Poplars That Cope With Salty
At 13 months, these salt-tolerant DN-34 poplar trees planted at Oremet
Titanium, Inc., in Albany, Oregon, are already 12 feet tall.
Fast-growing hybrid poplars--leafy, attractive relatives of cottonwoods and
aspens--can suck up leftover water drained from irrigated farmlands. This could
lessen the need for farmers to build and maintain evaporation ponds. And the
trees might provide a new source of cash.
Evaporation ponds burden growers and the environment alike. A 1-acre
evaporation pond for drainage disposal is needed for every 10 irrigated acres
in some parts of California, for instance.
But which poplars might make the best water blotters? Scientists with the
Agricultural Research Service in California pinpointed some top candidates in
an experiment with eight different kinds of hybrid poplars.
The idea of using hybrid poplars to recycle water from farms, factories, or
sewage treatment plants isn't new. But the ARS study is among the first to
scrutinize the hybrids' ability to withstand chloride salt, boron, and selenium
in amounts sometimes present in irrigation drainage water.
Conducting the experiment were Michael C. Shannon and John H. Draper of ARS
at Riverside, California; Gary S. Bañuelos and Husein A. Ajwa with ARS
at Fresno, California; and James L. Jordahl of CH2M Hill in Portland, Oregon.
The scientists used hybrid poplars furnished by Louis A. Licht of Ecolotree,
Inc., in Iowa City, Iowa.
The hybrids--offspring of two different poplar species--are usually more
robust than either parent. Some hybrids can grow nearly 100 feet tall in 6 to 8
years, whereas regular poplars might take two or three times as long to reach
Scientists planted poplar cuttings about 6 inches long outdoors at the U.S.
Salinity Laboratory in Riverside. When the study ended 5 months later, the
trees averaged 6 feet.
Salinity of water that was piped to the cuttings ranged from 400 to 14,000
parts per million, or "about that of tap water to about one-third as salty
as seawater," says lab director Shannon.
Boron and selenium levels were low to moderately high, according to
Bañuelos, a soil scientist with the ARS Water Management Research
Laboratory in Fresno.
The most salt-tolerant poplars were hybrids known as 49177 and DN-34.
Surprisingly, they appeared to use different salt-coping strategies.
"Hybrid 49177," says Shannon, "had the highest growth rate,
so it had more leaves and other natural sinks to store salt in. DN-34
apparently had a root-level mechanism that restricted salt uptake in the
Hybrid 49177, Bañuelos reports, accumulated the most boron. Another
hybrid, number 50194, took up the most selenium and was moderately
Shannon notes that hybrid poplars are less tolerant of salt than eucalyptus,
another extremely fast-growing species sometimes chosen for water re-use.
"But there's more of a market for hybrid poplar than eucalyptus,"
says Shannon. "Hybrid poplar is made into everything from toothpicks to
high-quality veneers and papers."
In the future, after drainwater is used for on-farm plantations of poplars,
secondary drainage from the poplars may be recycled onto small fields of
salt-tolerant plants--or halophytes--such as salt grass.
"With each step in this recycling sequence," says Shannon,
"you've decreased the volume of drainage water that you have to deal with,
but you've increased the saltiness. That's why you need a halophyte."
Bañuelos points out another option: thickets of the thirsty trees
might be irrigated with poor-quality well water pumped from natural aquifers.
"That way," he says, "groundwater unfit for drinking because of
the salt it contains could be used to grow a marketable crop."
"With either option," says Shannon, "you have to know which
hybrids are the most salt tolerant. Our study shows some of the genetically
controlled variability in the hybrids' capacities to tolerate salt or to take
up or exclude boron or selenium. And it provides clues for future breeding to
exploit these traits.
"Growers who recycle water to salt-tolerant poplars and then onto
halophytes," Shannon says, "might tomorrow handle all of their farm
drainwater with fewer and smaller evaporation ponds."--By
Marcia Wood, Agricultural
Research Service Information Staff.
Michael C. Shannon is with
the USDA-ARS U.S. Salinity Laboratory,
450 West Big Springs Rd., Riverside, CA 92507; phone (909) 369-4815, fax
Gary S. Banuelos is with the
USDA-ARS Water Management
Research Laboratory, 2021 S. Peach Ave., Fresno, CA 93727; phone (209)
453-3115, fax (209) 453-3122.
"Poplars That Cope With Salty Water" was published in the
June 1998 issue of Agricultural Research magazine. Click
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