Council subcommittee approves Inspector General framework, CBU women defend WAC title as UCR men energize home opener and August rains bring Riverside’s own tumbleweed species.
If Riverside has a monsoon season, it is in August. Typically, a tropical storm will slide up the Gulf of California and hit the continent about where Baja California and Sonora meet. If the storm is big enough, like a hurricane, it will plow into the Imperial Valley and then the Coachella Valley, spinning off huge chunks that flood arroyos in the lower desert as the eye continues into the Mojave. Other chunks go west, sending thunderstorms through the Banning Pass and over the Badlands into our Inland Valleys. For a few days, a monsoonal system pushes temperatures down and humidity up. Soon after, things return to normal. Almost.
Just as April showers bring May flowers, August rains bring a glow of green to local hills that lasts a month or more. In areas with native vegetation, leaves sprout from shrubs. More commonly in Riverside, with empty lots and invasive Mediterranean grasses, the green comes from summer annuals waiting for the chance to bloom. Much of that glow is composed of tumbleweeds.
“Tumbleweed” is a general term for any plant that matures, dies, dries, and detaches from its base. The spherical plant body carries seeds still attached to its branches. Wind sets it rolling, dispersing seeds for a mile or more, depending on wind direction and obstacles like fences, cars, or other tumbleweeds. Colliding with one on the freeway can create a satisfying crunch, sending tens of thousands of seeds flying. Dozens of plant species in at least 10 plant families use this dispersal method.
In Riverside and much of the United States, “tumbleweed” usually refers to Salsola species. Salsola belongs to the amaranth family, which includes both weeds like pigweed and edible crops like quinoa, epazote, beets and spinach. Young Salsola plants are edible and reportedly taste like spinach. Dried plants, spray-painted, can become unusual decorative objects.
Salsola species, however, cause problems. Caltrans considers them hazardous. Large tumbleweeds can block a driver’s view for several seconds, scratch or dent vehicles, or become lodged under a car, risking fire from the catalytic converter. Drivers often swerve dangerously to avoid them. They also compete with crops, reduce yields, and can pile up to block roads.
Salsola tragus, or Russian thistle, is the classic tumbleweed of the American West. Although now an icon of Western films and songs, it is not native. Originating in Russia, it spread across Eurasia and arrived in the United States in 1874 as a contaminant in flax seed in South Dakota. It reached California by 1893 and is now found in every state except Alaska and Florida.
Another species, Salsola australis, arrived in coastal California in the 20th century, moving eastward as S. tragus moved west. Native to Australia and South Africa, it was not recognized in California until 2000. It prefers southern coastal areas and inland valleys because it cannot tolerate cold.
The two species differ in chromosome number: S. australis has 18 chromosomes, S. tragus has 36. When they hybridize, the offspring have 27 chromosomes and are largely sterile. However, in California’s Central Valley, some hybrids doubled their chromosome number to 54, becoming fertile and forming a new species, Salsola ryanii, named after Fred Ryan, who discovered it. Because it originated in California, it is considered native, unlike its parents.
Early predictions suggested S. ryanii might go extinct due to small population sizes. But research by Dr. Shana Welles at UC Riverside showed otherwise. Of 54 populations sampled statewide, 15 contained S. ryanii, including in coastal areas and as far north as the San Francisco Bay Area. The species had spread rapidly just 10 years after its discovery.
Genetic evidence revealed that S. ryanii evolved independently at least three times, and common garden experiments showed it outperformed both parent species in weight, volume, and seed production. In its second year, an average plant produced about 300,000 seeds.
The data suggest S. ryanii is here to stay and may eventually replace its parents where they co-occur. Its rapid evolution offers a living example that evolution is ongoing, not just a phenomenon of the past.
This article was written with input from Dr. Shana Welles, Assistant Professor of Teaching, Department of Ecology, Evolution, and Organismal Biology, University of California, Riverside.
Resources and Relevant Readings
Borger, C. P. D. et al. 2008. Australian Journal of Botany 56: 600-608. Hrusa G. F. and J. F. Gaskin. 2008. Madroño 55: 113–131. Ryan, F. J. and D. R. Ayres. 2000. Canadian Journal of Botany 78: 59-67. Welles, S. R. and N. C. Ellstrand. 2016. American Journal of Botany 103: 663-667. Welles, S. R. and N. C. Ellstrand. 2016. Evolutionary Applications 9: 771-787. Welles, S. R. and N. C. Ellstrand. 2019. AOB Plants 12: doi:10.1093/aobpla/plz039 Young, F. L. 1988. Weed Science 36: 594-598.
Norm Ellstrand is an emeritus Distinguished Professor of Genetics and former Director of the California Agriculture and Food Enterprise at the University of California Riverside.
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