"How beautifully leaves grow old.
How full of light and colour are their last days."
- John Burroughs
Every year, I marvel with curiosity at the beauty of each leaf's death. How its green life is sucked back into the branches from where it grew, leaving behind a fibrous skeleton. I always think this transformation happens too soon, when the sun is still warm and the days shorter, but not too short. For a tree, it's the night that triggers this captivating transformation. When a tree detects a slightly longer night, it knows that delaying senescence - the death of its leaves - for just a few more weeks, it risks frost biting at the vulnerable soft flesh of every leaf and all nutrients will be lost, unrecycled.
There's a Lime Tree on Cotham Hill in Bristol where every year the leaves turn a beautiful sunshine yellow. As the wind blows, they scatter on the ground covering the black tarmac with a mesmerising carpet of curling yellow Post It notes. The bright yellow comes from a pigment called Carotenoids, the same pigment that makes carrots orange and lemons yellow. Its sudden appearance is like an autumnal surprise, but the colour was in the leaf the whole time! The green pigment chlorophyll stole the show over summer, but when autumn arrived, the tree stopped sending energy to its leaves to make chlorophyll. As it breaks down, the yellow pigment is unmasked in all its glory.
Red leaves tell a different story. Unlike yellow, the red pigment anthocyanin needs to be manufactured inside the leaf. This requires valuable energy and it's a tree's biggest contradiction. The purpose of a leaf is to make energy from sunlight. It's a living factory, and Like all factories, they need energy to run efficiently. When the days grow shorter, leaves begin to use more energy than they can make and so a tree cuts its losses and gets rid of its expensive green factories. So why, at the last moment do they spend valuable energy on something that's about to die? We've been trying to unravel this secret for years!
One thought is that this red pigment acts like sunscreen. Without the protection of chlorophyll, leaves are prone to sunburn. The red pigment protects the leaf. Like factor 50 sunscreen, it gives a tree more time to absorb as much nutrients as it can from its dying leaves. Others suggest the red lures in hungry birds - a tree's final attempt to sell its fruit and secure its lineage. Or perhaps the red has co-evolved with insects, warning pest species that this tree is far too healthy to infest. It'll have more chance surviving on a weaker tree!
This colour-changing spectacle is a glorious way to bid farewell to our summer memories. One final burst of colour before hunkering down to cold, winter nights.