True Colours

Day 5: With the onset of Autumn, trees and shrubs reveal their true colours. Yes, you read that right. Now, shorter days and cooler temperatures slow down the flow of nourishing carbohydrates and chlorophyll production is reduced. As existing chlorophyll breaks down, the natural colours of the leaves are no longer obscured by green and begin to shine through in all their glory. Eventually, a hard abscission layer is formed where petiole meets twig and, generally speaking, the leaf drops. Deciduous trees and shrubs are not all alike in the types of pigments present in their leaves. Those which turn red in Fall are long on anthocyanins. Large amounts of xanthophyll are present in those which turn yellow. Leaves which turn orange contain carotene. These pigments are not immune to shorter periods of sunlight and colder temps, though, and eventually even they will succumb and turn brown. Some leaves may hang on until the following spring. This phenomenon is called marcescence, and it will be the subject of a future post.

Pigments

Day 358: Every year, at least one person asks me why leaves change colour in the fall. Although there's a bit more to it, the basics are easy to understand. During the months of spring and summer, warm temperatures and length of daylight encourage the production of chlorophyll, the chemical substance which allows plants to convert carbon dioxide into starches and sugars. As nights grow cooler and daylight diminishes in autumn, the cells at the juncture of leaf and stem begin to thicken, restricting the uptake of carbohydrates. This process is called "abscission," i.e., "cutting off," and will eventually cause the leaves to fall. With the supply of carbohydrates limited, the chlorophyll begins to break down, allowing other pigments present in the leaf to become dominant visually. Different plants contain various levels of the different pigments, so those which have high concentrations of xanthophyll appear yellow, those with carotene appear orange, and those containing anthocyanins turn various shades of red. Other factors such as drought or an early frost may affect the colours as well.

Pigmentation

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.