February Botanical Definition: Abscission

I hope everyone had a good Valentines day. I figured this month I could talk about abscission.

Abscission is the term given for the process of losing leaves in the winter for deciduous trees. The process is actually pretty complex though, first the tree starts to pull all of the nutrients out of the leaf in preparation for the winter months. The chlorophyll in the leaf that makes it appear green begins to degrade so it can be easily moved into the tree for storage. Carotenoids, a different photosynthesizing pigment, are slower to degrade than chlorophyll and also absorbs different wavelengths of light, causing the color of the leaf to appear orange to us. Next, a protective layer starts to grow inside the base of the leafs stem (Petiole) and the cell walls begin to produce lignin to act as a waterproof barrier. Lastly the leaf detaches, this is the coolest part. The cell walls along the protective layer begin to secrete enzymes that melt the cells in the abscission zone and starve the cells now separated from the rest of the leaf from water. This quickly causes cell death, and the leaf is separated from the tree with loss of water and nutrients minimized.

Fun stuff:

Some trees (Oak, Beech, Hornbeam and a couple others) like to break the rules. It’s common for young deciduous trees to keep their dead leaves on through the winter (sometimes more than one) and shed them in the spring time. This process is called marcescence. Oak trees seem to carry this behavior on into adulthood, on intermittent years the oak trees will keep all, half or none of their leaves in the winters. The reason for this is also not yet fully understood. Some hypothesis’s range anywhere from collecting more snow in the winter (Evidence for this is seemingly more instances of marcesence occurring after a dry season) for extra water in the spring, to deterring herbivores that might attempt to eat the softer and more nutritious twigs during the winter months (would you want to eat a dry-ass leaf?). The answer is most likely a combination of the two, but there are many other theories out there as well. I’m planning on writing a blog post titled “The Mysteries of Botany” Maybe I’ll talk about marcescence there a little bit too, and when I do you’ll know what it means.

Until next time!