Why do some plants lose their leaves in autumn, and others not? Plants make leaves to to capture energy from the sun and transform it into chemicals that the plant can use for growth and reproduction. But leaves aren’t free – they cost the plant a large amount of resources to build and maintain. So it’s only worth having a leaf when the benefits outweigh the costs. In winter, when sunlight is weak and the risk of damage is high (from hungry animals, or frost, for example) leaves can become a problem. That’s why most plants get rid of their leaves before winter arrives – they’re not worth having! Of course some kinds of plants, like pines and other conifers, keep their leaves over the winter. These types of leaves are typically much stronger and have lower maintenance costs for the plant. Can you imagine a thin maple leaf surviving a winter as well as a pine needle?
Losing a leaf is costly to a plant, but these costs can be reduced. One way is to pull resources out of the leaf before it falls to the ground. Much of the autumn leaf colors we see are associated with the plant moving resources out of the leaf back into the roots for the winter. The leaf that falls to the ground is usually a shell of its former self. And plants can also recycle the last nutrients in these leaves as they decompose in the soil!
But what happens to a leaf that’s cut off from the plant before any of these things can happen? Does it change color too? I wasn’t sure. In my own experience, leaves I’ve picked have turned brown and then fallen apart into a crispy mess. But this summer I had the chance to collect over a thousand healthy leaves from all sorts of plants. I expected them all to turn brown and crispy. Imagine my surprise when I opened an envelope containing this aspen leaf!
Instead of turning brown uniformly, my leaf is changing colors in patches. It looks like each little section is outlined by a ring of veins, almost as if the chemical changes taking place in the leaf are kept localized by this vein network. I have no idea why this decay process wouldn’t proceed uniformly, and why some sections of the leaf still look almost fresh. I suspect that each section of the leaf was in a different chemical state when I harvested the leaf, and this state then influenced the rate of decomposition. But why should each part be in a different state? Were different parts of the leaf ‘on’ at the time? We know that the stomata in a leaf (the tiny ‘mouths’ through which the leaf breathes, that cover the leaf surface) can open and close in little patches. Maybe that has an influence here.
What’s for sure is that the leaf certainly looks different from when it was alive a few months before! I’m amazed by how much detail there is to see in something so common as a leaf. Doesn’t the network of veins make the leaf almost look like farm fields, or city blocks?
So why spend a summer collecting and looking at a thousand leaves? The answer, next time!