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NATURE HAPPENINGS - June 2002

Snow in Summer
by Damian Fagan

Mr. Fagan Fluffs of white swirl in the summer sun. Though the temperature is somewhere in the high eighties, the air is filled with white flecks. Snow in summer? Though the air resembles a winter blizzard when the wind blows, the “snow” is really just rain. Cottonwood rain, that is.

There comes a time each summer, generally coinciding with the peak of spring runoff, when the female cottonwood trees release their seeds. The tiny seeds are adorned with numerous fine white hairs that enable the seeds to “parachute” to earth and maintain some buoyancy when they hit the water. For that is where cottonwoods grow - along stream and river banks in the riparian zone.

So why do these trees release their seeds in summer when drier conditions exist? Cottonwood seeds need moist open ground for germination. Along the rivers this refers to the freshly deposited sediments that have settled out from spring runoff. Many visitors to Canyon Country may think that the Colorado or Green rivers which look “too muddy to drink” are bad things. But these sediments that the river transports during spring flows will form new sand bars or beaches, places where the next generation of cottonwoods may be able to become established.

But timing is everything to a cottonwood. The seeds do not have a great longevity - maybe two weeks - which is in stark contrast to other desert plants whose seeds may survive for years, awaiting proper soil conditions before they germinate. But since the cottonwoods grow along waterways, the possibility of moist soils is quite high.

As the seedlings become established on the sand bars and river banks, the roots grow quickly to keep pace with the descending water table. After one year, the roots may be fairly long. Even though the seedlings may survive that first year, there is no guarantee that next year won’t have a higher spring runoff, tearing out the sandbars laid the previous year. The odds of survival resemble my chance of winning the lottery - pretty slim.

The fast growth of the young seedlings enables the plants to increase their chances of survival season to season. A three-year-old tree may stand fifteen feet tall, with a deep tap root acting like an anchor. Growing in full-sun, cottonwoods don’t really like to grow in the shade of other trees as these trees compete for available nutrients, moisture and sunlight.

Unfortunately, some people consider cottonwoods to be “weed trees” because of their fast growth. But it is amazing, here in the Moab Valley, to see the difference in bird composition between a healthy cottonwood grove and one infested with tamarisk and Russian olives. The cottonwood grove unusually has a higher diversity of bird species than the habitat with the nonnative. This may be in part due to the diversity of insects found in the cottonwood grove, and it is this food supply that attracts birds like Wilson’s warblers, warbling vireos, yellow-billed cuckoos and Bullock’s orioles.

The tree’s structure, attracts great blue herons that may communally nest in one tree or a lone western screech owl that may be commander a nest cavity where an old limb broke off. Bald eagles and red-tailed hawks build their nests high in the crown of a cottonwood, even breaking off branches from lower limbs for construction. More than 70 different bird species build their nests in cottonwood trees - an assemblage of different individuals that would make a City planner smile.

Under pressure from invasive plants, fragmentation, and altered river flows, there are many variables that affect the health of riparian plant communities. However, it is impressive to see the linear ribbons of green snaking across the desert, anomalies in a landscape of little rain and summer “snows.”

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