Rand R. Evett

Stable carbon isotope composition of Poaceae pollen and its potential in paleovegetational reconstructions

Stable carbon isotope differences between ecologically distinct groups of Poaceae (C3 and C4 photosynthetic groups) provide a means of isotopically subdividing grass pollen in paleovegetation studies. We examined the isotopic composition of bulk grass plant tissue, untreated pollen, and chemically treated pollen, from several C3 and C4 grass species. Based on our data, untreated pollen is isotopically similar to the host plant from which it is derived, although small, random differences between plants and pollen occur. Methods of pollen concentration involving carbon-bearing compounds can alter the isotopic composition of recovered pollen, and in some cases, make pollen from different grass types isotopically indistinguishable. We conclude that the isotopic composition of physically separated Poaceae pollen should be an important means of determining the proportion of C3C4 grasses as long a carbon-bearing chemicals are not used in sample preparation. The carbon isotope composition of pollen should provide a new means of determining paleoclimatic conditions in grassland environments and aid in identifying the origin of the C4 photosynthetic pathway in the geologic past.

Anthropogenic Burning on the Central California Coast in Late Holocene and Early Historical Times: Findings, Implications, and Future Directions

Abstract In this final paper, we summarize the results of the eco-archaeological project, address five research questions concerning anthropogenic burning on the central California coast in Late Holocene and early historical times, and outline plans for future research.

Phytolith evidence for the extent and nature of prehistoric Californian grasslands

Californian grasslands have been radically transformed by cultivation and exotic species invasions that began with European settlement in 1769. Because native species contribute only a small percentage of total vegetation cover at most grassland sites in California today, there has been a long-standing controversy regarding the extent and composition of prehistoric grasslands. We used phytolith analysis of soil samples from 153 spatially diverse grassland sites currently cultivated or dominated by exotic annual grasses to provide the first direct evidence that, except for areas near the coast, grass was generally a minor component of the vegetation. Most of the arid, non-wetland Central Valley and surrounding foothills probably had very low grass cover. Combining our data with historical accounts of vegetation encountered by early explorers in California and relict analysis of native species in existing grassland patches, it appears that while the extent of prehistoric grassland was probably similar to the current extent of exotic annual grassland, most areas were likely dominated by a highly diverse assemblage of herbaceous species, composed largely of annual forbs adapted to exploit local environmental heterogeneity. Phytolith analysis should be routinely performed in California to guide grassland restoration efforts. The technique can also be used to reconstruct prehistoric vegetation of highly altered grassland ecosystems worldwide.

PHYTOLITH EVIDENCE FOR THE LACK OF A GRASS UNDERSTORY IN A SEQUOIADENDRON GIGANTEUM (TAXODIACEAE) STAND IN THE CENTRAL SIERRA NEVADA, CALIFORNIA

ABSTRACT Tree ring fire-scars in Sequoiadendron giganteum (giant sequoia) stands record a high frequency, low intensity, prehistoric fire regime. Difficulties achieving short prehistoric fire return intervals with prescribed burns at a S. giganteum stand in Calaveras Big Trees State Park, California, currently characterized by dense tree cover with little understory vegetation due to over a century of fire suppression, suggest that a prehistoric grass understory provided fine fuel required for frequent fire spread. We used phytolith analysis to test this hypothesis. Phytoliths, microscopic silica bodies found in many plants but produced in large quantities with distinctive morphotypes in grasses, are preserved for thousands of years in the soil. Soils under vegetation with extensive prehistoric grass cover retain a high concentration of grass phytoliths regardless of historic vegetation changes. Phytoliths were extracted from soil samples taken from pits dug at 14 plots throughout a S. giganteum stand in the South Grove Natural Preserve. Soil phytolith weight for most plots, currently without grass cover and comprising most of the stand, was less than 0.10%, consistent with reported values for forests with no grass in the understory. Soil phytolith weights for ridge-top plots and plots near the stream channel were significantly higher, suggesting localized areas with sparse grass cover. The hypothesis that there was substantial prehistoric grass cover in this S. giganteum stand was rejected.

Phytolith Evidence for a Grass-Dominated Prairie Landscape at Quiroste Valley on the Central Coast of California

Abstract Phytoliths are microscopic particles of silica formed in many plant taxa, particularly grasses. To better understand the extent of grass-dominated vegetation and the utilization of grasses by indigenous peoples in Quiroste Valley on the central coast of California, we used phytolith analysis of soil columns collected on the valley floor and columns collected within late Holocene archaeological site CA-SMA-113, located within the valley. Surface soil phytolith content in all soil columns is greater than 0.75 percent, much higher than normal for grassland in California. High phytolith content indicates extensive grass cover in the valley for several hundred to thousands of years, and cannot be explained by historical land use practices. The phytolith content–depth relationship of non-midden columns within the archaeological site mirrors the pattern observed for off-site soils; midden columns exhibit high variability at very small scales and consistently greater phytolith content below 35 cm than soils. Phytolith content of excavated ash features was highly variable; several features exhibited very high grass phytolith content. Phytolith evidence suggests there was considerable indigenous management of vegetation in Quiroste Valley; frequent burning was likely required to maintain grass-dominated grassland. Phytolith analysis is a useful tool to routinely include in archaeological investigations in California.

Distribution modelling of pre-Columbian California grasslands with soil phytoliths: New insights for prehistoric grassland ecology and restoration

Historical reconstructions of plant community distributions are useful for biogeographic studies and restoration planning, but the quality of insights gained depends on the depth and reliability of historical information available. For the Central Valley of California, one of the most altered terrestrial ecosystems on the planet, this task is particularly difficult given poor historical documentation and sparse relict assemblages of pre-invasion plant species. Coastal and interior prairies were long assumed to have been dominated by perennial bunchgrasses, but this hypothesis has recently been challenged. We evaluated this hypothesis by creating species distribution models (SDMs) using a novel approach based on the abundance of soil phytoliths (microscopic particles of biogenic silica used as a proxy for long-term grass presence) extracted from soil samples at locations statewide. Modeled historical grass abundance was consistently high along the coast and to a lesser extent in higher elevation foothills surrounding the Central Valley. SDMs found strong associations with mean temperature, temperature variability, and precipitation variability, with higher predicted abundance in regions with cooler, equable temperatures and moderated rainfall, mirroring the pattern for modern perennial grass distribution across the state. The results of this study strongly suggest that the pre-Columbian Central Valley of California was not dominated by grasses. Using soil phytolith data as input for SDMs is a promising new method for predicting the extent of prehistoric grass distributions where alternative historical datasets are lacking.