Scott A. Mensing

LONG-TERM FIRE HISTORY IN GREAT BASIN SAGEBRUSH RECONSTRUCTED FROM MACROSCOPIC CHARCOAL IN SPRING SEDIMENTS, NEWARK VALLEY, NEVADA

Abstract We use macroscopic charcoal analysis to reconstruct fire history in sagebrush (Artemisia tridentata var. wyomingensis and A. tridentata var. tridentata), in Newark Valley, Nevada. We analyzed charcoal at continuous 1-cm intervals (~7–127 years), and pollen at 2- to 10-cm intervals (~70–263 years) in a core spanning the last 5500 cal yr BP (calendar years before present). A charcoal peak in the historic period was associated with a >1400-ha fire dated to 1986 that burned in the watershed. We reconstructed the prehistoric fire history by inferring fires from similar charcoal peaks that were significantly greater than the background charcoal accumulation. Our results suggest the fire regime is climate and fuel driven. During periods of wetter climate, sagebrush increased and fires were more abundant, and during extended dry periods when sagebrush decreased, fires were less frequent. Our method does not allow calculation of a fire-return interval; however, our results support models that estimate a mean fire-return interval of up to a century in Artemisia tridentata var. wyomingensis. The charcoal record indicates that fires have increased within the historic period. This contrasts with pinyon/juniper studies that indicate an expansion of woodland associated with fewer fires in the historic period. We suggest that in the central Great Basin, a regime of frequent fires in sagebrush that limits woodland expansion is true for the sagebrush-woodland ecotone, but in sagebrush-dominated valleys with lower fuel loads, fires have always been less frequent. Protecting sagebrush-dominated valleys from frequent fire would appear to be consistent with the prehistoric fire regime.

Influence of the Pacific decadal oscillation on the climate of the Sierra Nevada, California and Nevada

Mono Lake sediments have recorded five major oscillations in the hydrologic balance between A.D. 1700 and 1941. These oscillations can be correlated with tree-ring-based oscillations in Sierra Nevada snowpack. Comparison of a tree-ring-based reconstruction of the Pacific Decadal Oscillation (PDO) index (D’Arrigo et al., 2001) with a coral-based reconstruction of Subtropical South Pacific sea-surface temperature (Linsley et al., 2000) indicates a high degree of correlation between the two records during the past 300 yr. This suggests that the PDO has been a pan-Pacific phenomena for at least the past few hundred years. Major oscillations in the hydrologic balance of the Sierra Nevada correspond to changes in the sign of the PDO with extreme droughts occurring during PDO maxima. Four droughts centered on A.D. 1710, 1770, 1850, and 1930 indicate PDO-related drought reoccurrence intervals ranging from 60 to 80 yr.

A Simple Method to Separate Pollen for AMS Radiocarbon Dating and its Application to Lacustrine and Marine Sediments

We present a simple method for manually separating pollen concentrates for radiocarbon accelerator mass spectrometry (AMS) dating using a mouth pipetting system. The required equipment is readily available from scientific equipment supply houses at minimal cost. Pollen samples from lake sediments required about 4 h of hand picking, whereas samples from marine sediments required about 8 h labor. Pollen dates from marine sediments were much older than expected. We are attempting to resolve whether this is due to contamination of the pollen or the presence of significant quantities of old reworked pollen. Pollen dates from lake sediments associated with Mazama Ash were consistent with other published ages; however, replicate dates on pollen samples from above the ash were consistently older than the surrounding sediment. Our results suggest that caution must be used when interpreting pollen dates if the potential for sediment reworking is present.

A 15,000 year record of vegetation and climate change from a treeline lake in the Rocky Mountains, Wyoming, USA:

Future climate projections predict warming at high elevations that will impact treeline species, but complex topographic relief in mountains complicates ecologic response, and we have a limited number of long-term studies examining vegetation change related to climate. In this study, pollen and conifer stomata were analyzed from a 2.3 m sediment core extending to 15,330 cal. yr BP recovered from a treeline lake in the Rocky Mountains of Wyoming. Both pollen and stomata record a sequence of vegetation and climate change similar in most respects to other regional studies, with sagebrush steppe and lowered treeline during the Late Pleistocene, rapid upward movement of treeline beginning about 11,500 cal. yr BP, treeline above modern between ~9000 and 6000 cal. yr BP, and then moving downslope ~5000 cal. yr BP, reaching modern limits by ~3000 cal. yr BP. Between 6000 and 5000 cal. yr BP sediments become increasingly organic and sedimentation rates increase. We interpret this as evidence for lower lake levels during an extended dry period with warmer summer temperatures and treeline advance. The complex topography of the Rocky Mountains makes it challenging to identify regional patterns associated with short term climatic variability, but our results contribute to gaining a better understanding of past ecologic responses at high elevation sites.

3000 years of environmental change at Zaca Lake, California, USA

Climatic variations of the last few millennia can reveal patterns of variability beyond that recorded by the instrumental record. In this study we use pollen and sediments to generate a high resolution 3000 year record of vegetation and climate along the southern California coast. An increase in Pinus and Quercus pollen found in the top 100 years of the record is a result of known planting and fire suppression by the forest service. In the pre-historic record, a period of high Salix percentages and high pollen concentration from 500-250 cal yr BP represents the wettest period of the record and coincides with the Little Ice Age. We also find evidence for 3 warm periods between 1350 and 650 cal yr BP which are identified in the record by the presence of Pediastrum boryanum var. boryanum. The latter two of these periods, dating from 1070-900 and 700–650 cal yr BP correspond to Medieval Climatic Anomaly droughts identified in other records. In addition to these events, we identify a multi-centennial scale drought between 2700 and 2000 cal yr BP in Zaca Lake, corroborating evidence from across the Great Basin and extending the regional spread of this multi-centennial drought to southern California. Corresponding wetter conditions in the northwest indicate that the modern ENSO precipitation dipole also occurred during this persistent drought. Today this dipole is associated with La Nina conditions and we note a coincidence with intriguing evidence for a change in ENSO dynamics from marine records in the tropical Pacific. This dry period is remarkably persistent and has important implications for understanding the possible durations of drought conditions in the past in California.

Late Holocene fire and vegetation reconstruction from the western Klamath Mountains, California, USA: A multi-disciplinary approach for examining potential human land-use impacts

The influence of Native American land-use practices on vegetation composition and structure has long been a subject of significant debate. This is particularly true in portions of the western United States where tribal hunter-gatherers did not use agriculture to meet subsistence and other cultural needs. Climate has been viewed as the dominant determinant of vegetation structure and composition change over time, but ethnographic and anthropological evidence suggests that Native American land-use practices (particularly through the use of fire) had significant landscape effects on vegetation. However, it is difficult to distinguish climatically driven vegetation change from human-caused vegetation change using traditional paleoecological methods. To address this problem, we use a multidisciplinary methodology that incorporates paleoecology with local ethnographic and archaeological information at two lake sites in northwestern California. We show that anthropogenic impacts can be distinguished at our Fish Lake site during the cool and wet ‘Little Ice Age’, when we have evidence for open-forest or shade-intolerant vegetation, fostered for subsistence and cultural purposes, rather than the closed-forest or shade-tolerant vegetation expected due to the climatic shift. We also see a strong anthropogenic influence on modern vegetation at both sites following European settlement, decline in tribal use, and subsequent fire exclusion. These results demonstrate that Native American influences on vegetation structure and composition can be distinguished using methods that take into account both physical and cultural aspects of the landscape. They also begin to determine the scale at which western forests were influenced by Native American land-use practices and how modern forests of northwestern California are not solely products of climate alone.

Historical ecology reveals landscape transformation coincident with cultural development in central Italy since the Roman Period

Knowledge of the direct role humans have had in changing the landscape requires the perspective of historical and archaeological sources, as well as climatic and ecologic processes, when interpreting paleoecological records. People directly impact land at the local scale and land use decisions are strongly influenced by local sociopolitical priorities that change through time. A complete picture of the potential drivers of past environmental change must include a detailed and integrated analysis of evolving sociopolitical priorities, climatic change and ecological processes. However, there are surprisingly few localities that possess high-quality historical, archeological and high-resolution paleoecologic datasets. We present a high resolution 2700-year pollen record from central Italy and interpret it in relation to archival documents and archaeological data to reconstruct the relationship between changing sociopolitical conditions, and their effect on the landscape. We found that: (1) abrupt environmental change was more closely linked to sociopolitical and demographic transformation than climate change; (2) landscape changes reflected the new sociopolitical priorities and persisted until the sociopolitical conditions shifted; (3) reorganization of new plant communities was very rapid, on the order of decades not centuries; and (4) legacies of forest management adopted by earlier societies continue to influence ecosystem services today.

The potential of paleoecology for functional forest restoration planning: lessons from Late Holocene Italian pollen records

Abstract We describe forest landscape transformations during the last two millennia in the Italian peninsula by analyzing local (Rieti basin – Lago Lungo) and regional (RF93-30 Adriatic Sea) sediment cores. We identify a dynamic forest ecosystem through paleoecologic reconstruction and consider potential interventions for effective restoration of the most ancient, least disturbed forest ecosystem. The most degraded ecosystems in consequence of human activities were hygrophilous (wet) and mesic forests. In the Rieti Basin, degraded forest ecosystems on mountain slopes are undergoing some degree of forest succession and have less need of restoration. However, management plans for biodiversity, ecosystem services and resources conservation are needed to achieve more sustainable development. In Rieti, the paleoecological investigation revealed a dramatic decrease of deciduous wet and mesic tree taxa through time due to human landscape transformation. The starting point for restoration of a Mediterranean forest ecosystem that preserves natural biodiversity and associated ecosystem services requires recreating some portion of the floodplain wetland ecological niche. Once floodplain forest ecological niche has been recreated, the original ecosystem composed of Alnus, Fraxinus excelsior, Tilia spp., Carpinus betulus and Acer spp., all species which today are rare, should be planted on the basis of microsite characteristics and tree autoecology.

Using Paleolandscape Modeling to Investigate the Impact of Native American–Set Fires on Pre-Columbian Forests in the Southern Sierra Nevada, California, USA

Ethnographic accounts document widespread use of low-intensity surface fires by Californias Native Americans to manage terrestrial resources, yet the effects of such practices on forest composition and structure remain largely unknown. Although numerous paleoenvironmental studies debate whether proxy interpretations indicate climatic or anthropogenic drivers of landscape change, available data sources (e.g., pollen, charcoal) are generally insufficient to resolve anthropogenic impacts and do not allow for hypothesis testing. We use a modeling approach with LANDIS-II, a spatially explicit forest succession and disturbance model, to test whether the addition of Native American–set surface fires was necessary to approximate vegetation change as reconstructed from fossil pollen. We use an existing 1,600-year pollen and charcoal record from Holey Meadow, Sequoia National Forest, California, as the empirical data set to which we compared modeled results of climatic and anthropogenic fire regimes. We found that the addition of anthropogenic burning best approximated fossil pollen–reconstructed vegetation change, particularly during periods of prolonged cooler, wetter periods coinciding with greater regional Native American activity (1550–1050 and 750–100 cal yr BP). For lightning-caused wildfires to statistically approximate the pollen record required at least twenty times more ignitions and 870 percent more area burned annually during the Little Ice Age (750–100 cal yr BP) than observed during the modern period (AD 1985–2006), a level of natural fire increase we consider highly improbable. These results demonstrate that (1) anthropogenic burning was likely an important cause of pre-Columbian forest structure at the site and (2) dynamic landscape models provide a valuable method for testing hypotheses of paleoenvironmental change.

Book review: The Great Basin: A Natural Prehistory

This volume presents the results of an ambitious five-year project (2005–2009) based at the University of Reading ‘to assess the changes in the hydrological climate of the Middle East and North Africa (MENA) region and their impact on human communities between 20,000 BP and ad 2100, with a case study of the Jordan Valley’. The research theme ‘Water, Life and Civilisation’ (and hence the volume’s title) stems from the editors’ perception of a global ‘water crisis’ now and in the foreseeable future. This all-embracing orientation fits well with the interdisciplinary research mission at Reading described by the editors. Indeed, the challenges inherent in the seemingly ubiquitous disciplinary reorganization of universities lead the editors to call for a ‘post-disciplinary academic world’. In keeping with the ambitious scope of this research agenda, no less than 40 authors contribute 27 chapters that are book-ended with detailed introductory and concluding discussions by the editors. In light of the academic disciplinary realities alluded to above, this project is an amalgam of five sub-projects in meteorology, palaeoenvironmental research, hydrology, archaeology and development studies, which correspond roughly to the thematic parts of this volume in which the chapters are organized. Although the volume considers evidence spanning the last 20,000 years, its social focus lies on ‘civilisation’, i.e. the agrarian societies of the last five millennia characterized by social, economic and political complexity, as defined by archaeologists. This project adds the attribute of sophisticated systems of water management for the purpose of understanding ‘the complex relationships between water availability, water management and the emergence of social complexity’. With this social orientation in mind, the editors turn much of their attention to the myriad impacts of climate change, arguing for the importance of testing Global and especially Regional Climate Models in the past to assess their applicability to the future. They note, for example, that Holocene aridification in the Middle East may parallel predicted regional decreases in rainfall. The contributions of the editors in particular (they co-author 15 of the 29 chapters), turn an eye from the past to predicted rises in global temperatures through the twenty-first century that threaten the future availability of water and food on a worldwide basis. This project’s interdisciplinary research programme calls for building a chain of models: a Global Circulation Model that informs a Regional Circulation Model, which informs a more localized hydrological model, which informs archaeological and ethnographic studies, especially of water utilization. The formal climate modelling encompasses the entire Mediterranean Basin and utilizes the Regional Climate Model HadRM3, which was developed originally to predict twenty-first century climate change in the UK. The volume’s large regional study area is entirely appropriate, given the westerly storm tracks that fuel the meteorological and climate dynamics of southwestern Asia. The palaeoenvironmental, hydrological, archaeological and developmental studies, on the other hand, are much more concentrated geographically in southern Jordan, especially along the Wadi Araba and southern Jordan Valley. This focus on the southern portion of the southern Levant raises an interesting paradox. On the one hand, this region, largely arid and agriculturally marginal through the Holocene, provides an ideal setting in which to monitor the effects of climate change on agrarian society. For these same reasons, however, this region was never as densely settled as the Levantine coastal plain and foothills (especially during the urbanized Middle Bronze Age), much less the Nile Valley or Mesopotamian floodplain. Thus, a major challenge for archaeologists lies in developing comparably multidisciplinary projects to address climatic, environmental and social dynamics in the less-marginal and more heavily populated heartlands of past and future civilisations. Various discussions throughout the volume underscore the importance of dramatic episodes of climate change (e.g. the increasingly celebrated 4.2 ka event) as major factors affecting Near Eastern cultural trajectories, while reviewing the inherent difficulties in substantiating climatic causes for social and economic effects. In the course of linking individual studies to sub-projects and larger project goals, the editors highlight hydrology as a key link between climate change and human society greatly in need of study (once again harkening back to the volume’s title). An equally significant inference, noted more subtly and addressed only modestly in this volume, notes the importance of vegetation dynamics that link climate perturbations to the changing landscapes that mould and are moulded by agrarian civilisations. Water, Life and Civilisation presents a first-rate compilation of the rationale, design, methods and results of a very ambitious research agenda. The editors and authors are forthright in discussing successes and difficulties in equal detail (a hallmark of sound science), thereby providing substantial intellectual contributions that can be explored and applied in other temporal or geographical contexts. This volume is a timely addition to current academic literature seeking the means to contribute meaningfully to future global discourse on environmental–cultural interaction, especially regarding the agricultural basis of modern civilisation.