January 26, 2010

The essence of marcescence

marcescent oak

Some trees stubbornly cling to their dead leaves throughout the winter. Pin oaks, scarlet oaks and American beeches are among a handful of local species that have marcescent leaves.

Marcescence, the persistence of withered tissue on a plant, occurs mostly on younger trees and on the juvenile parts (lower branches) of older trees, as shown at left.

Why would a tree evolve with this trait? Scientists think it may deter deer from feeding on a tree's nutritious twigs and buds. Dessicated leaves tend to be low in nutrients and difficult to digest, so their presence might cause a hungry deer to look elsewhere for food. By spring, when herbaceous greens have stolen the deer's attention, strong winds and expanding buds finally force a belated fall for the leaves with an added purpose.

marcescent beech

The leaves of the American beech, above, change from green to bronze in the autumn, ultimately becoming translucent white by winter's end.

That bleaching may be the result of prolonged exposure to sunlight's ultraviolet radiation, a notorious fader of pigments.

beech leaves

Snow may also help with the bleaching. "Snow removes many pollutants from the atmosphere when it falls," says Purdue University's Paul Shepson. "Some of these pollutants, like hydrogen peroxide and nitric acid, interact with sunlight to produce a reactive intermediate called the 'hydroxyl radical,' which is a potent [bleaching] agent."

SOURCES: "Effects of marcescent leaves on winter browsing by large herbivores in northern temperate deciduous forests," by Claus R. Svendsen (Alces, 2001); Paul Shepson, Purdue University Department of Chemistry

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