What causes leaves to change colour - again!

It's the kind of thing most people with any curiosity probably look up at some point. Me, I've looked it up several times over the years, and can never quite fix it in my brain. Maybe by typing it out I will remember it. So, once and for all, what causes tree leaves to change colour in the fall, and what makes for a good year of fall colours, like this year seems to be here in Toronto?

First, the basics. The green colour of most leaves us due to a chemical pigment called chlorophyll, which uses the blue and red wavelengths of sunlight to power the photosynthesis process by which trees grow, converting water and carbon dioxide into oxygen and sugars. The remaining green wavelenths are reflected back at us, creating the familiar green colour of most leaves. 

Chlorophyll is continuously being produced and replaced  throughout the growing season. But, with the shorter days and lower temperatures of fall, an abcission layer between branches and leaf stems starts to block the flow of nutrients, and chlorophyll ceases to be replaced in the leaves.

There are other leaf pigments that are also present in leaves throughout the growing season, including the carotenoids (e.g. xanthophyll) that produce yellow and orange colours, but these are usually masked by the more dominant green pigments, and are typically only noticeable in stressed or damaged trees. In the fall, though, when chlorophyll production slows and stops, these pigments come to the fore in trees like ash, beech, birch, poplar, Norway maples, gingkos, etc, and serve the purpose of using up excess sugars in the leaves not needed for growth late in the season.

On the other hand, anthocyanins, the pigments responsible for red and purple colours in leaves, are only produced in the fall, when excess sugars are trapped in the leaves. Like carotenoids, they help use up any excess energy as chlorophyll disappears. This creates the bright red colors we see in trees like sugar maples, red maples, sumac and mountain ash.

Eventually, even these colour pigments start to break down and the brown tannins are all that remain.

Clear, dry, sunny weather tends to produce more sugars in leaves and therefore yields more and brighter red colours, particularly if overnight temperatures fall to low, but not freezing, temperatures (freezing stops the process of creating red pigments). A dry spring or a drought in early summer stresses trees into creating the sealing abcission layers earlier, resulting in earlier (although not necessarily brighter) colours. Too much rain or cloud covering later in the growing season, on the other hand, means less sunlight and leads to more muted colours. And heavy and strong winds will, of course, cause the leaves to fall ealier, truncating the colour season.

So, ideal conditions for a good fall foliage season are a moist growing season in spring and early summer, a dry late summer, and sunny warm fall days with cold but not freezing nights. Now, let's see if I can remember it this time!