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Invasive plants have been making their way into the state since the Spanish were building missions in the 1700s, says Matthew Brooks, research ecologist at the USGS Western Ecological Research Center. As more people continued to settle in the area, so did stray seeds. The plants, most of Mediterranean origin, were often introduced accidentally through animal feed. https://www.popsci.com/california-fires-invasive-grasses/

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This comprehensive review of a century of scientific inquiry illuminates the causes and consequences of cheatgrass (Bromus tectorum) invasions, and evaluates solutions to restore healthy native ecosystems. Introduced to North America in the 1800s, this Eurasian annual was spread by railroads, vehicles, and livestock, colonizing lands disturbed and degraded by overgrazing and other factors. Today, millions of acres have been converted to cheatgrass monoculture. Tens of millions of acres more remain at high risk of invasion. Continuing expansion across vast areas of the West indicates that current livestock grazing remains responsible for cheatgrass expansion and dominance. Cheatgrass is a habitat generalist, has an extremely high reproductive rate, and germinates earlier than native grasses. It outcompetes seedlings of native plants for water and soil nutrients and alters soil chemistry and flora to its own advantage. Livestock trampling, grazing, and surface disturbance are the key ecological switches that transitions healthy arid ecosystems to cheatgrass- invaded systems, by eliminating the native bunchgrasses and biological soil crusts that are the natural defense against weeds. A livestock-cheatgrass-fire cycle now prevails across much of the public lands of the western United States, rendering lands susceptible to larger and more frequent fires. Cheatgrass invasion degrades or eliminates habitat for native wildlife and range for livestock. Climate change will likely shift the distribution of cheatgrass, and may exacerbate invasions. Solutions to restore native habitats remain elusive and expensive. Disking, targeted grazing, prescribed fire, fuelbreak construction risk a worsening of cheatgrass infestations; plantings of non-native forage species create invasive weed infestations of their own; while herbicides, natural parasites, seeding with native plants may fail on the regional scale demanded by the problem. Reduction or elimination of livestock grazing achieves results on a sufficiently large scale, but full restoration can take decades. Conversion of native rangelands to cheatgrass markedly decreases soil carbon, so returning cheatgrass infestations to native plant assemblages could play a key role in climate mitigation. We recommend rest from livestock grazing on an allotment scale until native species replace cheatgrass. On lands with light infestations, we recommend reducing livestock grazing to levels that promote the flourishing of native species and the maintenance of soil biocrusts. Cheatgrass invasions: History, causes, consequences, and solutions, Erik M. Molvar, Roger Rosentreter, Don Mansfield,6and Greta M. Anderson - PDF