Colour may help trees to store up nutrients before winter.
Autumn leaves turn fiery-red in an attempt to store up as much goodness as possible from leaves and soil before a tree settles down for the winter. The worse the quality of soil, the more effort a tree will put in to recovering nutrients from its leaves, and the redder they get.
That's the conclusion that Emily Habinck from the University of North Carolina, Charlotte, came to after looking at trees in a flood plain and in an adjacent upland area. The soil in the upland area was low in nutrients, and the leaves there were bright red. In the floodplain, where the soil was packed full of goodness, the autumn leaves remained yellow.
"In a nutshell: the redder a leaf is, the more nutrients it is going to recycle," explains Habinck, who presents her findings at the Geological Society of America?s annual meeting in Denver, Colorado, today.
It's not easy being red
Unlikely as it may seem, colour changes in leaves are not fully understood ? at least not when it comes to the redder hues.
As autumn approaches, trees begin to break down the green chlorophyll in their leaves and redistribute the nutrients contained there to their trunk and roots. This keeps them going throughout the winter, when sunlight is sparse.
The yellow colour seen in some autumn trees results from the loss of chlorophyll simply unmasking the yellow carotinoids that were there all along. But red coloration comes from a pigment called anthocyanin, which has to be made afresh as autumn takes hold.
Why trees would bother to spend energy doing this as things are winding down for the winter has been widely debated. Some researchers have suggested that these pigments act as antioxidants, which help a tree combat harsh conditions. Others say it helps to attract birds that can then disperse fruits. Or it might increase leaf temperature, helping to protect from the cold.
Some people have observed that trees tend to turn redder when an autumn is particularly bright and cold. In 2001, William Hoch, now at Montana State University, Bozeman, suggested that the pigment acts as a protective sunscreen, helping to keep leaves on the trees for longer so that more nutrients can be harvested from them. Photosynthesis becomes more difficult as chlorophyll is broken down, and leaves become more susceptible to damage from the Sun. Damaged leaves will fall more quickly, and rid the tree of a nutrient supply.
Hoch did a study in which he made mutant trees that couldn't produce anthocyanins. These dropped their leaves while they were still green when exposed to the high-stress environment of bright light and cold temperatures. The mutant trees were much less efficient at storing up nitrogen for the winter.
Habinck's study of natural sweetgum and red maple trees in a nature preserve in Charlotte supports this notion. Trees in the upland areas, where soils don't have much nitrogen, had much redder leaves than the trees in the flood-plain environment.
"A plant on a nutrient-poor soil is going to be more concerned about keeping the nutrients it has," says Hoch. So it will turn red to stop its leaves dropping prematurely.
Habinck's supervisor, Martha Eppes, now wants to look at satellite data to see whether there is a wider correlation between tree colour and soil type over large areas.