Autumn colours and leaf fall

---

Autumn colours and leaf fall

The leaves of deciduous trees in temperate regions change colour, and eventually drop off each year. We may take this process for granted, but it is far from simple. As day-length in autumn shortens, and temperatures drop, deciduous trees decrease chlorophyll pigment production, and other pigments present in the leaf become apparent. The other pigments include carotenoids, that are yellow, brown and orange, and anthocyanins which produce red and purple colours. Different tree species have different proportions and combinations of these various pigments, hence the wonderful, and changing, colours seen in autumn. As the leaf comes to the end of its function any remaining nutrients are withdrawn back to the stem and an ‘abscission’ layer forms at the junction of the petiole [leaf stalk] and stem. This facilitates a clean detachment of the leaf from the twig, especially after a frost, when the water in the abscission layer freezes [and expands] and then thaws, so reducing the ‘grip’ that the twig has on the leaf. The leaf scar that is left is often a characteristic shape – this is especially so in the case of Horse Chestnuts; the ‘nails’ in the ‘horseshoe’ are the scars of the vascular bundle that entered the leaf petiole from the stem – see the arrows in the picture below.



There is a large Sycamore [Acer pseudoplatanus] to the SW of my house, with a full rounded crown. I photographed it at intervals from mid-October, when it still appeared in full leaf, until mid-November. Even without a frost, however, the leaves will eventually fall and 2020 was one of those years – by 19 November we still had not had a frost at 115 m a.s.l. in Edinburgh – and the tree was almost bare. I selected six of the pictures taken over a 3-week period to show the progression of leaf fall in autumn 2020.

23 October 202025 October 202028 October 20205 November 202012 November 202016 November 2020.

---