All the leaves are brown and the skies are grey. So sang the Mamas and the Papas in their 1965 hit California Dreamin’, writes Dr Chris Andrews. But why do we see the change in colours at this time of year?
Well, the grey skies seem easy to explain as weather fronts race across the Atlantic. But why now? The shorter days means the air is heated less, meaning cold air from the Arctic can push southwards. There it meets the water of the Atlantic, still holding the warmth of the summer. This meeting generates the clouds of weather fronts, which then cross the ocean to dump lots of rain on top of us.
So much for the weather, what about the leaves? Concentrate, here comes the science bit! The leaves of trees are essentially solar panels. They trap the sun’s energy to give life to the plant. You may remember from science lessons that a chemical called chlorophyll is used to do this. It is this chemical which gives leaves their green colour. Well, actually there are another two chemicals which also help trap the sun’s energy. One of them has a yellow colour, another red (which is why you get some trees like copper beech that don’t have green leaves). Chlorophyll is quite a short-lived chemical; the tree needs to continually replace it, while the other two are more stable. When it gets to this time of year and there is less daylight, it is no longer effective for the tree to keep the process running. So they shut their leaves down. As the chlorophyll disappears, the green colour fades to be replaces with the reds and yellows of the other two chemicals. Eventually the leaves drop from the trees to form the multicoloured carpet we all know.
All seems a bit of a waste though, doesn’t it? The trees spend ages making the leaves, and then they just fall off again. But this seasonal striptease does have some very positive benefits. For example, it is a banquet for many smaller creatures. Attacked by fungi, mould and bacteria, the leaves start to break down and provide a lovely source of food for millipedes, woodlice and the like. All this breaks the leaves down further, returning the nutrients within them back to the soil, ready to be used the next spring. Both for more leaves for the trees, but also all the beautiful spring flowers that we love. It may seem that autumn is a time of things dying and disappearing. But in reality it is setting the scene for next year’s growth.
Dr Chris Andrews is Visitor Experience Manager at RSPB Frampton Marsh