Using phytolith analysis to reconstruct prehistoric fire regimes in central coastal California

Abstract

Over the last century, northern coastal scrub has encroached into open grasslands along the central California coast, increasing fire risk in coastal wildland–urban interfaces. Understanding prehistoric ecological conditions is crucial for fire mitigation projects. Current estimates of these conditions in coastal California grasslands and shrublands are largely speculative because tree ring data, lake sediment evidence and ethnographic information are sparse. Phytolith analysis is an alternative palaeoecological tool that has been successfully used to reconstruct the extent of prehistoric grass cover in California. Our study uses phytolith analysis of soil samples from the East Bay hills of the San Francisco Bay region as a novel approach to estimate prehistoric grassland distribution and infer fire frequency in central coastal California. Our data strongly indicate that many areas in the region were dominated by perennial bunchgrasses for at least several hundred years before European contact. Because grass-dominated grasslands in the East Bay hills are disturbance-dependent, our data suggest prehistoric fire frequency was of the order of 5 years or less in the region. Phytolith analysis is a useful technique for prehistoric fire regime reconstruction for grasslands and shrublands worldwide, leading to improved, data-based land management.

Burned phytoliths absorbing black carbon as a potential proxy for paleofire

“…The darkened color of burned phytoliths has been deemed to originate from fire (combustion) (Gu et al, 2008;Parr, 2006). The ratio of burned to unburned phytoliths (BPI -burned phytolith index) and/or darkening to all morphologies of phytolith has been widely used for fire reconstruction (Boyd, 2002;Gu et al, 2008;Li et al, 2010;Morris et al, 2010;Rutherford et al, 2020). Higher proportions of darkened phytolith were produced from BULLI-FORM FLABELLATE and ELONGATE morphologies relative to silica phytoliths, suggesting that particles with large size (surface area) more easily adsorb black carbon, and/or they contain more organic matter.…”

“…Whereas the bulk of paleoecological research has focused on forested ecosystems, paleoenvironmental archives are increasingly useful for elucidating how fire regimes in unforested ecosystems respond to climatic and ecological change (Leys et al, 2018;Pereboom et al, 2020;Sae-Lim et al, 2018). In particular, phytoliths are proving to be a recorder of valuable information about nonforested ecosystems of the past sources, but their ability to record fire history has been questioned (Evett and Cuthrell, 2017;Morris et al, 2010;Rutherford et al, 2020). Therefore, refining phytolithbased paleofire proxies are paramount to understanding how fires affect these unforested ecosystems.…”

“…The phytolith-related carbon value could be a potential fire (activity) indicator. Darkened and/or opaque (burned) phytolith can indicate the occurrence of fire activities (Boyd, 2002;Gu et al, 2008;Li et al, 2010;Morris et al, 2010;Rutherford et al, 2020 et al). However, the phytolith can adsorb some colored chemicals during burial in soil and sediments.…”

“…Phytoliths that have been exposed to fire often become darkened as compared with clear, unburned phytoliths (Boyd, 2002;Morris et al, 2010;Parr, 2006;Piperno and Becker, 1996). Researchers have used the ratio of burned to unburned phytoliths (BPI -burned phytolith index) in soil and sediment profiles to infer the fire occurrence (Boyd, 2002;Gu et al, 2008;Li et al, 2010;Morris et al, 2010;Rutherford et al, 2020). However, burned phytolith coloration likely changes during weathering in the soil, making it difficult to distinguish burned from weathered but unburned phytoliths (Evett and Cuthrell, 2017).…”

“…However, burned phytolith coloration likely changes during weathering in the soil, making it difficult to distinguish burned from weathered but unburned phytoliths (Evett and Cuthrell, 2017). Further, only relying on color to identify burnt phytoliths was found to be unreliable (Morris et al, 2010;Rutherford et al, 2020). The RI approach to identifying burned phytolith is based on the principle that heating of a phytolith exposed to fire expels some of the water incorporated in the phytolith matrix during its formation (Evett and Cuthrell, 2017).…”