Tuesday, June 16, 2009

Trembling Aspens

The bright green, roundish, finely-toothed leaves shake in the wind, its light colored bark carries on photosynthesis, and some are 80,000 years old. This is the quaking aspen (Populus tremuloides), which occurs across a broader range than any other tree in North America.

Its name, both Latin and common, expresses one of the aspen's most intriguing traits - the leaves flutter in the wind. The leaf petiole (or stem) is flat and oriented at a right angle to the leaf blade. This gives flexibility, allowing it to twist in the wind. Leaves of other hardwoods have a rounder petiole. Check it out next time you are walking by some trees.

The aspen's quaking leaves may reduce the effects of intense sunlight on outer leaves and allow sunlight through to the lower leaves, thus improving photosynthesis. Or perhaps it helps protect against strong winds, letting breezes pass through the tree evenly. Or maybe we just don't know.

Aspens are true pioneers; they colonize disturbed sites such as gravel pits, old fields, clear-cuts, and landslides. You often see them growing along with other early colonizers--gray birch and pin cherry. These trees like full sun. As other trees sprout around them and eventually over-top, aspens and other "early successional" species will fade out of the forest. Aspens start to look their age by 70 years or so.

In the Northeast, another aspen -- the bigtooth -- occurs less frequently, but shares many of the some traits as the quaking aspen. Bigtooth aspen, Populus grandidentata, has leaves with big teeth.

When aspens are young their bark is a smooth olive-green. In older age, the bark becomes furrowed as in these old bigtooth aspens.

You may notice aspens in the spring, when they send out their male and female fluffy catkins, which are borne on separate trees. All before the leaves emerge.

Although aspen disperse their seeds by wind, most new growth occurs from root suckering. A grove of aspen is often a clone, all the trees related and connected underground to an extensive root system. So the tree may die above-ground, but the genes live on. An "aspen" in southern Utah was aged at 80,000 years old. One technique for regenerating aspen is to cut down an old one, which spurs root suckering, and to cut other trees to create an opening. This provides full sun for aspen to sprout and grow.

Aspen is a favorite food of beaver. Ruffed grouse dine on the buds and flowers in winter and spring. Next year look for grouse perched on the sturdy branches high in the tree. Deer and moose and snowshoe hare browse the twigs. Watching aspen leaves flutter in the wind under a clear blue sky is a favorite past-time of mine.

In the East aspen is one of dozens and dozens of hardwoods. In the West it is nearly the only hardwood around. And they are beauties in the fall. A few falls ago we visited Rocky Mountain National Park, when aspen were in full glory.

Photos by S. Srinivasan

Elk are abundant in the Park, maybe too abundant. Years of browsing by elk leave deep scars on aspen trunks. As we huffed and puffed along the trails at high elevation, we watched the aspen leaves flutter in the wind and some fluttered to ground.

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