avoid laying eggs on leaves with anthocyanin, which plants create as a sign that warns of “danger – defensive chemical products.” Thus, the benefit is mutual: aphids know how to avoid the plants that could kill them, and the plants manage to avoid infestation. Proponents of this hypothesis have suggested it as an example of co-evolution between a parasite and its host.
Although there are other theories about the function of anthocyanin, this explanation is the one taken as the premise by the researchers Simcha Lev-Yadun of the University of Haifa-Oranim (Israel) and Jarmo Holopainen, then at the University of Kuopio (Finland) and today at the University of Eastern Finland. From this Scandinavian country, the Israeli scientist explains that in 2008 he came to Finland to try to understand the enigma of the different fall colours in Europe and America. “I rented a car and went to the forest to see for myself what was happening,” recalls Lev-Yadun.
“Nature teaches you things that books cannot”.
“On the second day I was in the center of the belt that was at the height of the golden- yellow colouration. I drove about 600 miles from Kuopio and saw millions of yellow trees, but also millions of red bushes growing under the trees,” says Lev-Yadun. “On one of my stops, while taking photographs and field notes, I realized the beginning of the solution.”
According to Lev-Yadun and his colleague Holopainen, who knows all the secrets of the local ecology, the trees and their insect attackers are exposed to extreme temperatures in winter, while the shrubs are covered with snow; “they have a natural igloo,” says Lev-Yadun. Thus, the trees do not need the red because their parasites die during the cold season, while the Scandinavian shrubs need to maintain this protection. This was “the first step in solving the puzzle,” says the researcher.
This being so, why have Asian and American trees been forced to keep their warning signs, while European trees have been able to do without them? To solve the mystery, the scientists expanded their study to focus on the geographic and climatic conditions in the recent history of the planet, and noticed a curious circumstance: in Europe the main mountain chains run from east to west, while in the Americas and Asia they go from north to south. We humans live in the Quaternary Glaciation, an ice age marked by glacial periods of intense cold, and other interglacial periods, like the present, with more moderate temperatures. “From other studies on global climate change, especially in the Pleistocene, we know that extinctions [due to cold] have been much stronger in Europe because of the direction of the mountains,” explains Lev-Yadun. In other words, while in the Americas the species, including plant parasites, could migrate north or south depending on the weather conditions, in Europe the mountain ridges prevented this and species died, trapped by the ice. Free from infestation, the European trees could dispense with the expensive burden of producing anthocyanin.
“Differences in the biological richness, unequal extinctions, the physiology of insects and the various defense mechanisms of the red and yellow leaves all combine to support the hypothesis,” says Lev-Yadun. The work of both researchers, published in the journal New Phytologist, “attracted much interest from the scientific community and the media, to the point that some saw it as the solution to many unknowns and gave up studying autumn colouration.” “But we still have many questions that will take years to answer,” concluded the scientist.