When we’re out for a Sunday afternoon walk with the dogs we often remark on the Autumn Colours. Arboretums make a lot of money this time of year charging us to marvel at one of the best spectacles of the year.
The natural world treats us to a last burst of colour before the onset of winter. For just a few weeks, the green leaves of deciduous trees and shrubs change colour to many shades of red, yellow, purple, black, orange, and brown.
But have you ever wondered why leaves change colour?
Well the colour of a leaf comes from pigments that are contained within it. These natural substances are produced by leaf cells to help them obtain food. The three pigments that colour leaves are chlorophyll (green), carotenes (yellow) and anthocyanins (reds and pinks).
The shorter days and cooler nights that we associate with the autumn triggers three major changes in the leaf which affect its colour.
Firstly, as the days become shorter the production of green chlorophyll slows down and eventually stops. Existing chlorophyll in the leaf breaks down and the green colour fades.
Secondly, in the spring and summer the green from the chlorophyll is more dominant and means that the yellow and orange carotenes aren’t visible. As the green colour fades the yellow or orange takes over and changes the colour of the leaves.
Finally, a layer of tougher cells start to form across the base of the stalk of the leaf to help it eventually fall. This means that the movement of sugars back to the main part of the tree is restricted and the sugars become trapped and concentrated in the leaf. They are eventually converted to anthocyanins giving the leaf a red, purple or pink colour.
Some years are better than others…….
The depth of colour is influenced by the blend of chemical processes and weather conditions.
Cold nights
Invariably this means lower temperatures that start to affect the chlorophyll so the green leaf fades to yellow. However if temperatures stay above freezing, anthocyanin production is enhanced and the leaves take on a red colour. Colder weather usually gives us more yellow leaves, warmer weather gives us more red leaves.
Dry weather
This results in sugars becoming more concentrated in the leaves. This means that more anthocyanin is produced and gives us redder leaves.
Bright sunny days
We’ve said that the production of new chlorophyll slows down in autumn and eventually stops. However photosynthesis can still occur on sunny autumn days, using the remaining chlorophyll. Sugar concentration increases, more anthocyanin is produced and we know that this means that the leaves are redder!
But why do trees lose their leaves?
Abscission is the technical name for when dead leaves and ripe fruit become detached from our plants. It starts when that layer of cells that exists between where the leaf stalk joins the stem starts to get thicker. This layer, known as the abscission layer, starts much earlier in the year – in the spring - during active new growth of the leaf.
In the autumn the hormones within our trees and plants start to change. The most notable one to change is something called auxin. During the growing season the production rates of auxin in the leaves are consistent with the rest of the tree.
When these rates are steady, the cells of the abscission layer remain connected, which means that the leaves remain attached.
However as the days shorten and temperatures cool the production of auxin in the leaves starts to decrease. This means that the cells within the abscission layer start to stretch. The elongation of the cells means they start to fracture, resulting in the leaf breaking away from the plant. The leaf will finally be blown off by the wind or will fall due to its own weight.
Is the loss of leaves good or bad for the tree?
Good! It means that the tree won’t dry out and die as it can preserve the moisture in its branches and trunk It also means that – being dormant - it needs less energy to remain alive.