Autumn Leaves
The fireweed and blueberry leaves are turning scarlet above treeline, and soon Autumn will paint the birches and aspens on the lower hillsides gold. Where do these brilliant colors come from? And why are Alaskan trees golden, instead of the burning crimson and purple of New England?
During the summer, most leaves are green. The color is due to chlorophyll, without which life on earth would not exist. It is chlorophyll, condensed into little packets called chloroplasts within the cells of plants, that allows a plant to use carbon dioxide from the air, water from the ground, and energy from sunlight to produce the sugars and oxygen that support animal life. This process only occurs, however, when the leaves are unfrozen. Since leaves are useless for producing food for a plant during cold winters, many plants drop their leaves in the autumn and regrow them each spring. The fall of the leaves is usually preceded by the destruction of the chlorophyll.
The chlorophyll destruction can be triggered in two ways, or by a combination. Plants that are native to an area, like the birches and fireweed of Alaska, will begin to prepare for their winter rest at about the same time each year, regardless of temperature. This is possible because plants can sense day length, or more accurately, night length. As the nights grow longer and colder in the fall, the trees stop growing, the circulation of sap to the leaves is cut off, and the chlorophyll in the leaves breaks down.
Imported ornamentals, on the other hand, are often from areas of the world where the proper time to start shedding leaves would be signaled by a much longer night. These trees are given an abrupt warning by the season's first frost. An unusually early frost may affect native plants in the same way, making them speed up their normal schedule.
There are other pigments in leaves in addition to chlorophyll. Many of the yellow and orange colors in flowers and vegetables such as tomatoes and carrots are caused by a group of pigments called carotenes. (Yes, they are named after carrots.) In leaves, these carotenes are packed in the chloroplasts with the chlorophyll, and in summer are hidden by the green chlorophyll. As the chlorophyll breaks down in fall the carotenes remain, producing the gold of birch and aspen leaves.
The red of fireweed and blueberry leaves is produced by a different set of pigments, the anthocyanins. Anthocyanins are responsible for most red, pink, purple, and blue flowers -- in fact the word means a flower (anthos) which is blue (cyan). They are also found in the cell sap of some but not all leaves. In Alaska, anthocyanins are common in the leaves as well as flowers and fruits of undergrowth and tundra plants, but not in the trees that grow here. Furthermore, the amount of anthocyanins in a plant which is able to produce them depends on environmental factors: cool but not freezing nights, drought, and sunshine. Fall in much of Alaska is the rainy season, and the transition from warm summers to sub-freezing nights is rapid in the drier areas. Our weather conditions would not promote the development of red fall color even if the raw materials were present in our trees.
In fact, there are only two places in the world with abundant trees (such as maples and oaks) with lots of anthocyanins in the leaves along with dry, crisp, autumns. These two areas are eastern North America and eastern Asia. Both areas are noted for their displays of crimson, scarlet, purple, and gold. Here in Alaska, we must be content with golden trees in the autumn.
