What makes leaves colorful in the fall?
This week’s column was written by my colleague Paul Hetzler. Every fall I write more or less the same information about the leaves changing color, but Paul’s spin on this topic is really well written. As you enjoy the harvest from your garden, I hope you will make a donation to your local food pantry, whether or not they accept produce and please consider donating some pet food too. *** Why would a prolonged lack of moisture affect autumn color? Let’s look at what makes leaves colorful in the first place. Among the things we learned— and probably forgot right away—in Junior High Biology is that leaves are green because of chlorophyll, the amazing molecule that converts light, water and carbon dioxide into sugar and oxygen. Its intense green tends to mask colors such as orange and yellow that are present in leaves in lower concentrations. When chlorophyll wanes in the fall, those “weaker” colors are revealed.
It’s not like yellow and orange are just randomly painted on the insides of leaves, though. Molecules other than chlorophyll are involved in various metabolic pathways within a leaf, and they happen to be colorful. By comparison, we are boring. Our hemoglobin is red, at least in the presence of oxygen, but we are not as flamboyant on a cellular level as leaves are.
Red, however, is a horse of a different color where leaves are concerned. Trees spend energy that they would otherwise save for next year’s growth to make the molecule responsible for red. It is called anthocyanin, mostly because short words embarrass scientists. Anthocyanin is “expensive” for trees to make, and no one knows why trees do this.
In wintertime, I make my own bread. Although quality varies because I never use a recipe, more than likely the bread would turn out worse than usual if I omitted water. Similarly, all the ingredients need to be there for photosynthesis to work properly. When water is in short supply, production at the sugar factory, also known as the chlorophyll molecule, drops off sharply.
Without sugars, many cellular processes slow or even stop. Damaged chlorophyll is not replaced, and that deep forest-green color starts to pale. Those yellow and orange molecules (xanthophylls and carotenes, if you are insecure about word length) also begin to disappear.
As trees dry out further, their leaves start to brown along their edges. This is called marginal scorching, not to be confused with marginally scorched, which describes my bread. In drought-prone locations with thin soils, some tree leaves will entirely brown and turn crisp, calling it quits for the year. This of course is not good for trees, because they are not able to plug up the vascular connections between leaves and twigs, making them prone to even more desiccation over the winter.
As if that doesn’t make for enough sepia tones in
one season, our sugar maples once again are looking tawdry due to yet another infestation of the native maple leafcutter. This is a tiny moth whose newly hatched larvae eat circular patterns inside leaves, eventually getting big enough to emerge onto the leaf surface and excise little holes in it to make a mini turtle-shell case for itself. A single infestation causes only minor harm to the maples, but repeated infestations can weaken them somewhat.
Between marginal scorch, brown leaves, holey maples and a general shortage of leaf pigments, we might not get the brightest display this fall. Cool nights and sunny days tend to favor the production of red in the few tree species capable of producing it, and this could at least offset the brown tinge that infuses our woodlands at present. Here’s hoping for a good crayon selection this autumn.
Bob Beyfuss lives and gardens in Schoharie County. Send him an e-mail to rlb14@cornell. edu.