Oct. 25, 2007 -- The hues of an autumnal hillside may owe as much to the soils under the trees as the trees themselves, according to a student researcher at North Carolina State University.
A preliminary study of fall leaf colors and the soils under sweet gum and red maple trees in hilly North Carolina show a significant nutrient difference that matches autumn tree color patterns.
The discovery matches the expectation of other researchers looking into why trees of the same species turn red with the pigment anthocyanin in some places and yellow without the pigment in others.
"Through the years biologists have debated whether it served a function," said soil researcher Martha Eppes of the University of North Carolina.
It was Eppes' student Emily Habinck who conducted the new work, which will be presented on Oct. 29 at the meeting of the Geological Society of America in Denver.
Habinck conducted an autumn inventory and laboratory leaf analyses of red maples and sweetgum trees along transects from a low-lying floodplain up to an adjacent valley divide in the UNC-Charlotte nature preserve in Charlotte, N.C. She confirmed that the trees were redder on steeper slopes and at the top of the divide when compared to the floodplains.
Likewise, Habinck found that the colors matched different soils types: the richer lowland soils corresponded to more yellow leaves and poorer highland soils correlated to redder leaves.
"It's very clear that there's a correlation," said plant physiologist Bill Hoch of Montana State University in Bozeman. What's more, it matches what he has discovered about the function of that stunning red anthocyanin.
Experiments make a pretty strong case for anthocyanin serving as a protective pigment that helps trees in nutrient-poor or stressed places to maximize the nutrients they can draw from the leaves before they are dropped to the ground, Hoch told Discovery News.
"They pull as many of the nutrients back into the plants as possible," said Hoch.
The red pigment protects any remaining green food-making chloroplasts in the leaves from damage. This is especially valuable for trees in nutrient-poor soils or stressful situations because this "photo-protection" allows the leaves to keep making sugars in their leaves longer.
This, in turn, is vital for pulling nutrients out of the leaves because the only way the nutrients can be extracted from leaf to trunk is by hitching a ride on the trunk-bound sugars.
The bottom line, Hoch explained, is that the longer photosynthesis can continue on an autumnal, coloring leaf, the more nutrients can be drawn out of it for re-use in the spring. So where every drop of nutrient counts the most -- like perhaps on some nutrient-poor hillsides of North Carolina -- red is the color of autumn.
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