YELLOW AND RED

Vienna, 21 November 2020

Just look at that maple! What a magnificent yellow its leaves turned!

My wife and I walked under it during a hike we did a couple of weeks ago. We were following the edge of a wood and lo and behold! there it was.

As I mentioned in an earlier post, as we have been walking the woods these last few weeks the trees have been putting on their autumnal colours. We have been bathed in yellows of all hues, turning to russet, and finally to dark brown.

But what we have not been bathed in is reds. We have not witnessed the wonders of a North American Fall

or the splendour of an East Asian Autumn.

“Why is that?” I asked myself as I sat there gazing at my photo of that yellow maple tree, “why is it that North Americans and East Asians have splendid red hues in their autumn colours and we in Europe do not?”

To answer this, we are going to use a version of Root Cause Analysis called the “5 Whys”. This was something invented by Sakichi Toyoda, the father of the founder of Toyota, who claimed that you had to ask “Why?” (more or less) five times before you got to the root cause of something. His son used it extensively in his car factories as a quality control tool, to discover the fundamental reason – the root cause – for a quality failure (and at a much more modest scale I have used it to discover the root cause of a source of pollution or waste). A simple example goes as follows:

“Why the hell isn’t my car working?!”
Because the alternator isn’t functioning.
“Well why is the bloody alternator not functioning?!”
Because the alternator belt has broken.
“Oh. Why did the alternator belt break?”
Because it was well beyond its useful service life but has never been replaced.
“Ah. Why wasn’t it ever replaced?”
Because you, idiot that you are, didn’t maintain your car according to the recommended service schedule.
“Ah, right, OK, sorry about that.”

OK, so now we can start using the method on our little problem:

“Why do the leaves of many species in North America and East Asia go red, whereas so few do so in Europe?”

We see leaves as green because of the chlorophyll they contain. But leaves also contain other pigments, which if the chlorophyll were not there would make the leaves look yellow, orange, or all hues in between. The chlorophyll simply masks them.

In Europe, when autumn comes and the chlorophyll begins to disappear, these other pigments are finally allowed to “express themselves”, giving the leaves the beautiful hues of yellow that we see. This explains the fact that the maple we came across went from green to lovely canary yellow.

In North America and East Asia, something else happens when the chlorophyll begins to disappear from the leaves. There, trees begin to produce – from scratch – a red pigment, anthocyanin, in their leaves. This pigment masks – or perhaps “mixes with” – the yellow or orange pigments already there, to give various shades of red. Thus do North American and East Asian maples go from green to red.

“OK, but why do North American and East Asian species produce this red pigment at the end of their leaves’ lives?”

Yes indeed, it does seem that the trees and bushes which do this are penalizing themselves. Just when their leaves are about to fall off, part of the general shut-down for their winter slumber, the trees start expending precious energy to pump their dying leaves full of red pigment. The reason for this apparently foolish behaviour has to do with pest control and especially control of aphids (which I happened to mention in an earlier post on wood ants). Aphids have this nasty habit (as far as trees are concerned) of sucking amino acids from them in the Fall season, and then laying their eggs on them; the eggs hibernate along with the trees and give birth to a new generation of aphids in the Spring. So the trees get hit twice: they lose precious amino acids to those pesky aphids, and then the next year they have to endure attacks by the next generation of aphids! Now, it so happens that aphids believe that a brightly-coloured tree is a tree that is chemically well defended against predators, so they tend to avoid laying their eggs on such trees. So of course trees in North America and East Asia have evolved to turn themselves bright red in the Fall, just when the aphids are laying their eggs, by pumping their dying leaves full of anthocyanin.

“Why do aphids think a brightly-coloured tree is a chemically well defended tree?”

I thought you might ask that. The answer is, I don’t know. Stop being a smart-ass and move on to the next question.

“A bit touchy are we? Well OK, why don’t European trees make their leaves go red then?”

Because they don’t they have aphids which prey on them.

“Why is that? How can it be that aphids prey on the North American and East Asian trees and not on the European trees? What’s so special about European trees?”

Yes indeed, this is where it gets really interesting. To answer this, we have to go back 35 million years. At about this time, the northern hemisphere began to go through a series of ice ages and dry spells. Most trees reacted to this by going from being evergreen to deciduous. They also retreated southwards when the ice sheets advanced and returned northwards when the ice sheets retreated. In North America and East Asia, their predators of course went with them, evolving to deal with the fact that trees now lost their leaves and went dormant during the winter. In turn, the trees evolved to fight off these predators by, among other things, turning their leaves red in the Fall. This struggle between tree and predator continued even as the trees moved northwards or southwards as the ice sheets advanced or retreated. Thus, still today, the trees in those parts of the world go a glorious red in the Fall.

But in Europe, there were the Alps and their lateral branches, which ran east-west. In North America and East Asia, the mountain ranges, where they existed, ran north-south, so the trees in their periodic advances and retreats could “flow around” these mountains. In Europe, though, as the trees moved southwards to escape the ice sheets they hit the barrier of the Alps; there, they could go no further and so perished in the piercing cold. And so of course did the predators which they harboured. Only seeds were carried southwards, by birds or the wind or in some other fashion, and of course these seeds harboured no predators. Thus it was that European trees did not need to make red leaves and so they give us glorious shades of yellow in the Autumn.

There is at least one exception to this rule, and these are dwarf shrubs that grow in Scandinavia. They still colour their leaves red in autumn. Unlike the trees, dwarf shrubs managed to survive the ice ages; in the winter they would be covered by a layer of snow, which protected them from the extreme conditions above. But that blanket of snow also protected the insect predators! So the plants had to continue their struggle with their predators, and thus evolved to colour their leaves red. We have here an example, the smooth dwarf birch.

Well, that was an interesting use of the 5 Whys method! I must see if there are other issues I could use it on.