While the leaves of many deciduous trees and shrubs turn red, orange or yellow in autumn, sometimes the colour can be poor even on plants noted for their autumn interest.
For most of the year, the leaf pigments that give the fiery autumn colours are masked by the green pigment chlorophyll. As the chlorophyll starts to break down just before the leaves fall, the other pigments are revealed: carotenoids and anthocyanins.
Chlorophyll is responsible for the predominantly green colour of leaves, the colour arising from the absorption of blue and red light in the process of photosynthesis (makes energy for the plant in the form of sugars).
Carotenoids are mainly yellow or orange pigments, such as found in carrots, which play a secondary role in photosynthesis. Their colour is usually masked by the green of chlorophyll.
Anthocyanins are red or purple pigments produced as by-products of photosynthesis. In some plants, such as copper beech, anthocyanins dominate, producing purplish-coloured leaves.
Different concentrations of these pigments causes the variation in colour – but the intensity can also be due to weather conditions and genetic differences between species and between individuals plants of the same species (see causes below).