UnbeLEAFable! The wonder of autumn foliage
An aerial rainbow of red, gold, purple, and green; brisk wind reddening cheeks, and leaves crunching with each step: fall is a robust season marked by bright foliage, jack-o-lanterns, and apple pie.
But have you ever wondered how trees morph into their bright array of colors?
Deciduous trees lose their leaves each year, growing new buds for the spring. Common deciduous trees of New England include hemlock, red maple, and white cedar. These trees all experience a short period of annual growth that finishes by June, in which each tree grows its leaves and becomes prepared with a set of buds for the next year. After the growth period is complete, trees spend the rest of the year working to produce and store carbohydrates that will help them grow in years to come. Leaves are vital parts of the process of producing carbohydrates.
Chlorophyll is the green pigment found in leaves whose job is to capture light energy from the sun through photosynthesis. This light energy can be used to combine carbon dioxide and water to produce the necessary carbohydrates. As chlorophyll is exposed to light, it gradually breaks down, and so always needs to be replaced. During this growth, constant chlorophyll production blocks orange and yellow pigments, carotenoids and xanthophylls.
As the growth season progresses further and the days begin to get shorter, deciduous trees respond to less light each day. When a certain threshold of day length is reached, the cells located at the base of the leaf begin to rapidly divide, which forms the abscission layer. This layer acts as a large wall, slowing chlorophyll production and blocking carbohydrate transportation from the leaf to its branch. Soon, the chlorophyll production is completely stopped, allowing for the reveal of the hidden orange and yellow pigments. Red and purple colors often appear as well, from anthocyanins — pigments produced from extra sugar trapped in the leaf.
Soon, the chlorophyll production is completely stopped, allowing for the reveal of the hidden orange and yellow pigments.
As the autumn months move forward, the abscission layer grows drier, eventually causing the leaf to break off from the tree and float down to the ground. As they lie on the ground at the mercy of leaf blowers and rakes, they eventually lose all their pigment except the tannins that show a brown color.
Seasonal temperatures and other environmental factors play an important part in determining the colors and intensities of them that we see each season.
The most ideal conditions for colorful trees are a dry, cool, sunny autumn. Lots of sunlight and lower temperatures (but above freezing) help chlorophylls get destructed more quickly, exposing the radiant colors underneath. Additionally, if temperatures are cool at night and the days are sunny, more anthocyanins are produced, giving more range of color.
An early frost can quickly end a season of foliage, since the freezing temperatures ruin any chance to produce anthocyanins. Heavy wind and rain also cause leaves to prematurely fall to the ground before they’re able to reach their full potential. On the contrary, drought conditions during the spring growing season causes the abscission layer to form much earlier and leaves may drop before they change color.
New England is not the only part of the world to see colorful leaves. Northeast Asia has similar deciduous species to new England and show a brief, intense display each fall. Since Europe spends most of its autumn under cloudy skies, their foliage responds with dull colors. In mixed forests of evergreens and deciduous trees seen at higher elevations, trees go from bright yellows to almost bare in about two weeks.
While New England isn’t the only place to see vivid fall color in the trees and swirling through in the wind, Bostonians are pretty lucky. When you look back on that scenic drive up north, or your walk through Harvard University’s Arnold Arboretum, remember all the xanthophylls, carotenoids, tannins, and anthocyanins.
