We all know it. Deciduous trees turn from green to shades of yellow/orange/red and eventually brown at the end of the growing season. But why? Do trees make loads of pigments in fall turning the tree into a festival of colour for our enjoyment? Or is something else happening?
Trees have the colours all year round
First of all, let us get one thing clear. The colours of the fall foliage are not due to your bonsai making more pigments. It is actually the reduction of some pigments that causes this. Because all the colour in the leaves is already there. You just do not see the colour. Chlorophyll in the leaves absorps the red and blue parts of the light, which can now not be reflected. As a result, the reflected light by chlorophyll is green. As chlorophyll is so much more abundant then the other main pigments (carotenoids and anthocyanins) and its efficiency in absorbing the reds and blues, leaves appear green.
So, why do bonsai turn red in fall?
In fall the tree is taking action to not loose valuable nutrients. Deciduous trees are not able to protect the leaves from freezing in winter. Instead, they have another strategy: They drop their leaves. As these are full of hard-gained nutrients, trees first relocate the nutrients they can re-use. Part of this process is a reduction of general transport between the leaves and the plant. As such, sugars produced by Chlorophyll start to build up in the leaves. Once this happens, the leaves stop rebuilding chlorophyll, and the concentration here is slowly reduced. As this reduces light absorption by chlorophyll, the other pigments become more visible. On top of this, under the right circumstances, plants will invest in these pigments, making some years much more colourfull than others.