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Huge, Rare Plant Population Found Near Solar Power Site

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One of the Rice Valley's crucifixion thorns | Photo: © 2009 Duncan S. Bell/Amanda Bell

Now that the Palen Solar Electric Generating System may have been dealt a death blow by regulators' concerns over concentrated solar flux's threat to wildlife, it's likely that similar questions will be asked about the Rice Solar Energy Project, a 150-megawatt solar power tower project planned for northeastern Riverside County, which would put a 653-foot tower among about 17,000 heliostats.

But Rice's owner Solar Reserve apparently has more to worry about than incinerated birds: a new paper indicates that the project may well damage the largest known population of a rare desert shrub, the crucifixion thorn (Castela emoryi).

Though the newly documented population is about six miles from the solar project site itself, the paper's authors express concern that pumping of groundwater to maintain the power tower project could lower the local water table, severely damaging the giant stand of rare shrubs.

The paper, entitled "A Newly Discovered Large and Significant Population of Castela emoryi in California," was written by botanists Duncan Bell and Tasya Herskovits and published in a recent issue of Aliso, the journal of the Rancho Santa Ana Botanic Garden.

According to Bell and Herskovits, something like 2,500 individual crucifixion thorn shrubs grow together in the sandy valley bottom downhill from the Rice Solar site, more than twice as many plants as there are in what had previously been the largest known group of the species in California, in Imperial County.

The species, classified by the California Native Plant Society as "rare or endangered in California," is more common elsewhere in its range in Arizona and northern Mexico. Growing up to 16 or so feet tall, the fiercely armed plant is an important larval food plant for at least one species of moth: Atteva exquisita, which apparently relies on the crucifixion thorn as the only place it lays its eggs. Its larvae then hatch out and eat the crucifixion thorn's leaves, which are laden with enough natural insecticides to repel other insects.

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Atteva exquisita | Photo: Zookeys/Wikimedia Commons/Creative Commons License

The plant's pollen apparently lacks those insecticidal contents. Bell and Herskovits suggest that a number of bee and wasp species may rely on Castela as a food source, given that few other plants in the area bloom when Castela does -- in the heat of summer.

Crucifixion thorn is generally found growing in desert soils where there's a bit more water than the average. Occasionally, this means at the edges of washes. In Rice Valley, it means at the boundary of the valley's dune habitat and a dry lake, where the lake's clay soil can hold a fair amount of water even after the surface water is long gone.

That habitat is downhill, and hence downstream, of the Rice Solar project site. Though the solar thermal project would be air-cooled, as is required of solar power tower projects in the state of California, Solar Reserve says the project would still consume up to 180 acre-feet of water each year for maintenance, which it would pump from wells on site. That's more than a third of the 500 acre-feet of annual recharge the Rice Valley aquifer gets according to a 1975 estimate by the California Department of Water Resources, and the desert has gotten quite a bit drier in the almost 40 years since.

Federal and state agencies signed off on the Rice Solar Power Project in 2011. It's unclear what effect, if any, the discovery of the state's largest population of crucifixion thorn downhill from the site will have on the project. The project's Biological Opinion from the U.S. Fish and Wildlife Service from that year nowhere mentions the crucifixion thorn. The California Energy Commission (CEC) Staff Assessment of the project mentions only that a few plants were seen on the site itself in the late 1990s.

And while the solar flux issue does rate some mention by the CEC staff in its assessment, that staff has learned a lot since then on the topic. Neither the USFWS Biological Opinion on the project nor the BLM's Environmental Impact Statements make much of the solar flux issue.

Solar Reserve is way behind schedule on Rice: the company hasn't yet begun construction on the project, which was originally slated to be delivering power by the third quarter of 2013. Financing difficulties are thought to be behind the delay.

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