Fall Changes: Leaves and What They Tell Us

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Leaves are an integral cue to us that fall has arrived. Today we’re going to explore the process of leaf colour change, what it does for tree health, and why some scientists are worried about the role of climate change on fall’s colours.

Every summer we get to a point here in Canada where you can sniff the air, and you know it’s coming: fall.  Maybe this conjures up images of hot chocolate, raking leaves, and pruning the garden back for the coming winter.  But what about those leaves?  Why do they change colour?

To truly understand the process, we must start in the spring, when the cycle of annual growth begins.  The tree opens its buds when it feels the warm weather coming on after the cold winter.  The buds spark the growth cycle, and the tree begins to produce new branches, leaves and store carbohydrates in next year’s buds.  This slows down around July for most deciduous species, and the tree begins to block the leaves and branches from using its carbohydrates stores.

Now, we all know that leaves are green due to chlorophyll, but did you know that it is constantly being replaced and replenished during the summer to keep those leaves looking and feeling fresh?  The chlorphyll actually fades in sunlight just like paper does.  Now, when the cholophyll stops being replaced due to many signal factors like night length, temperature, etc, the green of the leaf fades to reveal the underlying pigments that have been present in the leaf all along, just masked by the cholophyll.  So in essence, the “change” is actually a return to the original state of the leaf.  Different leaves change differently due to the underlying pigment.  Orange leaves have “cartenoids” which are in the same family as “carotene” aka the pigment that makes carrots that lovely crunchy orange.

So now that we’ve learned about the underlying cause of the leaf colour change, let’s try and understand why scientists are so concerned about climate change and how it might affect the cycle of deciduous trees. Climate change is altering the way water is moving throughout the world, with parts of the world that are normally wet and have a large amount of ground water experiencing drought.  Temperatures are also altered, which causes changes in both the timing of leaves in many tree species and changes what is tree types are able to grow and thrive.  This can alter the landscape in unexpected ways.  Trees during mild drought actually delay their leaf change longer, while trees experiencing massive drought move their leaf change up in the timetable, possibly to conserve energy for the next season where there will hopefully be more plentiful water.

As a species, trees are amazing organisms and will probably outlive most people.  So enjoy the fall colours and use this show of colour to reconnect with nature and enjoy some time outside.

What do you think?  Are the fall colours a significant part of your memories?  Let me know your favourite fall memory in the comments or on Facebook.