Day 19: Why do leaves change colour in autumn? As you already know, leaves contain chlorophyll, the substance which allows them to convert carbon dioxide into the starches and sugars which are their food. Sunlight and warmth encourage the production of chlorophyll, and in the autumn, this process slows down as temperatures begin to dip and even more specifically, to length of day. As nights grow longer, thicker cells form near the juncture of leaf and stem (technically, this is the "abscission layer"), blocking the transport of carbohydrates. Evenutally, this will be the point from which the leaf falls. As the chlorophyll in the body of the leaf breaks down in response to the decrease of carbs, pigments such as anthocyanin (red), xanthophyll (yellow) and carotene (orange) are quite literally unmasked, no longer obscured by an abundance of chlorophyll. After a freeze, these pigments also deteriorate until no pigment remains but brown tannins. Various external conditions can affect the intensity of colour, such as lack of water, steady low-range temperatures above the freezing point, or a hard frost. The amount of each of these pigments in the leaves is responsible for the hues typically seen in certain species, and of course gives each geographic area its own unique character.
