BELEAF IT OR NOT

Most people interested in nature know that the reason we have such wonderful autumn colours is that deciduous trees and other plants break down the chlorophyll in their leaves at this season. This results in carotenoids and other colourful pigments being left behind in the foliage for a while, and incidentally for our joy, until the leaves fall. I have been wondering, though, if there any ecological advantages for trees’ bright colours in autumn or whether is it merely an incidental result of end of seasonal chemistry as trees prepare for winter. A little research reveals the following: 

1. An ecological advantage from better nutrient conservation and stress protection before winter. This is the “photoprotection (physiological) hypothesis” which is currently the most widely supported explanation.

As chlorophyll degrades in autumn, trees tend to reabsorb valuable nutrients (especially nitrogen) from leaves before they’re shed. During this period, the photosynthetic system is still active but becomes unstable and prone to oxidative damage.

The pigments have benefits: carotenoids (the yellows & oranges) and anthocyanins (the reds) can protect leaf cells by screening out excess light, scavenging reactive oxygen species (free radicals), and reducing photo-oxidative stress. This allows more efficient nutrient resorption, improving the tree’s overall “nutrient economy” before the leaves fall.

It is thought that red leaves may undergo less photo-inhibition and more efficient nitrogen recovery under bright autumn light than do related species without red autumn leaves.

2. A possible ecological advantage in reduced loss of overwintering buds through herbivory. This is the “coevolution” or “insect signalling” hypothesis but, although interesting, is a more controversial theory[1].

Bright red or yellow autumn colours may act as a warning signal to herbivorous insects (like aphids) that the tree is well-defended chemically, or physiologically robust. Trees might thus deter insect colonisation and egg-laying before winter, thus protecting buds for the next spring.

In summary, the bright colours of autumn may not be merely cosmetic accidents; they may sometimes confer ecological advantages. However, the exact evolutionary pressures vary among species and environments.

[1] https://shorturl.at/hvyZN