Above: Image © tbralnina, iStockphoto.com

It’s fall. And while that means back to school, it also means cooler weather and shorter days. We’ve all started to notice these changes, and probably and beautiful phenomenon that accompanies it – autumn leaves! I love fall – and mostly because of the colours, but just as flowers serve a higher purpose than human enjoyment, so too does senescence (plant aging and death) and abscission (dropping of leaves).

Did you know? Shortened day length in the fall triggers senescence in trees and woody shrubs.

The purpose of senescence is to capture the remaining nutrients in the leaves that will soon be dropped and redistribute them to other plant tissues.

Did you know? chlorophyll gives leaves their green pigment. As it is broken down, other pigments become visible – yellow xanthophyll, orange carotenoid accessory pigments and anthocyanins are the scientific reason that we get autumn colours.

By recovering and recycling nutrients from their leaves, trees and shrubs make sure that they are losing as little as possible when they finally drop their leaves.

Abscission is necessary for trees and shrubs winter survival in Canada. This adaptive function has allowed deciduous plants (those that lose their leaves every year) to survive cold winters.

Did you know? Without senescence and abscission, plants are in danger of becoming dehydrated or snapping during the winter.

Dehydration could occur in plants with leaves during the winter because the frozen ground doesn’t allow water to move up the plant, but the leaves try to continue photosynthesis (releasing water from the stoma). The other problem with leaves in winter is that they can catch snow and ice and weigh them down more than usual and can cause branches to snap or break. If you’ve ever seen an unexpected snow or ice storm in the fall you know what kind of damage that can do.

So while leaves changing colour can be a nice distraction while you’re daydreaming out the window, it is also a tested and exact science that allows deciduous trees to come back year after year.

Learn More!

Graham, Graham and Wilcox. 2003. Plant Biology. Pearson Education, Inc: Upper Saddle River, New Jersey.

How Trees Get Naked – Leaf Senescence and Why Leaves Fall off a Tree

Article first published October 26, 2009.

Rebecca Spring

I am a science communication graduate. I work at an environmental organization in Toronto. In my free time, I am learning Spanish so I can travel and work in South America.

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