Why do leaves change colour in autumn? There are scientific explanations as to why leaves change colour but there are also many myths and folk tales surrounding this phenomenon. Each year, hundreds of Americans go to New Hampshire just to admire the beauty of the autumn foliage.
According to Native Americans, in autumn, the Great Bear constellation (the Plough) was killed and its blood dripped on to some leaves, making them red. While its meat was cooking, fat dripped down and coloured other leaves yellow.
Even today, scientists are not 100 per cent sure what causes leaves to change colour. The primary factor is the photoperiod - the amount of sunlight a region receives during the course of a year.
As autumn comes, the days become shorter. The decrease in sunlight intensity, shorter hours and cooler temperatures reduce the growth and food-making processes that normally flourish during spring and summer.
Plants rely on sunlight for energy and nutrients. Leaves contain chlorophyll, which absorbs energy from sunlight to make sugar and carbohydrates through photosynthesis. Several weeks before leaves change colour, a hardened cell layer forms at the base of the leaf due to decreased photosynthesis. This layer blocks nutrients and moisture flow to the leaf, inhibiting further production of chlorophyll.
The chlorophyll begins to break down as the tree absorbs nutrients into the trunk and eventually the roots. Since chlorophyll reflects green light waves, the green colour starts to diminish. The yellow/orange pigments, which have existed in the leaf since spring but were not visible because of the large amount of chlorophyll, now become apparent.