Northern California

Northern California is a geographic and cultural region that generally comprises the northern portion of the U.S. state of California. Spanning the state's northernmost 48 counties, its main population centers include the San Francisco Bay Area (anchored by the cities of San Jose, San Francisco, and Oakland), the Greater Sacramento area (anchored by the state capital Sacramento), the Redding, California, area south of the Cascade Range, and the Metropolitan Fresno area (anchored by the city of Fresno). Northern California also contains redwood forests, along with most of the Sierra Nevada, including Yosemite Valley and part of Lake Tahoe, Mount Shasta (the second-highest peak in the Cascade Range after Mount Rainier in Washington), and most of the Central Valley, one of the world's most productive agricultural regions. The 48-county definition is not used for the Northern California Megaregion, one of the 11 megaregions of the United States. The megaregion's area is instead defined fr

Positive litter feedbacks of an introduced species reduce native diversity and promote invasion in Californian grasslands

Pierre Rémi Mariotte

Questions: Californian grasslands have a long history of invasion, starting with the introduction of exotic forage species a century ago, and followed by newer waves of invaders. Both exotic and invasive species produce large amounts of litter, but the importance of litter accumulation on the growth and expansion of these species has not been rigorously assessed. We addressed the following questions: (1) do litter type (exotic or invasive species) and litter quantity affect exotic and invasive plant performance; (2) at which plant life stages does litter accumulation effects occur; and (3) is litter accumulation impacting native species recruitment and diversity? Location: Semi-natural grasslands, northern California. Methods: We created litter quantity gradients (0, 62.5, 125, 250 or 500 g.m(-2)) of three litter types (exotic or invasive species litter or a mixture of both). We evaluated litter effects on the germination, recruitment, production of flowers and biomass production of the exotic Avena fatua, the invasive Elymus caput-medusae and a mixture of native species. We used litter depth as a proxy for litter accumulation and measured litter impacts on light availability at ground level, soilmoisture, soil andmicrobial C and N. Results: We found a significant litter depth impact on all species, but no litter type effects. Litter accumulation principally reduced light availability at ground level and positively affected exotic and invasive species at different life-history stages. Above-ground biomass production of the exotic species Avena increased with increased litter depth, suggesting a litter accumulation feedback. The invasive species Elymus responded to increased litter with increased seed production, rather than biomass production. In contrast, litter negatively affected the recruitment and diversity of native species. Conclusion: Overall, our findings point to cascading effects in invasion with positive litter accumulation feedbacks of an exotic species simultaneously reducing native diversity and promoting seed production of a more recent invasive arrival. These results have strong consequences for invasion management and suggest that litter removal practices, such as burning and grazing, would better succeed at limiting the growth and spread of invasive species than herbicide application or seeding.

Wiley2017

Osmium isotopic compositions of Os-rich platinum group element alloys from the Klamath and Siskiyou Mountains

Reto Frei, Anders Meibom

[1] We present new measurements of Os-186/Os-188 and Os-187/Os-188 in 10 Os-rich platinum group element (PGE) alloys from placer deposits formed by the mechanical erosion of peridotite-bearing ophiolites in the Klamath and Siskiyou Mountains in northern California and southwestern Oregon. These data nearly double our database of high-precision Os-186/Os-188 measurements on such samples. Together with previously published data, our new results reinforce the conclusion that the radiogenic Os-186/Os-188 compositions of these PGE alloys are very difficult to reconcile with a derivation of their Os from the outer core. Such a model requires extremely early growth of the inner core to its present size, within several hundred million years after accretion of the Earth, which is geophysically implausible. Collectively, our data suggest instead that partial melting or metasomatic processes in the upper mantle play a primary role in controlling the Os isotopic systematics of these Os-rich PGE alloys and suggest the existence of upper mantle components characterized by radiogenic Os-186/Os-188 ratios. Pyroxene-rich lithologies are possible candidates.

2004

Positive litter feedbacks of an introduced species reduce native diversity and promote invasion in Californian grasslands

Pierre Rémi Mariotte

Wiley2017

Positive litter feedbacks of an introduced species reduce native diversity and promote invasion in Californian grasslands

Osmium isotopic compositions of Os-rich platinum group element alloys from the Klamath and Siskiyou Mountains

Osmium isotope compositions of mantle-derived Os-rich PGE grains from northern California and SW Oregon

Osmium isotope compositions of mantle-derived Os-rich PGE grains from northern California and SW Oregon

Positive litter feedbacks of an introduced species reduce native diversity and promote invasion in Californian grasslands

Osmium isotopic compositions of Os-rich platinum group element alloys from the Klamath and Siskiyou Mountains