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.