• Question: Why do leaves change colour in Autum?

    Asked by aileenjcat to Colin, John, Kevin, Shikha, Triona on 14 Nov 2014.
    • Photo: John Wenger

      John Wenger answered on 14 Nov 2014:

      Trees and plants produce a chemical called chlorophyll which absorbs the blue and red parts of the light that we get from the sun (remember that sunlight can be split into many colours – rainbow).
      Chlorophyll mostly reflects green light and that is why leaves are green.

      The chlorophyll is in fact really important because the red and blue light it absorbs is used by the plants for photosynthesis, i.e. production of sugars to fuel the growth of plants.

      In autumn the plant recognises that the amount of sunlight is getting lower and stops producing chlorophyll. The result is that the leaves lose their green colour.
      In some trees, like maples, sugar is trapped in the leaves after photosynthesis stops. Sunlight and the cool nights of autumn cause the leaves turn this glucose into a red colour.

      The brown colour of leaves on trees like oaks is made from wastes left in the leaves.

    • Photo: Kevin Motherway

      Kevin Motherway answered on 19 Nov 2014:

      We live in a windy country and trees can do one of two things. Have an aerodynamic shape and have leaves that are also aerodynamic but not as efficient at photosynthesis but can stay on the tree all year round as the tree has a narrow profile that can stand up in storms…..these are evergreen trees. The other strategy is to have a really big wide tree with big wide leaves that gather lots of sunlight and make lots of food during the summer and then the tree lets the leaves die and drop so the tree effectively “lowers it sails” and becomes aerodynamic to cope with winter storms. If a bad storm strikes in summer when all the deciduous trees still have their leaves the wind can knock lots of them over. The leaves turning brown is a org of the deciduous trees process of letting the leaves die and drop off!

    • Photo: Shikha Sharma

      Shikha Sharma answered on 20 Nov 2014:

      Hi aileenjcat,
      Leaves appear green because they contain lot of chlorophyll. There high amount of chlorophyll in an active leaf masks other pigment colors. As we know that sunlight regulates chlorophyll production, in autumn, days grow shorter, less chlorophyll is produced. The breakdown rate of chlorophyll remains constant, so the green color starts to fade from leaves. At the same time, surging sugar concentrations cause increased production of another pigment called anthocyanins which gives red color.