“Autumn is a second spring when every leaf is a flower.” – Albert Camus
This is certainly true of the autumn here in Ontario. It’s spectacular.
Parks and woods are alive with colour; yellows, oranges, golds and reds so impossibly deep it’s like a new kind of black. Some trees are a near uniform gold, others are a tortoiseshell mottling of all the hues of the autumnal palette.
As the daylight hours dwindle and temperatures drop the trees respond; these are the signals to make ready for the coming winter. Veins in the leaves start to close, reducing the flow of sap, and production of the green pigment chlorophyll halts as trees reclaim nutrients and energy from leaves before finally shedding them. As it does so, the veil of chlorophyll is lifted; the tree line turns yellow, orange and gold.
For years the reds and purples—due to a group of pigments called anthocyanins—were thought to appear in the same way as the yellows and oranges, always there, just masked by chlorophyll. The conventional wisdom was that this was simply a vestigial colouring, a leftover as leaves die off. Pretty, but of no other function or interest.
But the reds are not like the yellows and oranges, the reds are actively made as the leaves begin to fall.
Why then, when leaves are to be shed, do some trees invest valuable winter resources into producing this bold spectacle only to have it drop to the floor and decay? This grabbed the interest of plant biologists; the dumb process of evolution is (generally) no fan of waste, so could it be these colours actually play an important role in a trees life?
Chlorophyll is the major workhorse of photosynthesis for plants—absorbing energy from sunlight and passing it on to make sugars and oxygen from water and CO2. Photosynthesis takes its toll on chlorophyll though, breaking it down, so it constantly needs to be replaced. The yellows and oranges come from other important photosynthesis machinery, the more stable pigments called ‘carotenoids’; they hang about longer once chlorophyll stops being made.
The sugars from photosynthesis provide the energy for the trees metabolic activities, like growth, and are stored away to see it safely through the winter, ready for the eruption of spring. These sugars are also one of the building blocks of the red anthocyanins.
The amount of anthocyanins made, and therefore how red the leaves are, depends on a number of factors, including the amount of sugar in the leaf, soil moisture, the amount of light and what the temperature is. More brilliant autumn colours are seen when warm sunny days are followed by cooler—but not freezing—nights; as leaves veins close, less sugar from daytime photosynthesis moves into the trees body, leaving more for anthocyanin production.
Biologists have been scratching their heads over why this happens for some time; fortunately the excess dandruff wasn’t in vain, and some good ideas have emerged.
One thought is that the reddening of leaves is a warning to insects, such as aphids, that essentially says: “Back off. I’m fit, I’m healthy, and I’m ready for you.” Much like the bold colouring of poison frogs seem to.
Aphids, and their ilk, also need to prepare for the winter. They do this by laying their eggs in trees during the autumn and their larvae emerge in the spring to feed on their host, damaging the tree. Some trees have evolved ways to help combat this, producing their own kinds of insecticides or otherwise finding ways to make themselves unpalatable.
The idea is that there is a link between a trees preparedness to fight off insects and the production of anthocyanins. A redder leaf serves to show that the trees arsenal is stocked up, and the insects would be better off seeking less prepared trees.
There is good evidence for this. One study, looking at apple trees in the autumn, suggests that aphids prefer to lay their eggs in yellow or green leaved trees rather than red leaved trees, and that those that do lay in red leaved trees tend to fair worse in the spring. Another study showed that an evergreen shrub with red-fringed leaves suffered less from damage by insects the more red, and the more anthocyanin, they contained.
Another idea, that’s been kicking about for some time, is that the anthocyanins act like sunscreen for trees. As chlorophyll levels start to drop, the energy usually harvested from sunlight can damage the leaves internal machinery—producing reactive oxygen species (ROS)—making it more difficult to re-absorb the precious nutrients. Anthocyanins are good antioxidants, so could mop up the excess ROSs, thereby shielding the more important parts of the leaf.
One curious point to mention is that there are a lot more species making these glorious autumn reds in North America, at least 89 species, than there are in Northern Europe, which can only muster a measly four indigenous species. Why should this be? One idea is that it has to do with glaciations and mountains.
It could be that trees—and their aphid type pests—‘fled’ south, over the years and seasons of Earth’s history, as the northern glaciers grew. In North America the mountains tend to run North/South, allowing a southerly escape route for both trees and pests, whereas in Europe the Alps run East/West, creating a barrier for the fleeing trees. Backed into a corner, with nowhere left to go, both the red leaved trees and their aphid foes likely perished.
This actually adds a little more weight to the idea that pests may have driven the evolution of red coloured autumn leaves; since they too may have perished—aphids are pretty susceptible to cold—with the trees they called host. No more pests may have meant the evolutionary pressure to keep the red leaves going was lifted, no longer needed.
This puzzle is still being unravelled, and none of these ideas are mutually exclusive. It’s entirely possible, desirable even, in nature for adaptations to have multiple benefits, even if they were originally driven by just a single specific factor.
But if the benefits are so great, why aren’t there more trees doing it?
All we can say for sure right now is that, whatever its purpose, the reds and crimsons of anthocyanin are a beautiful addition to natures brief, explosive, autumnal palette.
P.S. Yes. Yes I did just totally spam you with pics I’ve taken here in Ontario…