November 3, 2011

Gardener Steph: Science of Autumn

As the days get shorter and the nights grow longer, people start dreading the coming of winter and the possibility of not experiencing much daylight during the work week.   Deciduous plants are also affected by the length of darkness each day. When nights are long enough, an added layer of cells form between the leaves and branches of a plant. This layer begins to block the transport of carbohydrates from the leaf and essentially starts the color change process. Since this process is initiated by day length, the window for fall color roughly starts at the same time every year. How bright and showy the change is determined by a variety of other environmental factors.

Chlorophyll is a green pigment that synthesizes carbohydrates in leaves, but also breaks down in sunlight. It is replaced constantly throughout the growing season in order to keep photosynthesis active. Once the extra cell layer between the branches and leaves is formed, chlorophyll can no longer be replaced within the leaves. Once Chlorophyll is no longer produced, the yellow pigments and orange pigments that are hidden by chlorophyll become visible.

While yellow and orange pigments are present because they are important in capturing light energy, scientists have yet to determine the reason behind why red and purple pigments are found in autumn leaves. These pigments aren’t usually found in leaves during the growing season and they require a lot of energy to produce. There are currently two popular theories behind why the energy draining red and purple pigments are present; one theory states that the pigments act to protect leaves against water loss and frost injury, while another theory believes that the red color warns off pests.

As time goes on, the cell layer between the leaves and the branches become dry and the connections weaken causing the leaves to break off. Since secondary pigments such as yellow, orange and red pigments are also sensitive to light, they too eventually break down leaving brown tannins that stay until the leaf decomposes.

While the biological process for color change is the same for every plant, environmental factors can mean the difference between lasting color and a quick change to brown. Abundant sunlight and low temperatures destroy chlorophyll, but if cool night temperatures are present, the production of secondary pigments is promoted. These two conditions allow for the best fall color production.

So what does this all mean for Mad. Sq. Park?

Due to warm nights and cloudy skies, you shouldn’t expect too many brilliant reds and purples this fall. So even if the colors aren’t as brilliant as can be, sit back, relax, and enjoy one of nature’s finest shows.