Autumn is when the quaking aspen trees begin to show off their mysterious side. Questions fall into place, but answers are hard to come by.
At lower elevations in the Rockies, aspen (Populus tremuloides [Salicaceae]) is everywhere. Its clones dominate the landscape, creating a sea of white trunks and dark green leaves that rustle softly in the wind. At first glance, they all look the same, though we know they actually represent a complex mosaic of forms and genotypes.
Autumn is when they begin to invite questions. Fall colors come, and suddenly the clones don’t look so similar any longer. The received wisdom is that individual clones change color all at once, and asynchronously with their neighbors. But after some wandering around, I don’t think this is really true – and I’m not sure why it isn’t.
Individual trunks (ramets) don’t always change color at the same time. Some do. But not most. It is common to see single small branches turn yellow while the rest of the branches stay green, as if they are in a rush. I’ve heard proposals that these branches are water-stressed or damaged, and are ones likely to die in the coming year, but it’s hard to know if this is always the case.
And individual leaves don’t all turn yellow, either. Some, and probably the majority, do.
But some go from green to just a bit of yellow, then to black, with the color and pigments retreating down the major veins.
Others seem to become wholly black and crisp and dead without any intervening colored stages.
And others – my favorite – transition from green to red directly, giving the trees a fiery appearance.
The overall effect is a rainbow of colors. I haven’t built any intuition for why some ramets turn one color or another, except that the leaves growing on the driest-looking soils seem to simply crisp up, turn black, and fall off.
The other notion that whole clones change color synchronously also doesn’t seem to be true. I have noticed that in many cases there are sharp delineations of color between adjacent trunks, but there are also long smooth gradients. I think the microclimate each leaf experiences may has as much of an effect on fall coloration as the genotype does.
The autumn phenology of these leaves effectively determines the end of the growing season, and puts a strong cap on the amount of carbon each leaf can capture, ultimately limiting these clones’ growth and survival.
So what drives these patterns, and what are they telling us about these trees’ lives? I’m not sure yet, but the mystery is well worth considering, and the patterns inspiring for dreaming up future research plans…