Every year thousands of tourists flock to the Northeastern part of the United States and Canada, particularly the areas of New England and Vermont, to see the spectacular array of Fall colors; nearly 40% of Vermont's annual tourism relies on the Autumn show of changing tree colors. However, Fall color is changing and it is thought that climate change, in addition to the usual factors which may differentiate leaf colorings, could be to blame.
Leaves of Green, Yellow and Orange
Leaves are green in color due to the presence of a pigment called chlorophyll. In Summer, when a tree is growing, the green pigment is more dominant than other pigments and thus other pigments stay hidden. As the end of the growing period is reached, leaves don't require as much food intake, such as water and minerals, and the leaf vein along which they are carried is gradually closed off.
As chlorophyll decreases other pigments, such as carotenoids, begin to show again. Carotenoids are the pigments which give a leaf its colorings of yellow and orange; Carotenoids are present in many things such as carrots, buttercups, daffodils and bananas. The hardwood species of trees such as Ash, Maple, Sycamore and Cottonwood owe their yellow and orange colors to the presence of carotenoids.
Red and Purple Leaf Colorings
Red and purple leaves are caused by the pigment anthocyanin; not present throughout the growing season of the leaf, anthocyanins develop late in the season in the sap of the leaf's cells and therefore a number of factors can influence the development of anthocyanins. These factors include sunlight and temperature.
Anthocyanins give color to many fruits such as blueberries, strawberries, plums and cranberries; trees species such as Oak, Maple and Dogwood contain anthocyanins, as well as combining with carotenoids in the hardwood trees, resulting in a mixture of colors of deep oranges and intense reds. Early Spring buds may contain anthocyanins too.
Climate Change Studies on Fall Color Changes
It is assumed by scientists that temperature influences the color of a leaf; given that global warming is causing temperatures to rise around the world, a change may be afoot in the Fall tree colors. The Proctor Maple Research Center at the University of Vermont is carrying out a three year study exposing leaves to differing temperatures and monitoring the changes in leaf pigmentation colors.
The study is focusing specifically on anthocyanin pigments, as it is already assumed that temperature effects the resulting shade of red in a leaf. Warm, sunny days followed by cold nights are said to produce more vivid red colors. The study will monitor the effect of warmer temperatures on the length of the Fall season, the timing and the brilliance (or lack of) Fall colors. It will be three years before the conclusive results of this study are known.
Another study in the journal Global Change Biology reveals that increased carbon dioxide levels in the atmosphere may be delaying the start of the Fall season; research indicated that both the forests of Europe and North America remained green for longer, regardless of any changes in temperature. The study was deemed too brief to forecast the long term impact.
There is speculation at this stage as to whether Fall color is being affected by climate change but it is known that some trees prefer a colder environment, such as sugar maples (Acer saccharum) , and a warmer temperature would inhibit the growth of new sugar maple trees. The change in weather patterns may result not only in a change to the Autumn tree colors but to the overall future of some trees.
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