Autumn Leaf Physics
How And Why Leaves Turn Red And Yellow In The Autumn
With the 22nd of September marking the Autumn Equinox, we know the season is now upon us. But this is more than just a date; it’s driven by solar orbits and our relative positioning to and distance from the sun. This leads to longer days, higher temperatures and more intense UV radiation in the summer and less in winter.
Why Are Leaves Green In The First Place?
It is these physical effects that explain the green leaves that we observe on trees throughout the summer. During the spring and summer, the biochemical substance known as Chlorophyll dominates leaf chemistry. Its purpose is to absorb the energy from the Sun to create energy-storing molecules in the form of sugars. During this process of photosynthesis, Chlorophyll absorbs blue and red wavelengths and reflects green light, which is why it appears green.
Why Do The Leaves Change Colour?
As the temperature drops and the days shorten in Autumn, the leaves begin to receive less UV radiation from the sun, which triggers the breakdown of Chlorophyll. The tree does not replenish the Chlorophyll, and other chemicals start to dominate leaf chemistry. The beautiful yellow and orange leaves that we see in Autumn come from the xanthophyll and carotenoids, (now free from the masking effect of chlorophyll) which have a yellow and red pigment.
However, one of the striking botanical sites of Autumn is the fiery red leaves, also explainable by physics. This occurs in specific species of tree in years when there has been lots of sunlight and dry weather, meaning photosynthesis has been super-charged and there are very high concentrations of sugar in the tree sap. This causes the tree to produce anthocyanins, which have a red pigment, to rapidly extract the sugar from the leaves before they fall, to maximise energy reserves for the winter.
Once this process is over, the leaves die, and trees shed them as these leaves are unable to produce energy. It also makes way for new growth in Spring and the start of the new energy transducing system of photosynthesis.
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I am a practising HR consultant working with several start-ups on an ongoing and ad-hoc basis in the London and M4 area, and am a Chartered Member of the Chartered Institute of Personnel and Development or CIPD. I am the Director of thecareercafe.co.uk; thecareercafe.co.uk is a resource for start-ups and small business. It includes a blog containing career advice, small business advice articles, HR software reviews, and contains great resources such as HR Productivity Apps.
