Now that autumn has spoilt us with its palette of dry oranges and spiced browns, I want to reflect on a question I always find myself asking when walking through a canopied forest path in late October. Why do the leaves fall from the trees?

A reductionist biological explanation won’t cut it. Although I am sure the biological processes involving tree loss are fascinating, it is like answering the question, “why does ice melt?” by replying that ice melts because the molecules’ bonds weaken allowing them to separate into liquid form. But the separation of the bonds is the result of a rise in temperature of the substance, causing bonded molecules to be jostled violently until they break free. So ice melts because it is heated by an external source of molecules that are jostling more than the ice is on average. Now then I ask again, why do the leaves fall from the trees?

Biology strives for efficiency, though it doesn’t always get there. Physics is where efficiency reigns (e.g. principal of least action). The loss of leaves requires a tree to regrow them, which consumes energy supplies and therefore requires the tree to gather more resources. A tree, therefore, will not choose to lose leaves unless the net gain is positive concerning energy. Did I just say a tree can choose? Perhaps retaining leaves in the winter is wasteful? Keeping leaves healthy requires supplying nutrients to their vascular bundles, but if there is no sunlight to photosynthesise, the leaf will not return to the tree what it has taken from it; the relationship between tree and leaf becomes antibiotic.

Or perhaps I am overly pedantic with this trivial question. After all, do we cut our hair only when the net gain in energy from doing so will be positive?

An answer to my question, for those interested. A tree which loses its leaves at some point in its lifecycle is called ‘deciduous’ meaning “tending to fall off”, and the process of losing its parts is ‘abscission’¹. From the same source: a few possible advantages of loosing leaves include protection from insects which would normally eat the leaves, water conservation (reduction of resource use), and that leaf repair could be more costly than just losing the leaves. All very interesting.

1: https://en.wikipedia.org/wiki/Deciduous