Autumn in this part of the world (Sweden at 58.7000° N) is always a riot of colour. A kaleidescope with every possible shade of yellow and brown with a small splash of red. Never quite the dominating reds of New England but glorious in the late autumn sunlight. But this year I felt that there was much more red around than “usual”. A purely subjective perception of course and one which I put aside as being just the vagaries of visual memory.
But yesterday the local paper reported that this was a perception being shared by many others. Trees and bushes did seem to be redder than usual. And “reds”were varying from deep rusts and maroons to bright scarlets and bleeding crimsons. But science did not really know why the colours might vary from year to year – just that they did vary.
Is it due just to how the weather has been through the summer?
Or is it because of the weather anticipated for the winter?
The three main pigments that color leaves are, chlorophyll (green), carotenoid (yellow, orange, and brown), and anthocyanin (red). Chlorophyll and carotenoid are always present in leaf cells. More sunlight means more chlorophyll and green summers. The reducing sunlight in autumn reduces the chlorophyll allowing the yellows and browns of carotenoid to show through more strongly. But high levels of sunlight may lead to an increase of anthocyanins.
In photosynthetic tissues (such as leaves and sometimes stems), anthocyanins have been shown to act as a “sunscreen”, protecting cells from high-light damage by absorbing blue-green and ultraviolet light, thereby protecting the tissues from photoinhibition, or high-light stress.
This summer of 2013 started late after a long winter and a very late spring. The number of sunshine hours have not been unusually high nor the amount of rain unusually low. Temperatures have not been particularly noteworthy. So just the number of sunshine hours seems inadequate as an explanation of the greater levels of red I perceive.
The Daily Green: Unlike the ever-present yellows that simply become unmasked when chlorophyll recedes, red pigments are actually created as a tree is going dormant. But why would a tree expend energy to produce a new pigment just as it’s hunkering down for the winter? And why do some trees make red pigments, when others don’t? Further, the reds of New England are so famous in part because they are unique to the new world. Why are European autumns so predominantly yellow?
…… yellow trees are those that colonize open land first – so-called pioneer species that are tolerant of direct sunlight. Those that turn red are species that follow in the succession of species that come to dominate a landscape, and they tend to benefit from more protection from the sun. It’s not that the red leaves lack the yellow pigment; the red pigment is an addition, and in fall it is so intense that it masks the yellow, just as green does in summer. Those pioneer species are less susceptible to light damage,
But why are European trees more yellow? It could be that falls there tend to be warmer and cloudier, so there was never any selective advantage for trees to evolve red pigments that would be protective of the sun.
According to the U.S. Forest Service:
A succession of warm, sunny days and cool, crisp but not freezing nights seems to bring about the most spectacular color displays. During these days, lots of sugars are produced in the leaf but the cool nights and the gradual closing of veins going into the leaf prevent these sugars from moving out. These conditions-lots of sugar and lots of light-spur production of the brilliant anthocyanin pigments, which tint reds, purples, and crimson. Because carotenoids are always present in leaves, the yellow and gold colors remain fairly constant from year to year.
So I don’t really know why the leaves look redder this autumn. But I think trees may be better predictors of weather than we give them credit for. There are some signs that Europe may be in for another hard, long winter this year. Perhaps the trees already know this and are busy storing nutrients in their roots. Therefore they need more anthocyanin to allow them to do this.
The trees already know what to expect this winter. At least, that’s what I would like to think!