Summer flooding may mean a drought of fall color

By Jim Barlow

Heavy summer rains that swelled Midwest rivers to record levels may leave
a dull reminder to tree lovers this fall. Trees in the flood plain may not
be as colorful as usual, says a UI forestry researcher.

Trees in saturated pockets of upland areas also may wear milder colors, 
but the diverse species of deciduous trees in higher elevations may, on 
the other hand, be thriving from the rains and display vivid colors, said
Jeff Dawson, a forestry professor who studies chemical changes in tree 
foliage.
 
"The trees that show the best colors are healthy trees. Sometimes mild 
drought in late summer will promote brilliant fall colors," he said. "But
severe drought or other stresses, such as those brought about by summer 
floods, can reduce tree vigor and fall coloration."

Trees in river basins can more easily weather the more common spring 
floods, because their metabolic activity is low as the growing season 
begins. But summer flooding, when temperatures are high and metabolism is
peaking, can partially asphyxiate tree roots, Dawson said.

However, there will be plenty of color for sightseers who drive miles for
their annual color fix, Dawson said. He sees two sides to fall coloration;
that seen in the leaves and the invisible chemical changes occurring 
inside the trees.

Fall colors explode as the result of biochemical changes within trees as 
temperatures fall and days get shorter. Most trees stop producing 
chlorophyll, the green pigment involved in photosynthesis, revealing the 
gold, orange, russet, red, yellow, flaming scarlet, bronze and purple 
pigments within the leaves. Leaves with yellow carotenoid pigments and the
genetic ability to produce the red-to-purplish anthocyanins, another 
pigment, spark the brightest colors.

"The leaves that change colors before dropping off tend to be on trees 
that are storing nitrogen, phosphorous, carbohydrates and other 
nutrients," Dawson said. "The nutrients of the leaves are resorbed and 
stored in branches and bark. They are reused to fuel leaf expansion the 
following spring."

Most of the nutrients come from the soil. Some trees, such as those that 
produce nitrogen with the help of symbiotic bacteria in root nodules, 
retain green leaves. Nitrogen-producing trees grow well in infertile soil,
replenishing it as their nitrogen-rich green leaves freeze and fall to the
ground.

In the April issue of the Canadian Journal of Forest Research, Dawson 
reported that "nitrogen-fixing" black locusts and autumn olives, like the
alders he had previously documented, absorb less nitrogen but more 
phosphorous than most other deciduous trees. The finding suggests that 
storing phosphorous, which fuels nitrogen fixation, may be more important
than storing nitrogen in some nitrogen-fixing plants, he said.

To Dawson, fall foliage provides both a sparkling beauty to warm the soul
and a physiological gateway to understand how trees survive and evolve.