We were rather taken with this stunning ensemble of trees turning from green to gold with red-accented vines in a parking lot near home—there’s happily quite a spectacle to see with the changing of the seasons but sometimes there’s the most contrast when it’s removed from the forest a bit. The chloroplasts in plants would be optimised for absorbing light across all spectra should leaves be black and while there’s a wide range in colouration, botanists aren’t sure exactly why most vegetation is green and not a darker shade. I wondered if the changing colours was just the onset of shedding them, the parts dying—or whether the process weren’t something more poetic, like the death of a star with the different phases and outcome it goes through as its energy sources dwindle.
I don’t think one can quite bear out that metaphor but it turns out that it’s a gross over-simplification to say that trees shed their leaves because of the cost of maintaining a green mantle during the winter months outweighs the photosynthetic benefits. The chemical responsible for the yellow and orange hues is always present in the leaves but is masked by renewed chlorophyll during the growing season.
The chemicals responsible for purples and reds are produced at the end of summer and slowly become a part of the tree’s complexion. Brown is the absence of pigment altogether.Trees undergo this transformation to prevent water loss primarily and in certain climes to stave off freezing of extremities but there’s a whole host of other reasons including foiling the camouflage of herbivores, avoiding infestation, advertising its seeds and berries and to even stunt the growth of close neighbours. The clusters of dead leaves that remain attached and aren’t dropped, called marcescent, are even kept around by design as in the Spring they are a store of nutrients and they mask growing buds and ensure that any animal foraging for these new shoots gets a nasty taste for the effort.