Identification of fossil juniper seeds from Rancho La Brea (California, USA): drought and extirpation in the Late Pleistocene

Introduction

The asphaltic fossil deposits at the Rancho La Brea (RLB) locality in Los Angeles, California, USA (Fig. 1) are internationally known for the preservation of Pleistocene mega-mammals such as sabertoothed cats (Smilodon fatalis), dire wolves (Aenocyon dirus), and Columbian mammoths (Mammuthus columbi). What is less known is that the asphaltic seeps also captured and preserved an abundance of plant macrofossils, including seeds, leaves, and wood, over the site's c. 60 000 yr (60 ka) depositional history. This provides an exceptional opportunity for long-term and taxonomically highly resolved vegetation reconstructions to be made across the Late Pleistocene and Holocene for southern California. While plant material has been identified in the past with species aligning to a broad diversity of California plant communities such as closed-cone conifer forests, coastal sage scrub, oak woodland, and chaparral (Frost, 1927; Templeton, 1956, 1964; Warter, 1976), before the present study, no effort has been made to radiocarbon date plant fossils or place them into any chronological context across the 60 ka preservational window at RLB. Such a record is critical in understanding the ecology of the RLB fauna.

Fig. 1

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Map of the location of the La Brea Tar Pit (Rancho La Brea) fossil deposits (a) within the Los Angeles Basin, (b) California, and (c) United States.

During the past 60 ka, covering marine isotope stages (MISs) 3-1, significant long-term shifts in global climate occurred with the growth and decline of continental ice sheets. Abrupt millennial-scale climatic events including 17 Dansgaard–Oeschger (D-O) warming events and five of the more extreme cold intervals known as Heinrich stadials punctuated the glacial and interglacial phases, culminating in the Bølling–Allerød and Younger Dryas at the start of Holocene warming (Asmerom et al., 2010; Wagner et al., 2010; Renssen et al., 2018). The environmental upheaval occurring at this time includes the spread of humans in North America (Bennett et al., 2021) and the disappearance of much of the world's megafauna (Barnosky et al., 2011; O'Keefe et al., 2023). Plant macrofossils recovered from RLB provide a unique opportunity to track species-level responses to the extreme climatic and environmental shifts of the Late Quaternary, which in turn can offer key insights into the climate and environment during past megafauna extinction and potential vegetation range shifts with future anthropogenic warming.

Seeds, leaves, and wood of Juniperus spp. are among the most commonly found plant fossils at RLB. Juniperus is a geographically widespread genus (Adams, 2014) whose species are considered keystone taxa in woodland habitats as they modulate hydrology, nitrogen cycling (Miller & Wigand, 1994), and land surface temperatures (Wang et al., 2021), and provide food and habitat for a diversity of wildlife (Miller et al., 2019). Within fossil contexts, particularly in packrat middens (Betancourt et al., 2001, 2016; Holmgren et al., 2006, 2010; Inman et al., 2018), Juniperus spp. remnants serve as important paleoecological and paleoenvironmental indicators as they are particularly sensitive to changes in temperature, winter precipitation, and fire (Stevens et al., 2020; Loehman et al., 2023).

Juniper on the run

Recent decades have witnessed dramatic changes to juniper populations in the Northern Hemisphere, from slowed recruitment, dramatic die off, and fragmentation in their current geographic range (Fisher, 1997; Breshears et al., 2005; Lloret & García, 2016; Lu et al., 2019; Kannenberg et al., 2021; Baker et al., 2024), to active encroachment as invasive plants (Jackson et al., 2002; Mueller et al., 2005; Willson et al., 2008; Wang et al., 2021). Juniperus as a genus is considered drought tolerant, since many species demonstrate substantial resistance to water stress-induced xylem cavitation and the leaves can withstand long periods of negative water potential (Linton et al., 1998; Maherali et al., 2004; Mueller et al., 2005; Willson et al., 2008; Long et al., 2023).

Yet, juniper's ability to tolerate drought has its limits: Today, significant mortality of Utah juniper (J. osteosperma) is occurring in the American southwest due to extreme drought coupled with increased summer and annual temperatures (MacDonald, 2010; Kannenberg et al., 2021), particularly at the lower elevations (< 2000 m) of its geographical range. This same pattern has occurred repeatedly in the lowlands of southern California throughout glacial/interglacial cycles of the Late Quaternary (Woolfenden, 1996; Heusser, 1998; Davis, 1999; Mensing, 2001; Koehler et al., 2005; Heusser et al., 2015; McGann, 2015).

Sometime between the Late Pleistocene and today, naturally growing Juniperus spp. disappeared completely from the lowlands of southern California (O'Keefe et al., 2023). The nearest populations of established juniper are scattered stands of J. californica in mountainous areas and rocky washes outside the Los Angeles Basin c. 20 km north of RLB at elevations between 900 and 2700 m (Calflora, 2023), though isolated stands have been observed at lower elevations (< 100 m) in the nearby Santa Monica Mountains (Rundel & Stürmer, 1998; Calflora, 2023). However, during the Late Pleistocene, dominance of Cupressaceae pollen, likely of Juniperus spp., indicates that juniper was the most prevalent tree species growing at elevations as low as 377 m across southern California during glacial periods (Heusser, 1998; Davis, 1999; Mensing, 2001; Heusser et al., 2015). The great abundance of macrofossils preserved at RLB confirm Juniperus spp. as the dominant tree of Late Pleistocene woodlands in the region at even lower elevations (c. 58 m). In contrast to pollen, the occurrence of juniper seeds and branchlets at RLB allows for species-level identifications to be made and is representative of vegetation growing in the immediate area. Juniperus as a genus demonstrates significant interspecific variation in environmental tolerances (Miller & Wigand, 1994; Lyford et al., 2003). Therefore, having species-level identifications from fossil assemblages is necessary to understand how climatic changes have impacted ecosystems in the past, and how they are likely to impact them in the future (Schupp et al., 1997; Dimitri et al., 2017).

Juniper of Rancho La Brea

RLB's exceptional fossil record is the product of the combined presence of surficial pooling of asphalt from buried, Miocene age, oil-bearing strata, and alluvial deposition from the surrounding Santa Monica Mountains (Quinn, 1992). Throughout the Late Pleistocene and Holocene, sticky asphalt pools trapped flora and fauna at the surface of the northern Los Angeles Basin (Akersten et al., 1983; Spencer et al., 2003). In areas near to seasonal streams, flood deposits buried entrapped organic tissue, and with the aid of continued asphalt seepage, tissues such as bone, chitin, calcium carbonate, cellulose, and lignin were preserved. Renewed asphalt seepage to the surface would eventually start the entrapment process again. The resulting ‘pockets’ of fossil material have no easily discernable stratigraphy or temporal associations and may represent periods of tens of thousands of years (Friscia et al., 2008; Holden et al., 2017). As a result of this age-mixing, determining relative abundances of species at discrete time intervals is not possible, hence we use presence/absence data for our study.

Plant fossils recovered from RLB include seeds of two species of Juniperus: the large-seeded J. californica and a small-seeded juniper of uncertain affinity. Previously, the small-seeded juniper was described as J. hanseni n. sp., an extinct species (Templeton, 1964). This identification was based on seed size and a qualitative assessment of resin pit arrangement when compared to extant juniper species with cones containing one or two seeds. However, Templeton's J. hanseni n. sp. description was never officially published, and occurrences of this taxon have not been reported outside of RLB. Given that only one plant species extinction has been documented from Late Quaternary macrofossil records in North America (Jackson & Weng, 1999), it may be more likely that the small-seeded juniper of La Brea is from an extant juniper species now extirpated from southern California.

The goals of this study are to identify the small-seeded juniper to species and to track the two juniper species occurrences at RLB through time. To determine the taxonomic affinity of the unknown juniper species, we compared branchlet and seed morphology of the fossils to selected extant taxa and produced hindcasted species distribution models (SDMs) for the last glacial maximum (LGM) for morphologically similar Juniperus species using their respective modern climate envelopes. To develop a timeline for juniper occurrences at RLB, we radiocarbon dated individual Juniperus seeds. Combined, these data allow us to trace juniper's history in the basin and investigate potential causes of its disappearance.