Making Sense of Senescence
Why do leaves change colour in autumn?
John Casey, Teagasc forester, explains what 'senescence' means and how leaf and fruit fall in autumn is important to all, whether in the garden or in the wild.
Leaf senescence refers to when the normal green leaves of many deciduous trees and shrubs take on shades of yellow, orange, red, purple, and brown in the autumn.
A green leaf is green because of a pigment known as chlorophyll. During the spring and summer, the chlorophyll's green colour masks the colours of the other pigments.
As autumn approaches, with daylight hours shortening and temperatures falling, a layer of special cells forms at the base of each leaf. Water and mineral intake into the leaf is reduced. The amount of chlorophyll in the leaf begins to decrease.
When this happens, the hidden pigments of red, yellow and orange are revealed. As the tree sleeps during winter dormancy, amino acids are stored in the tree's roots, branches, stems, and trunk until the next spring, when the tree begins to grow anew.
In the meantime, both the leaves and the fruit of the tree fall to the ground, acting as a food resource for animals and insects over the winter and to provide seed for the next year.