• Question: why do leaves change colour in autumn?

    Asked by rompom to Joel, Kristian, Tim, Venus, Zachary on 19 Nov 2013.
    • Photo: Tim Hollowood

      Tim Hollowood answered on 19 Nov 2013:

      Nice question…but as a particle physicists I’m out of my comfort zone! However, the answer is quite simple. Leaves are green because of an amazing chemical called chlorophyll. It converts the energy in sunlight into chemical energy. In the autumn the tree shuts down the production of chlorophyll since the sunlight is reduced and then the other chemicals in the leaf can show through and it’s these other compounds that give rise to the beautiful colours.

    • Photo: Kristian Harder

      Kristian Harder answered on 19 Nov 2013:

      Why leaves change colour in autumn?
      Well, midlife crisis, obviously. 😉

      Yeah, sorry, I don’t know more than Tim either. I googled around to see whether there is a more detailed explanation, and of course there’s plenty to read there, for example:
      http://chemistry.about.com/od/howthingsworkfaqs/f/fallleafcolor.htm
      But the point is really what Tim says.

    • Photo: Zachary Williamson

      Zachary Williamson answered on 21 Nov 2013:

      Hi rompom. In winter, most trees shed their leaves because there’s very little light for photosynthesis. The amount of energy required to keep the leaves alive is more than the energy gained from photosynthesis, so trees shed their leaves.

      The leaves turn brown because the tree is sucking up all the chlorophyll in the leaves (which give them their green colour). Chlorophyll is hard to make and there’s no point wasting it by leaving it in the soon-to-be-shed leaf.

      So that’s where most of the brown colour comes from. It gets that autumn crispiness because the tree is literally starving its leaves so they die and fall off. That includes sucking up all the moisture in the leaf, making it crispy.

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