Drought May Kill Off Endangered Desert Plant, Fear Botanists

The Lane Mountain milkvetch | Photo: © Duncan S. Bell

Though California's deserts haven't been as hard-hit by the state's drought as the rest of the state, relatively speaking, botanists from UCLA are still worried the dry spell may spell the end of a plant species that's been in trouble for a long time.

The Lane Mountain milkvetch (Astragalus jaegerianus), which grows in just four patches in the Mojave Desert near Barstow, can do better in drought conditions than many other California native plants. Nonetheless, it's rare enough that the federal government listed the plant as Endangered in 1998.

A woody vining perennial plant, the Lane Mountain milkvetch grows only within the shelter of certain other desert shrubs in a couple restricted areas between Barstow and Fort Irwin. It's faced threats from off-road vehicle trampling, grazing, and military activities, but it may well be the current drought that sends it over extinction's cliff edge.

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At least that's what's making a group of UCLA botanists nervous, enough so that they'll be hauling water into the desert come February to give the plants a shot at survival.

Botanists from the Department of Ecology and Evolutionary Biology in the UCLA College of Letters and Science have been tracking the plant's numbers for the last 13 years, and the news isn't good. Even before the current drought, at least two years long in most of the Mojave Desert, the number of surviving Lane Mountain milkvetch plants has been dwindling.

Botanist Tom Huggins and his UCLA colleagues have been propagating the plants from collected seed and transplanting them out in hopes of boosting the species' numbers, but it's not straightforward. For one thing, though the milkvetch can't survive the rigors of the desert without a sheltering shrub, the shrub most readily available in that part of the Mojave is off-limits. Creosote bush (Larrea tridentata), is the most common shrub across the Mojave desert, but it turns out to be unsuitable shelter for the milkvetch.

Creosote bush is known to secrete chemicals from its roots that can inhibit the growth of other plants, but it's also a voracious consumer of what little water is available in desert soils. It's uncertain whether either of these factors is why the Lane Mountain milkvetch can't grow under a creosote bush, or whether there's some other mechanism at work. either way, being unable to use the most common plant in the Mojave Desert definitely limits the milkvetch's chances for survival.

"That's why we think the milkvetch is so endangered," Huggins said in a UCLA publication in January. "It's restricted to an area where the number of creosote bush is very low."

Another variable in the species' long-term survival is its reliance on insect pollinators in order to produce more seed. The desert solitary bee Anthidium dammersi, which makes its nests in small holes in the soil, is one of the milkvetch's main pollinators. The bees are as susceptible to drought as any other species. Without a good bloom in a dry year the bees' population may well decline, making the milkvetch's long-term survival even less likely.

Huggins and his colleagues have boosted the milkvetch's population a bit through their propagation efforts, but not by much. Of 160 seedlings transplanted into the desert in the last three years, only 20 yet survive: a 12.5 percent success rate. "If we don't get at least four inches of rain, we really can't say how many are alive, dead or dormant," says Huggins.

The Lane Mountain milkvetch has been the focus of one campaign after another over the last dozen years, with environmental activists challenging the U.S. Army over tank training exercises and suing the U.S. Fish and Wildlife Service to designate Critical Habitat for the plant. That designation finally happened in 2011, 13 years after USFWS listed the species.

It may turn out that the most important milkvetch-saving campaign of all will involve botanists carrying water into the middle of the Mojave to slake the plant's thirst.

For ongoing environmental coverage in March 2017 and afterward, please visit our show Earth Focus, or browse Redefine for historic material.
KCET's award-winning environment news project Redefine ran from July 2012 through February 2017.

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