Fall is a time to prepare for winter. Freezing temperatures and reduced sunlight make nutrients hard to come by, so animals and plants have developed mechanisms to reduce their need for energy.
Deciduous trees sense the change in day-length and, in response, they trigger a process known as leaf senescence. The tree cuts off the flow of nutrients to the leaves and reabsorbs valuable minerals such as nitrogen and phosphorous.
The leaves eventually die and fall from the tree, leaving the tough woody trunk, stems and twigs to survive the winter.
Although the reason trees lose their leaves is well known, it is still a mystery why trees display a range of colours during senescence.
According to an expert on the evolution of autumn colours, Dr. Marco Archetti of Oxford University, it is "a common misconception" that autumn colours are simply the result of leaves dying before they are shed.
A closer look at leaf colour is required to understand this puzzle. Leaves are normally green due to the presence of a pigment called chlorophyll.
Chlorophyll plays a key role in photosynthesis - enabling leaves to capture energy from sunlight and transform it into chemical energy in the form of sugars. During leaf senescence chlorophyll is degraded and not replaced.
Pigments called carotenoids are responsible for shades of yellow and orange. Carotenoids are present in mature leaves throughout the year but their colour only becomes visible in fall when chlorophyll production stops.
On the other hand, the pigments that cause red and purple leaves - anthocyanins - are actively synthesized in fall leaves. This proposes an interesting question: why do trees expend valuable energy making leaves red when they will be shed in a short while?
Red coloured leaves are present in 10 to 70 per cent of deciduous trees depending on the region of the world under study. Scientists believe that the red pigment display must have evolved for a reason.
One theory is that the red pigments protect the leaves from light damage at low temperatures. The red pigments may also act as antioxidants, reducing oxidative stress in the leaves.
The healthier the leaf, the more nutrients can be reabsorbed by the tree to help it survive the winter.
Another popular theory is that red leaves act as a warning signal to animals and insects. Insects such as aphids lay eggs on trees in fall and when the eggs hatch, the young can cause considerable damage to the new leaves.
If red leaves indicate leaves are low in nutrients, about to fall or high in toxins, insects are likely to avoid these trees and find more suitable hosts for their young.
It is curious that, despite our best efforts, we do not completely understand why trees have evolved to show a variety of autumn colours.
Scientists are currently collecting information on deciduous forests around the world and combing the literature for clues. Hopefully, soon we will be able to unravel one of the mysteries of fall.
