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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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