Ah, the bittersweet moments near the end of the summer. The nights are starting to cool, but the lake is still an incredible 76.5 degrees, at least here in Burlington, and the first of fall colors are poking into the green canopy. In this series, we celebrate the rainbow of fall (though a bit out of the ROYGBIV order) and explore the reasons behind the different colors we see in plants.
Much of what I wrote about purple pigments in fall foliage applies here as well, particularly since the purple and red colors on plants are both produced by anthocyanins (anthocyanins take on a purplish hue in the presence of more alkaline sap and are redder at lower pHs). In addition to painting a plant red or purple, these pigments also absorb high energy light in the UV end of the spectrum (source), providing protection from harsh UV radiation. Not surprisingly then, anthocyanins in leaves and on stems are produced in response to sun exposure. This is why a raspberry or blackberry cane is red on the top and green on the bottom (see image below). Bring a raspberry into the shade and it ceases to produce anthocyanins and the stem soon reverts to green.
We see this in how many species, like red maples, change colors in the fall. Initially, the upper part of the canopy begins to take on a brilliant red, while the lower, shaded part of the crown stays green (more on how leaves change colors). As the upper leaves fall are shed or ripped off by the wind (and the sun’s position in the sky at noon draws closer to the horizon), the lower leaves are exposed to more sunlight and begin to produce anthocyanins. The following series shows the changes in the canopy of a silver x red maple hybrid from September 29 – November 6, 2018. Since anthocyanins cost energy and nutrients to fabricate, producing pigments in the absence of sun is a waste of resources. By postponing producing the pigments in lower leaves, the tree can be flexible and responsive to weather conditions over the 2 month period where leaves are being deconstructed and ultimately shed.
The same energy cost-benefit analysis happens on a forest wide scale. Canopy trees (e.g. sugar maple) are more likely to have reds in their foliage (again, the anthocyanins help protect a leaf from UV radiation while it is recouping energy and nutrients), while understory trees (e.g. hophornbeam) are more likely to turn yellow in the fall. The same pattern repeats across seasons. When fall passes under an overcast sky, forests forego anthocyanin production and fall colors are more muted.
Alternative reasons for red
This isn’t the whole story with reds, however. In our discussion of blue colors, I wrote that plants will signal when fruits are ready by changing color from green to a contrasting color, like bright red or blue. If a leaf actively turns red in the fall, and this costs energy, it’s reasonable to hypothesize that the plant may also be signaling to other organisms some information about its current condition. While a staghorn sumac might entice a robin to eat its bright red fruits, it would certainly be less keen on a deer browsing on the red fall foliage. So what’s it communicating?
There is compelling evidence that red pigments in fall leaves act as a deterrent to insect parasites. Much like the brilliantly colored plumage of a male scarlet tanager (male is pictured below, females are a drab olive brown) signals to females, “Hey, look, I can be about as obnoxiously obvious as you can imagine and still avoid predators so I must be pretty fit,” so too does a bright red leaf signal to insect predators, “Hey, look, I can waste all this extra energy on pigments, imagine how incredible my chemical defenses must be.” It also highlights to potential parasites that might be looking for a host to lay their eggs on that the leaves will soon be shed by the tree.
And it works. Aphids will preferentially avoid laying eggs and feeding on leaves that have begun to turn red in the fall. As with the red maple series above, color change can begin as early as August while the leaves won’t drop until as late as November. That’s a 2+ month gap where a leave could be photosynthesizing. This early color change acts as a false fall. It’s too early for the leaves to have begun the process of breaking down, but the insect doesn’t know that. The leaves are effectively tricking the potential parasites into thinking that fall has begun on that tree and that the leaves therefore would be less nutritious than on a neighboring tree.